diff --git a/en_US.ISO8859-1/books/handbook/security/chapter.sgml b/en_US.ISO8859-1/books/handbook/security/chapter.sgml index 67047edfa6..0b1ab88b9b 100644 --- a/en_US.ISO8859-1/books/handbook/security/chapter.sgml +++ b/en_US.ISO8859-1/books/handbook/security/chapter.sgml @@ -1,4562 +1,5300 @@ Matthew Dillon Much of this chapter has been taken from the security(7) manual page by Security security Synopsis This chapter will provide a basic introduction to system security concepts, some general good rules of thumb, and some advanced topics under FreeBSD. A lot of the topics covered here can be applied to system and Internet security in general as well. The Internet is no longer a friendly place in which everyone wants to be your kind neighbor. Securing your system is imperative to protect your data, intellectual property, time, and much more from the hands of hackers and the like. FreeBSD provides an array of utilities and mechanisms to ensure the integrity and security of your system and network. After reading this chapter, you will know: Basic system security concepts, in respect to FreeBSD. About the various crypt mechanisms available in FreeBSD, such as DES and MD5. How to set up one-time password authentication. How to set up Kerberos, another alternative authentication system. How to create firewalls using IPFW. How to configure IPsec and create a VPN between FreeBSD/&windows; machines. How to configure and use OpenSSH, FreeBSD's SSH implementation. How to configure and load access control extension modules using the TrustedBSD MAC Framework. What file system ACLs are and how to use them. Before reading this chapter, you should: Understand basic FreeBSD and Internet concepts. Introduction Security is a function that begins and ends with the system administrator. While all BSD &unix; multi-user systems have some inherent security, the job of building and maintaining additional security mechanisms to keep those users honest is probably one of the single largest undertakings of the sysadmin. Machines are only as secure as you make them, and security concerns are ever competing with the human necessity for convenience. &unix; systems, in general, are capable of running a huge number of simultaneous processes and many of these processes operate as servers – meaning that external entities can connect and talk to them. As yesterday's mini-computers and mainframes become today's desktops, and as computers become networked and internetworked, security becomes an even bigger issue. Security is best implemented through a layered onion approach. In a nutshell, what you want to do is to create as many layers of security as are convenient and then carefully monitor the system for intrusions. You do not want to overbuild your security or you will interfere with the detection side, and detection is one of the single most important aspects of any security mechanism. For example, it makes little sense to set the schg flags (see &man.chflags.1;) on every system binary because while this may temporarily protect the binaries, it prevents an attacker who has broken in from making an easily detectable change that may result in your security mechanisms not detecting the attacker at all. System security also pertains to dealing with various forms of attack, including attacks that attempt to crash, or otherwise make a system unusable, but do not attempt to compromise the root account (break root). Security concerns can be split up into several categories: Denial of service attacks. User account compromises. Root compromise through accessible servers. Root compromise via user accounts. Backdoor creation. DoS attacks Denial of Service (DoS) security DoS attacks Denial of Service (DoS) Denial of Service (DoS) A denial of service attack is an action that deprives the machine of needed resources. Typically, DoS attacks are brute-force mechanisms that attempt to crash or otherwise make a machine unusable by overwhelming its servers or network stack. Some DoS attacks try to take advantage of bugs in the networking stack to crash a machine with a single packet. The latter can only be fixed by applying a bug fix to the kernel. Attacks on servers can often be fixed by properly specifying options to limit the load the servers incur on the system under adverse conditions. Brute-force network attacks are harder to deal with. A spoofed-packet attack, for example, is nearly impossible to stop, short of cutting your system off from the Internet. It may not be able to take your machine down, but it can saturate your Internet connection. security account compromises A user account compromise is even more common than a DoS attack. Many sysadmins still run standard telnetd, rlogind, rshd, and ftpd servers on their machines. These servers, by default, do not operate over encrypted connections. The result is that if you have any moderate-sized user base, one or more of your users logging into your system from a remote location (which is the most common and convenient way to login to a system) will have his or her password sniffed. The attentive system admin will analyze his remote access logs looking for suspicious source addresses even for successful logins. One must always assume that once an attacker has access to a user account, the attacker can break root. However, the reality is that in a well secured and maintained system, access to a user account does not necessarily give the attacker access to root. The distinction is important because without access to root the attacker cannot generally hide his tracks and may, at best, be able to do nothing more than mess with the user's files, or crash the machine. User account compromises are very common because users tend not to take the precautions that sysadmins take. security backdoors System administrators must keep in mind that there are potentially many ways to break root on a machine. The attacker may know the root password, the attacker may find a bug in a root-run server and be able to break root over a network connection to that server, or the attacker may know of a bug in a suid-root program that allows the attacker to break root once he has broken into a user's account. If an attacker has found a way to break root on a machine, the attacker may not have a need to install a backdoor. Many of the root holes found and closed to date involve a considerable amount of work by the attacker to cleanup after himself, so most attackers install backdoors. A backdoor provides the attacker with a way to easily regain root access to the system, but it also gives the smart system administrator a convenient way to detect the intrusion. Making it impossible for an attacker to install a backdoor may actually be detrimental to your security, because it will not close off the hole the attacker found to break in the first place. Security remedies should always be implemented with a multi-layered onion peel approach and can be categorized as follows: Securing root and staff accounts. Securing root – root-run servers and suid/sgid binaries. Securing user accounts. Securing the password file. Securing the kernel core, raw devices, and filesystems. Quick detection of inappropriate changes made to the system. Paranoia. The next section of this chapter will cover the above bullet items in greater depth. Securing FreeBSD security securing FreeBSD Command vs. Protocol Throughout this document, we will use bold text to refer to a command or application. This is used for instances such as ssh, since it is a protocol as well as command. The sections that follow will cover the methods of securing your FreeBSD system that were mentioned in the last section of this chapter. Securing the <username>root</username> Account and Staff Accounts su First off, do not bother securing staff accounts if you have not secured the root account. Most systems have a password assigned to the root account. The first thing you do is assume that the password is always compromised. This does not mean that you should remove the password. The password is almost always necessary for console access to the machine. What it does mean is that you should not make it possible to use the password outside of the console or possibly even with the &man.su.1; command. For example, make sure that your pty's are specified as being insecure in the /etc/ttys file so that direct root logins via telnet or rlogin are disallowed. If using other login services such as sshd, make sure that direct root logins are disabled there as well. You can do this by editing your /etc/ssh/sshd_config file, and making sure that PermitRootLogin is set to NO. Consider every access method – services such as FTP often fall through the cracks. Direct root logins should only be allowed via the system console. wheel Of course, as a sysadmin you have to be able to get to root, so we open up a few holes. But we make sure these holes require additional password verification to operate. One way to make root accessible is to add appropriate staff accounts to the wheel group (in /etc/group). The staff members placed in the wheel group are allowed to su to root. You should never give staff members native wheel access by putting them in the wheel group in their password entry. Staff accounts should be placed in a staff group, and then added to the wheel group via the /etc/group file. Only those staff members who actually need to have root access should be placed in the wheel group. It is also possible, when using an authentication method such as Kerberos, to use Kerberos' .k5login file in the root account to allow a &man.ksu.1; to root without having to place anyone at all in the wheel group. This may be the better solution since the wheel mechanism still allows an intruder to break root if the intruder has gotten hold of your password file and can break into a staff account. While having the wheel mechanism is better than having nothing at all, it is not necessarily the safest option. An indirect way to secure staff accounts, and ultimately root access is to use an alternative login access method and do what is known as starring out the encrypted password for the staff accounts. Using the &man.vipw.8; command, one can replace each instance of an encrypted password with a single * character. This command will update the /etc/master.passwd file and user/password database to disable password-authenticated logins. A staff account entry such as: foobar:R9DT/Fa1/LV9U:1000:1000::0:0:Foo Bar:/home/foobar:/usr/local/bin/tcsh Should be changed to this: foobar:*:1000:1000::0:0:Foo Bar:/home/foobar:/usr/local/bin/tcsh This change will prevent normal logins from occurring, since the encrypted password will never match *. With this done, staff members must use another mechanism to authenticate themselves such as &man.kerberos.1; or &man.ssh.1; using a public/private key pair. When using something like Kerberos, one generally must secure the machines which run the Kerberos servers and your desktop workstation. When using a public/private key pair with ssh, one must generally secure the machine used to login from (typically one's workstation). An additional layer of protection can be added to the key pair by password protecting the key pair when creating it with &man.ssh-keygen.1;. Being able to star out the passwords for staff accounts also guarantees that staff members can only login through secure access methods that you have set up. This forces all staff members to use secure, encrypted connections for all of their sessions, which closes an important hole used by many intruders: sniffing the network from an unrelated, less secure machine. The more indirect security mechanisms also assume that you are logging in from a more restrictive server to a less restrictive server. For example, if your main box is running all sorts of servers, your workstation should not be running any. In order for your workstation to be reasonably secure you should run as few servers as possible, up to and including no servers at all, and you should run a password-protected screen blanker. Of course, given physical access to a workstation an attacker can break any sort of security you put on it. This is definitely a problem that you should consider, but you should also consider the fact that the vast majority of break-ins occur remotely, over a network, from people who do not have physical access to your workstation or servers. Kerberos Using something like Kerberos also gives you the ability to disable or change the password for a staff account in one place, and have it immediately effect all the machines on which the staff member may have an account. If a staff member's account gets compromised, the ability to instantly change his password on all machines should not be underrated. With discrete passwords, changing a password on N machines can be a mess. You can also impose re-passwording restrictions with Kerberos: not only can a Kerberos ticket be made to timeout after a while, but the Kerberos system can require that the user choose a new password after a certain period of time (say, once a month). Securing Root-run Servers and SUID/SGID Binaries ntalk comsat finger sandboxes sshd telnetd rshd rlogind The prudent sysadmin only runs the servers he needs to, no more, no less. Be aware that third party servers are often the most bug-prone. For example, running an old version of imapd or popper is like giving a universal root ticket out to the entire world. Never run a server that you have not checked out carefully. Many servers do not need to be run as root. For example, the ntalk, comsat, and finger daemons can be run in special user sandboxes. A sandbox is not perfect, unless you go through a large amount of trouble, but the onion approach to security still stands: If someone is able to break in through a server running in a sandbox, they still have to break out of the sandbox. The more layers the attacker must break through, the lower the likelihood of his success. Root holes have historically been found in virtually every server ever run as root, including basic system servers. If you are running a machine through which people only login via sshd and never login via telnetd or rshd or rlogind, then turn off those services! FreeBSD now defaults to running ntalkd, comsat, and finger in a sandbox. Another program which may be a candidate for running in a sandbox is &man.named.8;. /etc/defaults/rc.conf includes the arguments necessary to run named in a sandbox in a commented-out form. Depending on whether you are installing a new system or upgrading an existing system, the special user accounts used by these sandboxes may not be installed. The prudent sysadmin would research and implement sandboxes for servers whenever possible. sendmail There are a number of other servers that typically do not run in sandboxes: sendmail, popper, imapd, ftpd, and others. There are alternatives to some of these, but installing them may require more work than you are willing to perform (the convenience factor strikes again). You may have to run these servers as root and rely on other mechanisms to detect break-ins that might occur through them. The other big potential root holes in a system are the suid-root and sgid binaries installed on the system. Most of these binaries, such as rlogin, reside in /bin, /sbin, /usr/bin, or /usr/sbin. While nothing is 100% safe, the system-default suid and sgid binaries can be considered reasonably safe. Still, root holes are occasionally found in these binaries. A root hole was found in Xlib in 1998 that made xterm (which is typically suid) vulnerable. It is better to be safe than sorry and the prudent sysadmin will restrict suid binaries, that only staff should run, to a special group that only staff can access, and get rid of (chmod 000) any suid binaries that nobody uses. A server with no display generally does not need an xterm binary. Sgid binaries can be almost as dangerous. If an intruder can break an sgid-kmem binary, the intruder might be able to read /dev/kmem and thus read the encrypted password file, potentially compromising any passworded account. Alternatively an intruder who breaks group kmem can monitor keystrokes sent through pty's, including pty's used by users who login through secure methods. An intruder that breaks the tty group can write to almost any user's tty. If a user is running a terminal program or emulator with a keyboard-simulation feature, the intruder can potentially generate a data stream that causes the user's terminal to echo a command, which is then run as that user. Securing User Accounts User accounts are usually the most difficult to secure. While you can impose Draconian access restrictions on your staff and star out their passwords, you may not be able to do so with any general user accounts you might have. If you do have sufficient control, then you may win out and be able to secure the user accounts properly. If not, you simply have to be more vigilant in your monitoring of those accounts. Use of ssh and Kerberos for user accounts is more problematic, due to the extra administration and technical support required, but still a very good solution compared to a crypted password file. Securing the Password File The only sure fire way is to * out as many passwords as you can and use ssh or Kerberos for access to those accounts. Even though the encrypted password file (/etc/spwd.db) can only be read by root, it may be possible for an intruder to obtain read access to that file even if the attacker cannot obtain root-write access. Your security scripts should always check for and report changes to the password file (see the Checking file integrity section below). Securing the Kernel Core, Raw Devices, and Filesystems If an attacker breaks root he can do just about anything, but there are certain conveniences. For example, most modern kernels have a packet sniffing device driver built in. Under FreeBSD it is called the bpf device. An intruder will commonly attempt to run a packet sniffer on a compromised machine. You do not need to give the intruder the capability and most systems do not have the need for the bpf device compiled in. sysctl But even if you turn off the bpf device, you still have /dev/mem and /dev/kmem to worry about. For that matter, the intruder can still write to raw disk devices. Also, there is another kernel feature called the module loader, &man.kldload.8;. An enterprising intruder can use a KLD module to install his own bpf device, or other sniffing device, on a running kernel. To avoid these problems you have to run the kernel at a higher secure level, at least securelevel 1. The securelevel can be set with a sysctl on the kern.securelevel variable. Once you have set the securelevel to 1, write access to raw devices will be denied and special chflags flags, such as schg, will be enforced. You must also ensure that the schg flag is set on critical startup binaries, directories, and script files – everything that gets run up to the point where the securelevel is set. This might be overdoing it, and upgrading the system is much more difficult when you operate at a higher secure level. You may compromise and run the system at a higher secure level but not set the schg flag for every system file and directory under the sun. Another possibility is to simply mount / and /usr read-only. It should be noted that being too Draconian in what you attempt to protect may prevent the all-important detection of an intrusion. Checking File Integrity: Binaries, Configuration Files, Etc. When it comes right down to it, you can only protect your core system configuration and control files so much before the convenience factor rears its ugly head. For example, using chflags to set the schg bit on most of the files in / and /usr is probably counterproductive, because while it may protect the files, it also closes a detection window. The last layer of your security onion is perhaps the most important – detection. The rest of your security is pretty much useless (or, worse, presents you with a false sense of safety) if you cannot detect potential incursions. Half the job of the onion is to slow down the attacker, rather than stop him, in order to give the detection side of the equation a chance to catch him in the act. The best way to detect an incursion is to look for modified, missing, or unexpected files. The best way to look for modified files is from another (often centralized) limited-access system. Writing your security scripts on the extra-secure limited-access system makes them mostly invisible to potential attackers, and this is important. In order to take maximum advantage you generally have to give the limited-access box significant access to the other machines in the business, usually either by doing a read-only NFS export of the other machines to the limited-access box, or by setting up ssh key-pairs to allow the limited-access box to ssh to the other machines. Except for its network traffic, NFS is the least visible method – allowing you to monitor the filesystems on each client box virtually undetected. If your limited-access server is connected to the client boxes through a switch, the NFS method is often the better choice. If your limited-access server is connected to the client boxes through a hub, or through several layers of routing, the NFS method may be too insecure (network-wise) and using ssh may be the better choice even with the audit-trail tracks that ssh lays. Once you give a limited-access box, at least read access to the client systems it is supposed to monitor, you must write scripts to do the actual monitoring. Given an NFS mount, you can write scripts out of simple system utilities such as &man.find.1; and &man.md5.1;. It is best to physically md5 the client-box files at least once a day, and to test control files such as those found in /etc and /usr/local/etc even more often. When mismatches are found, relative to the base md5 information the limited-access machine knows is valid, it should scream at a sysadmin to go check it out. A good security script will also check for inappropriate suid binaries and for new or deleted files on system partitions such as / and /usr. When using ssh rather than NFS, writing the security script is much more difficult. You essentially have to scp the scripts to the client box in order to run them, making them visible, and for safety you also need to scp the binaries (such as find) that those scripts use. The ssh client on the client box may already be compromised. All in all, using ssh may be necessary when running over insecure links, but it is also a lot harder to deal with. A good security script will also check for changes to user and staff members access configuration files: .rhosts, .shosts, .ssh/authorized_keys and so forth… files that might fall outside the purview of the MD5 check. If you have a huge amount of user disk space, it may take too long to run through every file on those partitions. In this case, setting mount flags to disallow suid binaries and devices on those partitions is a good idea. The nodev and nosuid options (see &man.mount.8;) are what you want to look into. You should probably scan them anyway, at least once a week, since the object of this layer is to detect a break-in whether or not the break-in is effective. Process accounting (see &man.accton.8;) is a relatively low-overhead feature of the operating system which might help as a post-break-in evaluation mechanism. It is especially useful in tracking down how an intruder has actually broken into a system, assuming the file is still intact after the break-in occurs. Finally, security scripts should process the log files, and the logs themselves should be generated in as secure a manner as possible – remote syslog can be very useful. An intruder tries to cover his tracks, and log files are critical to the sysadmin trying to track down the time and method of the initial break-in. One way to keep a permanent record of the log files is to run the system console to a serial port and collect the information on a continuing basis through a secure machine monitoring the consoles. Paranoia A little paranoia never hurts. As a rule, a sysadmin can add any number of security features, as long as they do not effect convenience, and can add security features that do effect convenience with some added thought. Even more importantly, a security administrator should mix it up a bit – if you use recommendations such as those given by this document verbatim, you give away your methodologies to the prospective attacker who also has access to this document. Denial of Service Attacks Denial of Service (DoS) This section covers Denial of Service attacks. A DoS attack is typically a packet attack. While there is not much you can do about modern spoofed packet attacks that saturate your network, you can generally limit the damage by ensuring that the attacks cannot take down your servers. Limiting server forks. Limiting springboard attacks (ICMP response attacks, ping broadcast, etc.). Kernel Route Cache. A common DoS attack is against a forking server that attempts to cause the server to eat processes, file descriptors, and memory, until the machine dies. inetd (see &man.inetd.8;) has several options to limit this sort of attack. It should be noted that while it is possible to prevent a machine from going down, it is not generally possible to prevent a service from being disrupted by the attack. Read the inetd manual page carefully and pay specific attention to the , , and options. Note that spoofed-IP attacks will circumvent the option to inetd, so typically a combination of options must be used. Some standalone servers have self-fork-limitation parameters. Sendmail has its option, which tends to work much better than trying to use sendmail's load limiting options due to the load lag. You should specify a MaxDaemonChildren parameter, when you start sendmail, high enough to handle your expected load, but not so high that the computer cannot handle that number of sendmails without falling on its face. It is also prudent to run sendmail in queued mode () and to run the daemon (sendmail -bd) separate from the queue-runs (sendmail -q15m). If you still want real-time delivery you can run the queue at a much lower interval, such as , but be sure to specify a reasonable MaxDaemonChildren option for that sendmail to prevent cascade failures. Syslogd can be attacked directly and it is strongly recommended that you use the option whenever possible, and the option otherwise. You should also be fairly careful with connect-back services such as tcpwrapper's reverse-identd, which can be attacked directly. You generally do not want to use the reverse-ident feature of tcpwrappers for this reason. It is a very good idea to protect internal services from external access by firewalling them off at your border routers. The idea here is to prevent saturation attacks from outside your LAN, not so much to protect internal services from network-based root compromise. Always configure an exclusive firewall, i.e., firewall everything except ports A, B, C, D, and M-Z. This way you can firewall off all of your low ports except for certain specific services such as named (if you are primary for a zone), ntalkd, sendmail, and other Internet-accessible services. If you try to configure the firewall the other way – as an inclusive or permissive firewall, there is a good chance that you will forget to close a couple of services, or that you will add a new internal service and forget to update the firewall. You can still open up the high-numbered port range on the firewall, to allow permissive-like operation, without compromising your low ports. Also take note that FreeBSD allows you to control the range of port numbers used for dynamic binding, via the various net.inet.ip.portrange sysctl's (sysctl -a | fgrep portrange), which can also ease the complexity of your firewall's configuration. For example, you might use a normal first/last range of 4000 to 5000, and a hiport range of 49152 to 65535, then block off everything under 4000 in your firewall (except for certain specific Internet-accessible ports, of course). ICMP_BANDLIM Another common DoS attack is called a springboard attack – to attack a server in a manner that causes the server to generate responses which overloads the server, the local network, or some other machine. The most common attack of this nature is the ICMP ping broadcast attack. The attacker spoofs ping packets sent to your LAN's broadcast address with the source IP address set to the actual machine they wish to attack. If your border routers are not configured to stomp on ping's to broadcast addresses, your LAN winds up generating sufficient responses to the spoofed source address to saturate the victim, especially when the attacker uses the same trick on several dozen broadcast addresses over several dozen different networks at once. Broadcast attacks of over a hundred and twenty megabits have been measured. A second common springboard attack is against the ICMP error reporting system. By constructing packets that generate ICMP error responses, an attacker can saturate a server's incoming network and cause the server to saturate its outgoing network with ICMP responses. This type of attack can also crash the server by running it out of mbuf's, especially if the server cannot drain the ICMP responses it generates fast enough. The FreeBSD kernel has a new kernel compile option called which limits the effectiveness of these sorts of attacks. The last major class of springboard attacks is related to certain internal inetd services such as the udp echo service. An attacker simply spoofs a UDP packet with the source address being server A's echo port, and the destination address being server B's echo port, where server A and B are both on your LAN. The two servers then bounce this one packet back and forth between each other. The attacker can overload both servers and their LANs simply by injecting a few packets in this manner. Similar problems exist with the internal chargen port. A competent sysadmin will turn off all of these inetd-internal test services. Spoofed packet attacks may also be used to overload the kernel route cache. Refer to the net.inet.ip.rtexpire, rtminexpire, and rtmaxcache sysctl parameters. A spoofed packet attack that uses a random source IP will cause the kernel to generate a temporary cached route in the route table, viewable with netstat -rna | fgrep W3. These routes typically timeout in 1600 seconds or so. If the kernel detects that the cached route table has gotten too big it will dynamically reduce the rtexpire but will never decrease it to less than rtminexpire. There are two problems: The kernel does not react quickly enough when a lightly loaded server is suddenly attacked. The rtminexpire is not low enough for the kernel to survive a sustained attack. If your servers are connected to the Internet via a T3 or better, it may be prudent to manually override both rtexpire and rtminexpire via &man.sysctl.8;. Never set either parameter to zero (unless you want to crash the machine). Setting both parameters to 2 seconds should be sufficient to protect the route table from attack. Access Issues with Kerberos and SSH ssh Kerberos There are a few issues with both Kerberos and ssh that need to be addressed if you intend to use them. Kerberos V is an excellent authentication protocol, but there are bugs in the kerberized telnet and rlogin applications that make them unsuitable for dealing with binary streams. Also, by default Kerberos does not encrypt a session unless you use the option. ssh encrypts everything by default. ssh works quite well in every respect except that it forwards encryption keys by default. What this means is that if you have a secure workstation holding keys that give you access to the rest of the system, and you ssh to an insecure machine, your keys are usable. The actual keys themselves are not exposed, but ssh installs a forwarding port for the duration of your login, and if an attacker has broken root on the insecure machine he can utilize that port to use your keys to gain access to any other machine that your keys unlock. We recommend that you use ssh in combination with Kerberos whenever possible for staff logins. ssh can be compiled with Kerberos support. This reduces your reliance on potentially exposable ssh keys while at the same time protecting passwords via Kerberos. ssh keys should only be used for automated tasks from secure machines (something that Kerberos is unsuited to do). We also recommend that you either turn off key-forwarding in the ssh configuration, or that you make use of the from=IP/DOMAIN option that ssh allows in its authorized_keys file to make the key only usable to entities logging in from specific machines. Bill Swingle Parts rewritten and updated by DES, MD5, and Crypt security crypt crypt DES MD5 Every user on a &unix; system has a password associated with their account. It seems obvious that these passwords need to be known only to the user and the actual operating system. In order to keep these passwords secret, they are encrypted with what is known as a one-way hash, that is, they can only be easily encrypted but not decrypted. In other words, what we told you a moment ago was obvious is not even true: the operating system itself does not really know the password. It only knows the encrypted form of the password. The only way to get the plain-text password is by a brute force search of the space of possible passwords. Unfortunately the only secure way to encrypt passwords when &unix; came into being was based on DES, the Data Encryption Standard. This was not such a problem for users resident in the US, but since the source code for DES could not be exported outside the US, FreeBSD had to find a way to both comply with US law and retain compatibility with all the other &unix; variants that still used DES. The solution was to divide up the encryption libraries so that US users could install the DES libraries and use DES but international users still had an encryption method that could be exported abroad. This is how FreeBSD came to use MD5 as its default encryption method. MD5 is believed to be more secure than DES, so installing DES is offered primarily for compatibility reasons. Recognizing Your Crypt Mechanism Before FreeBSD 4.4 libcrypt.a was a symbolic link pointing to the library which was used for encryption. FreeBSD 4.4 changed libcrypt.a to provide a configurable password authentication hash library. Currently the library supports DES, MD5 and Blowfish hash functions. By default FreeBSD uses MD5 to encrypt passwords. It is pretty easy to identify which encryption method FreeBSD is set up to use. Examining the encrypted passwords in the /etc/master.passwd file is one way. Passwords encrypted with the MD5 hash are longer than those encrypted with the DES hash and also begin with the characters $1$. Passwords starting with $2$ are encrypted with the Blowfish hash function. DES password strings do not have any particular identifying characteristics, but they are shorter than MD5 passwords, and are coded in a 64-character alphabet which does not include the $ character, so a relatively short string which does not begin with a dollar sign is very likely a DES password. The password format used for new passwords is controlled by the passwd_format login capability in /etc/login.conf, which takes values of des, md5 or blf. See the &man.login.conf.5; manual page for more information about login capabilities. One-time Passwords one-time passwords security one-time passwords S/Key is a one-time password scheme based on a one-way hash function. FreeBSD uses the MD4 hash for compatibility but other systems have used MD5 and DES-MAC. S/Key has been part of the FreeBSD base system since version 1.1.5 and is also used on a growing number of other operating systems. S/Key is a registered trademark of Bell Communications Research, Inc. From version 5.0 of FreeBSD, S/Key has been replaced with the functionally equivalent OPIE (One-time Passwords In Everything). OPIE uses the MD5 hash by default. There are three different sorts of passwords which we will discuss below. The first is your usual &unix; style or Kerberos password; we will call this a &unix; password. The second sort is the one-time password which is generated by the S/Key key program or the OPIE &man.opiekey.1; program and accepted by the keyinit or &man.opiepasswd.1; programs and the login prompt; we will call this a one-time password. The final sort of password is the secret password which you give to the key/opiekey programs (and sometimes the keyinit/opiepasswd programs) which it uses to generate one-time passwords; we will call it a secret password or just unqualified password. The secret password does not have anything to do with your &unix; password; they can be the same but this is not recommended. S/Key and OPIE secret passwords are not limited to 8 characters like old &unix; passwordsUnder &os; the standard login password may be up to 128 characters in length., they can be as long as you like. Passwords of six or seven word long phrases are fairly common. For the most part, the S/Key or OPIE system operates completely independently of the &unix; password system. Besides the password, there are two other pieces of data that are important to S/Key and OPIE. One is what is known as the seed or key, consisting of two letters and five digits. The other is what is called the iteration count, a number between 1 and 100. S/Key creates the one-time password by concatenating the seed and the secret password, then applying the MD4/MD5 hash as many times as specified by the iteration count and turning the result into six short English words. These six English words are your one-time password. The authentication system (primarily PAM) keeps track of the last one-time password used, and the user is authenticated if the hash of the user-provided password is equal to the previous password. Because a one-way hash is used it is impossible to generate future one-time passwords if a successfully used password is captured; the iteration count is decremented after each successful login to keep the user and the login program in sync. When the iteration count gets down to 1, S/Key and OPIE must be reinitialized. There are three programs involved in each system which we will discuss below. The key and opiekey programs accept an iteration count, a seed, and a secret password, and generate a one-time password or a consecutive list of one-time passwords. The keyinit and opiepasswd programs are used to initialize S/Key and OPIE respectively, and to change passwords, iteration counts, or seeds; they take either a secret passphrase, or an iteration count, seed, and one-time password. The keyinfo and opieinfo programs examine the relevant credentials files (/etc/skeykeys or /etc/opiekeys) and print out the invoking user's current iteration count and seed. There are four different sorts of operations we will cover. The first is using keyinit or opiepasswd over a secure connection to set up one-time-passwords for the first time, or to change your password or seed. The second operation is using keyinit or opiepasswd over an insecure connection, in conjunction with key or opiekey over a secure connection, to do the same. The third is using key/opiekey to log in over an insecure connection. The fourth is using key or opiekey to generate a number of keys which can be written down or printed out to carry with you when going to some location without secure connections to anywhere. Secure Connection Initialization To initialize S/Key for the first time, change your password, or change your seed while logged in over a secure connection (e.g., on the console of a machine or via ssh), use the keyinit command without any parameters while logged in as yourself: &prompt.user; keyinit Adding unfurl: Reminder - Only use this method if you are directly connected. If you are using telnet or rlogin exit with no password and use keyinit -s. Enter secret password: Again secret password: ID unfurl s/key is 99 to17757 DEFY CLUB PRO NASH LACE SOFT For OPIE, opiepasswd is used instead: &prompt.user; opiepasswd -c [grimreaper] ~ $ opiepasswd -f -c Adding unfurl: Only use this method from the console; NEVER from remote. If you are using telnet, xterm, or a dial-in, type ^C now or exit with no password. Then run opiepasswd without the -c parameter. Using MD5 to compute responses. Enter new secret pass phrase: Again new secret pass phrase: ID unfurl OTP key is 499 to4268 MOS MALL GOAT ARM AVID COED At the Enter new secret pass phrase: or Enter secret password: prompts, you should enter a password or phrase. Remember, this is not the password that you will use to login with, this is used to generate your one-time login keys. The ID line gives the parameters of your particular instance: your login name, the iteration count, and seed. When logging in the system will remember these parameters and present them back to you so you do not have to remember them. The last line gives the particular one-time password which corresponds to those parameters and your secret password; if you were to re-login immediately, this one-time password is the one you would use. Insecure Connection Initialization To initialize or change your secret password over an insecure connection, you will need to already have a secure connection to some place where you can run key or opiekey; this might be in the form of a desk accessory on a &macintosh;, or a shell prompt on a machine you trust. You will also need to make up an iteration count (100 is probably a good value), and you may make up your own seed or use a randomly-generated one. Over on the insecure connection (to the machine you are initializing), use the keyinit -s command: &prompt.user; keyinit -s Updating unfurl: Old key: to17758 Reminder you need the 6 English words from the key command. Enter sequence count from 1 to 9999: 100 Enter new key [default to17759]: s/key 100 to 17759 s/key access password: s/key access password:CURE MIKE BANE HIM RACY GORE For OPIE, you need to use opiepasswd: &prompt.user; opiepasswd Updating unfurl: You need the response from an OTP generator. Old secret pass phrase: otp-md5 498 to4268 ext Response: GAME GAG WELT OUT DOWN CHAT New secret pass phrase: otp-md5 499 to4269 Response: LINE PAP MILK NELL BUOY TROY ID mark OTP key is 499 gr4269 LINE PAP MILK NELL BUOY TROY To accept the default seed (which the keyinit program confusingly calls a key), press Return. Then before entering an access password, move over to your secure connection or S/Key desk accessory, and give it the same parameters: &prompt.user; key 100 to17759 Reminder - Do not use this program while logged in via telnet or rlogin. Enter secret password: <secret password> CURE MIKE BANE HIM RACY GORE Or for OPIE: &prompt.user; opiekey 498 to4268 Using the MD5 algorithm to compute response. Reminder: Don't use opiekey from telnet or dial-in sessions. Enter secret pass phrase: GAME GAG WELT OUT DOWN CHAT Now switch back over to the insecure connection, and copy the one-time password generated over to the relevant program. Generating a Single One-time Password Once you have initialized S/Key or OPIE, when you login you will be presented with a prompt like this: &prompt.user; telnet example.com Trying 10.0.0.1... Connected to example.com Escape character is '^]'. FreeBSD/i386 (example.com) (ttypa) login: <username> s/key 97 fw13894 Password: Or for OPIE: &prompt.user; telnet example.com Trying 10.0.0.1... Connected to example.com Escape character is '^]'. FreeBSD/i386 (example.com) (ttypa) login: <username> otp-md5 498 gr4269 ext Password: As a side note, the S/Key and OPIE prompts have a useful feature (not shown here): if you press Return at the password prompt, the prompter will turn echo on, so you can see what you are typing. This can be extremely useful if you are attempting to type in a password by hand, such as from a printout. MS-DOS Windows MacOS At this point you need to generate your one-time password to answer this login prompt. This must be done on a trusted system that you can run key or opiekey on. (There are versions of these for DOS, &windows; and &macos; as well.) They need both the iteration count and the seed as command line options. You can cut-and-paste these right from the login prompt on the machine that you are logging in to. On the trusted system: &prompt.user; key 97 fw13894 Reminder - Do not use this program while logged in via telnet or rlogin. Enter secret password: WELD LIP ACTS ENDS ME HAAG For OPIE: &prompt.user; opiekey 498 to4268 Using the MD5 algorithm to compute response. Reminder: Don't use opiekey from telnet or dial-in sessions. Enter secret pass phrase: GAME GAG WELT OUT DOWN CHAT Now that you have your one-time password you can continue logging in: login: <username> s/key 97 fw13894 Password: <return to enable echo> s/key 97 fw13894 Password [echo on]: WELD LIP ACTS ENDS ME HAAG Last login: Tue Mar 21 11:56:41 from 10.0.0.2 ... Generating Multiple One-time Passwords Sometimes you have to go places where you do not have access to a trusted machine or secure connection. In this case, it is possible to use the key and opiekey commands to generate a number of one-time passwords beforehand to be printed out and taken with you. For example: &prompt.user; key -n 5 30 zz99999 Reminder - Do not use this program while logged in via telnet or rlogin. Enter secret password: <secret password> 26: SODA RUDE LEA LIND BUDD SILT 27: JILT SPY DUTY GLOW COWL ROT 28: THEM OW COLA RUNT BONG SCOT 29: COT MASH BARR BRIM NAN FLAG 30: CAN KNEE CAST NAME FOLK BILK Or for OPIE: &prompt.user; opiekey -n 5 30 zz99999 Using the MD5 algorithm to compute response. Reminder: Don't use opiekey from telnet or dial-in sessions. Enter secret pass phrase: <secret password> 26: JOAN BORE FOSS DES NAY QUIT 27: LATE BIAS SLAY FOLK MUCH TRIG 28: SALT TIN ANTI LOON NEAL USE 29: RIO ODIN GO BYE FURY TIC 30: GREW JIVE SAN GIRD BOIL PHI The requests five keys in sequence, the specifies what the last iteration number should be. Note that these are printed out in reverse order of eventual use. If you are really paranoid, you might want to write the results down by hand; otherwise you can cut-and-paste into lpr. Note that each line shows both the iteration count and the one-time password; you may still find it handy to scratch off passwords as you use them. Restricting Use of &unix; Passwords S/Key can place restrictions on the use of &unix; passwords based on the host name, user name, terminal port, or IP address of a login session. These restrictions can be found in the configuration file /etc/skey.access. The &man.skey.access.5; manual page has more information on the complete format of the file and also details some security cautions to be aware of before depending on this file for security. If there is no /etc/skey.access file (this is the default on FreeBSD 4.X systems), then all users will be allowed to use &unix; passwords. If the file exists, however, then all users will be required to use S/Key unless explicitly permitted to do otherwise by configuration statements in the skey.access file. In all cases, &unix; passwords are permitted on the console. Here is a sample skey.access configuration file which illustrates the three most common sorts of configuration statements: permit internet 192.168.0.0 255.255.0.0 permit user fnord permit port ttyd0 The first line (permit internet) allows users whose IP source address (which is vulnerable to spoofing) matches the specified value and mask, to use &unix; passwords. This should not be considered a security mechanism, but rather, a means to remind authorized users that they are using an insecure network and need to use S/Key for authentication. The second line (permit user) allows the specified username, in this case fnord, to use &unix; passwords at any time. Generally speaking, this should only be used for people who are either unable to use the key program, like those with dumb terminals, or those who are uneducable. The third line (permit port) allows all users logging in on the specified terminal line to use &unix; passwords; this would be used for dial-ups. OPIE can restrict the use of &unix; passwords based on the IP address of a login session just like S/Key does. The relevant file is /etc/opieaccess, which is present by default on FreeBSD 5.0 and newer systems. Please check &man.opieaccess.5; for more information on this file and which security considerations you should be aware of when using it. Here is a sample opieaccess file: permit 192.168.0.0 255.255.0.0 This line allows users whose IP source address (which is vulnerable to spoofing) matches the specified value and mask, to use &unix; passwords at any time. If no rules in opieaccess are matched, the default is to deny non-OPIE logins. Mark Murray Contributed by Mark Dapoz Based on a contribution by Kerberos Kerberos Kerberos is a network add-on system/protocol that allows users to authenticate themselves through the services of a secure server. Services such as remote login, remote copy, secure inter-system file copying and other high-risk tasks are made considerably safer and more controllable. The following instructions can be used as a guide on how to set up Kerberos as distributed for FreeBSD. However, you should refer to the relevant manual pages for a complete description. Installing Kerberos MIT Kerberos installing Kerberos is an optional component of FreeBSD. The easiest way to install this software is by selecting the krb4 or krb5 distribution in sysinstall during the initial installation of FreeBSD. This will install the eBones (KerberosIV) or Heimdal (Kerberos5) implementation of Kerberos. These implementations are included because they are developed outside the USA/Canada and were thus available to system owners outside those countries during the era of restrictive export controls on cryptographic code from the USA. Alternatively, the MIT implementation of Kerberos is available from the ports collection as security/krb5. Creating the Initial Database This is done on the Kerberos server only. First make sure that you do not have any old Kerberos databases around. You should change to the directory /etc/kerberosIV and check that only the following files are present: &prompt.root; cd /etc/kerberosIV &prompt.root; ls README krb.conf krb.realms If any additional files (such as principal.* or master_key) exist, then use the kdb_destroy command to destroy the old Kerberos database, or if Kerberos is not running, simply delete the extra files. You should now edit the krb.conf and krb.realms files to define your Kerberos realm. In this case the realm will be EXAMPLE.COM and the server is grunt.example.com. We edit or create the krb.conf file: &prompt.root; cat krb.conf EXAMPLE.COM EXAMPLE.COM grunt.example.com admin server CS.BERKELEY.EDU okeeffe.berkeley.edu ATHENA.MIT.EDU kerberos.mit.edu ATHENA.MIT.EDU kerberos-1.mit.edu ATHENA.MIT.EDU kerberos-2.mit.edu ATHENA.MIT.EDU kerberos-3.mit.edu LCS.MIT.EDU kerberos.lcs.mit.edu TELECOM.MIT.EDU bitsy.mit.edu ARC.NASA.GOV trident.arc.nasa.gov In this case, the other realms do not need to be there. They are here as an example of how a machine may be made aware of multiple realms. You may wish to not include them for simplicity. The first line names the realm in which this system works. The other lines contain realm/host entries. The first item on a line is a realm, and the second is a host in that realm that is acting as a key distribution center. The words admin server following a host's name means that host also provides an administrative database server. For further explanation of these terms, please consult the Kerberos manual pages. Now we have to add grunt.example.com to the EXAMPLE.COM realm and also add an entry to put all hosts in the .example.com domain in the EXAMPLE.COM realm. The krb.realms file would be updated as follows: &prompt.root; cat krb.realms grunt.example.com EXAMPLE.COM .example.com EXAMPLE.COM .berkeley.edu CS.BERKELEY.EDU .MIT.EDU ATHENA.MIT.EDU .mit.edu ATHENA.MIT.EDU Again, the other realms do not need to be there. They are here as an example of how a machine may be made aware of multiple realms. You may wish to remove them to simplify things. The first line puts the specific system into the named realm. The rest of the lines show how to default systems of a particular subdomain to a named realm. Now we are ready to create the database. This only needs to run on the Kerberos server (or Key Distribution Center). Issue the kdb_init command to do this: &prompt.root; kdb_init Realm name [default ATHENA.MIT.EDU ]: EXAMPLE.COM You will be prompted for the database Master Password. It is important that you NOT FORGET this password. Enter Kerberos master key: Now we have to save the key so that servers on the local machine can pick it up. Use the kstash command to do this: &prompt.root; kstash Enter Kerberos master key: Current Kerberos master key version is 1. Master key entered. BEWARE! This saves the encrypted master password in /etc/kerberosIV/master_key. Making It All Run Two principals need to be added to the database for each system that will be secured with Kerberos. Their names are kpasswd and rcmd. These two principals are made for each system, with the instance being the name of the individual system. These daemons, kpasswd and rcmd allow other systems to change Kerberos passwords and run commands like &man.rcp.1;, &man.rlogin.1; and &man.rsh.1;. Now let us add these entries: &prompt.root; kdb_edit Opening database... Enter Kerberos master key: Current Kerberos master key version is 1. Master key entered. BEWARE! Previous or default values are in [brackets] , enter return to leave the same, or new value. Principal name: passwd Instance: grunt <Not found>, Create [y] ? y Principal: passwd, Instance: grunt, kdc_key_ver: 1 New Password: <---- enter RANDOM here Verifying password New Password: <---- enter RANDOM here Random password [y] ? y Principal's new key version = 1 Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ? Max ticket lifetime (*5 minutes) [ 255 ] ? Attributes [ 0 ] ? Edit O.K. Principal name: rcmd Instance: grunt <Not found>, Create [y] ? Principal: rcmd, Instance: grunt, kdc_key_ver: 1 New Password: <---- enter RANDOM here Verifying password New Password: <---- enter RANDOM here Random password [y] ? Principal's new key version = 1 Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ? Max ticket lifetime (*5 minutes) [ 255 ] ? Attributes [ 0 ] ? Edit O.K. Principal name: <---- null entry here will cause an exit Creating the Server File We now have to extract all the instances which define the services on each machine. For this we use the ext_srvtab command. This will create a file which must be copied or moved by secure means to each Kerberos client's /etc/kerberosIV directory. This file must be present on each server and client, and is crucial to the operation of Kerberos. &prompt.root; ext_srvtab grunt Enter Kerberos master key: Current Kerberos master key version is 1. Master key entered. BEWARE! Generating 'grunt-new-srvtab'.... Now, this command only generates a temporary file which must be renamed to srvtab so that all the servers can pick it up. Use the &man.mv.1; command to move it into place on the original system: &prompt.root; mv grunt-new-srvtab srvtab If the file is for a client system, and the network is not deemed safe, then copy the client-new-srvtab to removable media and transport it by secure physical means. Be sure to rename it to srvtab in the client's /etc/kerberosIV directory, and make sure it is mode 600: &prompt.root; mv grumble-new-srvtab srvtab &prompt.root; chmod 600 srvtab Populating the Database We now have to add some user entries into the database. First let us create an entry for the user jane. Use the kdb_edit command to do this: &prompt.root; kdb_edit Opening database... Enter Kerberos master key: Current Kerberos master key version is 1. Master key entered. BEWARE! Previous or default values are in [brackets] , enter return to leave the same, or new value. Principal name: jane Instance: <Not found>, Create [y] ? y Principal: jane, Instance: , kdc_key_ver: 1 New Password: <---- enter a secure password here Verifying password New Password: <---- re-enter the password here Principal's new key version = 1 Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ? Max ticket lifetime (*5 minutes) [ 255 ] ? Attributes [ 0 ] ? Edit O.K. Principal name: <---- null entry here will cause an exit Testing It All Out First we have to start the Kerberos daemons. Note that if you have correctly edited your /etc/rc.conf then this will happen automatically when you reboot. This is only necessary on the Kerberos server. Kerberos clients will automatically get what they need from the /etc/kerberosIV directory. &prompt.root; kerberos & Kerberos server starting Sleep forever on error Log file is /var/log/kerberos.log Current Kerberos master key version is 1. Master key entered. BEWARE! Current Kerberos master key version is 1 Local realm: EXAMPLE.COM &prompt.root; kadmind -n & KADM Server KADM0.0A initializing Please do not use 'kill -9' to kill this job, use a regular kill instead Current Kerberos master key version is 1. Master key entered. BEWARE! Now we can try using the kinit command to get a ticket for the ID jane that we created above: &prompt.user; kinit jane MIT Project Athena (grunt.example.com) Kerberos Initialization for "jane" Password: Try listing the tokens using klist to see if we really have them: &prompt.user; klist Ticket file: /tmp/tkt245 Principal: jane@EXAMPLE.COM Issued Expires Principal Apr 30 11:23:22 Apr 30 19:23:22 krbtgt.EXAMPLE.COM@EXAMPLE.COM Now try changing the password using &man.passwd.1; to check if the kpasswd daemon can get authorization to the Kerberos database: &prompt.user; passwd realm EXAMPLE.COM Old password for jane: New Password for jane: Verifying password New Password for jane: Password changed. Adding <command>su</command> Privileges Kerberos allows us to give each user who needs root privileges their own separate &man.su.1; password. We could now add an ID which is authorized to &man.su.1; to root. This is controlled by having an instance of root associated with a principal. Using kdb_edit we can create the entry jane.root in the Kerberos database: &prompt.root; kdb_edit Opening database... Enter Kerberos master key: Current Kerberos master key version is 1. Master key entered. BEWARE! Previous or default values are in [brackets] , enter return to leave the same, or new value. Principal name: jane Instance: root <Not found>, Create [y] ? y Principal: jane, Instance: root, kdc_key_ver: 1 New Password: <---- enter a SECURE password here Verifying password New Password: <---- re-enter the password here Principal's new key version = 1 Expiration date (enter yyyy-mm-dd) [ 2000-01-01 ] ? Max ticket lifetime (*5 minutes) [ 255 ] ? 12 <--- Keep this short! Attributes [ 0 ] ? Edit O.K. Principal name: <---- null entry here will cause an exit Now try getting tokens for it to make sure it works: &prompt.root; kinit jane.root MIT Project Athena (grunt.example.com) Kerberos Initialization for "jane.root" Password: Now we need to add the user to root's .klogin file: &prompt.root; cat /root/.klogin jane.root@EXAMPLE.COM Now try doing the &man.su.1;: &prompt.user; su Password: and take a look at what tokens we have: &prompt.root; klist Ticket file: /tmp/tkt_root_245 Principal: jane.root@EXAMPLE.COM Issued Expires Principal May 2 20:43:12 May 3 04:43:12 krbtgt.EXAMPLE.COM@EXAMPLE.COM Using Other Commands In an earlier example, we created a principal called jane with an instance root. This was based on a user with the same name as the principal, and this is a Kerberos default; that a <principal>.<instance> of the form <username>.root will allow that <username> to &man.su.1; to root if the necessary entries are in the .klogin file in root's home directory: &prompt.root; cat /root/.klogin jane.root@EXAMPLE.COM Likewise, if a user has in their own home directory lines of the form: &prompt.user; cat ~/.klogin jane@EXAMPLE.COM jack@EXAMPLE.COM This allows anyone in the EXAMPLE.COM realm who has authenticated themselves as jane or jack (via kinit, see above) to access to jane's account or files on this system (grunt) via &man.rlogin.1;, &man.rsh.1; or &man.rcp.1;. For example, jane now logs into another system using Kerberos: &prompt.user; kinit MIT Project Athena (grunt.example.com) Password: &prompt.user; rlogin grunt Last login: Mon May 1 21:14:47 from grumble Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994 The Regents of the University of California. All rights reserved. FreeBSD BUILT-19950429 (GR386) #0: Sat Apr 29 17:50:09 SAT 1995 Or jack logs into jane's account on the same machine (jane having set up the .klogin file as above, and the person in charge of Kerberos having set up principal jack with a null instance): &prompt.user; kinit &prompt.user; rlogin grunt -l jane MIT Project Athena (grunt.example.com) Password: Last login: Mon May 1 21:16:55 from grumble Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994 The Regents of the University of California. All rights reserved. FreeBSD BUILT-19950429 (GR386) #0: Sat Apr 29 17:50:09 SAT 1995 - + + + + + + Tillman + Hodgson + Contributed by + + + + + Mark + Murray + Based on a contribution by + + + + + <application>Kerberos5</application> + + Every &os; release beyond &os;-5.1 includes support + only for Kerberos5. Hence + Kerberos5 is the only version + included, and its configuration is similar in many aspects + to that of KerberosIV. The following + information only applies to + Kerberos5 in post &os;-5.0 + releases. Users who wish to use the + KerberosIV package may install the + security/krb4 port. + + Kerberos is a network add-on + system/protocol that allows users to authenticate themselves + through the services of a secure server. Services such as remote + login, remote copy, secure inter-system file copying and other + high-risk tasks are made considerably safer and more + controllable. + + Kerberos can be described as an + identity-verifying proxy system. It can also be described as a + trusted third-party authentication system. + Kerberos provides only one + function — the secure authentication of users on the network. + It does not provide authorization functions (what users are + allowed to do) or auditing functions (what those users did). + After a client and server have used + Kerberos to prove their identity, they + can also encrypt all of their communications to assure privacy + and data integrity as they go about their business. + + Therefore it is highly recommended that + Kerberos be used with other security + methods which provide authorization and audit services. + + The following instructions can be used as a guide on how to set + up Kerberos as distributed for &os;. + However, you should refer to the relevant manual pages for a complete + description. + + For purposes of demonstrating a Kerberos + installation, the various namespaces will be handled as follows: + + + + The DNS domain (zone) + will be EXAMPLE.ORG. + + + + The Kerberos realm will be + EXAMPLE.ORG. + + + + + Please use real domain names when setting up + Kerberos even if you intend to run + it internally. This avoids DNS problems + and assures inter-operation with other + Kerberos realms. + + + + History + + Kerberos was created by + MIT as a solution to network security problems. + The Kerberos protocol uses strong + cryptography so that a client can prove its identity to a server + (and vice versa) across an insecure network connection. + + Kerberos is both the name of a + network authentication protocol and an adjective to describe + programs that implement the program + (Kerberos telnet, for example). The + current version of the protocol is version 5, described in + RFC 1510. + + Several free implementations of this protocol are available, + covering a wide range of operating systems. The Massachusetts + Institute of Technology (MIT), where + Kerberos was originally developed, + continues to develop their Kerberos + package. It is commonly used in the US + as a cryptography product, as such it + has historically been affected by US export + regulations. The MIT + Kerberos is available as a port + (security/krb5). Heimdal + Kerberos is another version 5 + implementation, and was explicitly developed outside of the + US to avoid export + regulations (and is thus often included in non-commercial &unix; + variants). The Heimdal Kerberos + distribution is available as a port + (security/heimdal), and a + minimal installation of it is included in the base &os; + install. + + In order to reach the widest audience, these instructions assume + the use of the Heimdal distribution included in &os;. + + + + + Setting up a Heimdal <acronym>KDC</acronym> + + The Key Distribution Center (KDC) is the + centralized authentication service that + Kerberos provides — it is the + computer that issues Kerberos tickets. + The KDC is considered trusted by + all other computers in the Kerberos + realm, and thus has heightened security concerns. + + Note that while running the Kerberos + server requires very few computing resources, a dedicated machine + acting only as a KDC is recommended for security + reasons. + + To begin setting up a KDC, ensure that your + /etc/rc.conf file contains the correct + settings to act as a KDC (you may need to adjust + paths to reflect your own system): + + kerberos5_server_enable="YES" +kadmind5_server_enable="YES" +kerberos_stash="YES" + + + The is only available in + &os; 4.X. + + + Next we will set up your Kerberos + config file, /etc/krb5.conf: + + [libdefaults] + default_realm = EXAMPLE.ORG +[realms] + EXAMPLE.ORG = { + kdc = kerberos.EXAMPLE.ORG + } +[domain_realm] + .EXAMPLE.ORG = EXAMPLE.ORG + + Note that this /etc/krb5.conf file implies + that your KDC will have the fully-qualified + hostname of kerberos.EXAMPLE.ORG. + You will need to add a CNAME (alias) entry to your zone file to + accomplish this if your KDC has a different + hostname. + + + For large networks with a properly configured + BIND DNS server, the + above example could be trimmed to: + + [libdefaults] + default_realm = example.org + + With the following lines being appended to the + example.org zonefile: + + _kerberos._udp IN SRV 01 00 88 kerberos.example.org. +_kerberos._tcp IN SRV 01 00 88 kerberos.example.org. +_kpasswd._udp IN SRV 01 00 464 kerberos.example.org. +_kerberos-adm._tcp IN SRV 01 00 749 kerberos.example.org. +_kerberos IN TXT EXAMPLE.ORG. + + Next we will create the Kerberos + database. This database contains the keys of all principals encrypted + with a master password. You are not + required to remember this password, it will be stored in a file + (/var/heimdal/m-key). To create the master + key, run kstash and enter a password. + + Once the master key has been created, you can initialize the + database using the kadmin program with the + -l option (standing for local). + This option instructs kadmin to modify the + database files directly rather than going through the + kadmind network service. This handles the + chicken-and-egg problem of trying to connect to the database + before it is created. Once you have the kadmin + prompt, use the init command to create your + realms initial database. + + Lastly, while still in kadmin, create your + first principal using the add command. Stick + to the defaults options for the principal for now, you can always + change them later with the modify command. + Note that you can use the ? command at any + prompt to see the available options. + + A sample database creation session is shown below: + + &prompt.root; kstash + Master key: xxxxxxxx + Verifying password - Master key: xxxxxxxx + + &prompt.root; kadmin -l + kadmin> init EXAMPLE.ORG + Realm max ticket life [unlimited]: + kadmin> add tillman + Max ticket life [unlimited]: + Max renewable life [unlimited]: + Attributes []: + Password: xxxxxxxx + Verifying password - Password: xxxxxxxx + + Now it's time to start up the KDC services. + Run /etc/rc.d/kerberos start and + /etc/rc.d/kadmind start to bring up the + services. Note that you won't have any Kerberized daemons running + at this point but you should be able to confirm the that the + KDC is functioning by obtaining and listing a + ticket for the principal (user) that you just created from the + command-line of the KDC itself: + + &prompt.user;k5init tillman + tillman@EXAMPLE.ORG's Password: + + &prompt.user;k5list + Credentials cache: FILE:/tmp/krb5cc_500 + Principal: tillman@EXAMPLE.ORG + + Issued Expires Principal + Aug 27 15:37:58 Aug 28 01:37:58 krbtgt/EXAMPLE.ORG@EXAMPLE.ORG + Aug 27 15:37:58 Aug 28 01:37:58 krbtgt/EXAMPLE.ORG@EXAMPLE.ORG + + v4-ticket file: /tmp/tkt500 + k5list: No ticket file (tf_util) + + + + + <application>Kerberos</application> enabling a server with + Heimdal services + + First, we need a copy of the Kerberos + configuration file, /etc/krb5.conf. To do + so, simply copy it over to the client computer from the + KDC in a secure fashion (using network utilities, + such as &man.scp.1;, or physically via a + floppy disk). + + Next you need a /etc/krb5.keytab file. + This is the major difference between a server providing + Kerberos enabled daemons and a + workstation — the server must have a + keytab file. This file + contains the servers host key, which allows it and the + KDC to verify each others identity. It + must be transmitted to the server in a secure fashion, as the + security of the server can be broken if the key is made public. + This explicitly means that transferring it via a clear text + channel, such as FTP, is a very bad idea. + + Typically, you transfer to the keytab + to the server using the kadmin program. + This is handy because you also need to create the host principal + (the KDC end of the + krb5.keytab) using + kadmin. + + Note that you must have already obtained a ticket and that this + ticket must be allowed to use the kadmin + interface in the kadmind.acl. See the section + titled Remote administration in the Heimdal info + pages (info heimdal) for details on designing + access control lists. If you do not want to enable remote + kadmin access, you can simply securely connect + to the KDC (via local console, + &man.ssh.1; or Kerberos + &man.telnet.1;) and perform administration locally + using kadmin -l. + + After installing the /etc/krb5.conf file, + you can use kadmin from the + Kerberos server. The + add --random-key command will let you add the + servers host principal, and the ext command + will allow you to extract the servers host principal to its own + keytab. For example: + + &prompt.root; kadmin + kadmin> add --random-key host/myserver.EXAMPLE.ORG + Max ticket life [unlimited]: + Max renewable life [unlimited]: + Attributes []: + kadmin> ext host/myserver.EXAMPLE.ORG + kadmin> exit + + Note that the ext command (short for + extract) stores the extracted key in + /etc/krb5.keytab by default. + + If you do not have kadmind running on the + KDC (possibly for security reasons) and thus + do not have access to kadmin remotely, you + can add the host principal + (host/myserver.EXAMPLE.ORG) directly on the + KDC and then extract it to a temporary file + (to avoid over-writing the /etc/krb5.keytab + on the KDC) using something like this: + + &prompt.root; kadmin + kadmin> ext --keytab=/tmp/example.keytab host/myserver.example.org + kadmin> exit + + You can then securely copy the keytab to the server + computer (using scp or a floppy, for + example). Be sure to specify a non-default keytab name + to avoid over-writing the keytab on the + KDC. + + At this point your server can communicate with the + KDC (due to it's krb5.conf + file) and it can prove it's own identity (due to the + krb5.keytab file). It's now ready for + you to enable some Kerberos services. + For this example we will enable the telnet + service by putting a line like this into your + /etc/inetd.conf and then restarting the + &man.inetd.8; service with + /etc/rc.d/inetd restart: + + telnet stream tcp nowait root /usr/libexec/telnetd telnetd -a user + + The critical bit is that the -a + (for authentication) type is set to user. Consult for the + &man.telnetd.8; manual page for more details. + + + + + <application>Kerberos</application> enabling a client with Heimdal + + Setting up a client computer is almost trivially easy. As + far as Kerberos configuration goes, + you only need the Kerberos + configuration file, located at /etc/krb5.conf. + Simply securely copy it over to the client computer from the + KDC. + + Test your client computer by attempting to use + kinit, klist and + kdestroy from the client to obtain, show, and + then delete a ticket for the principal you created above. You + should also be able to use Kerberos + applications to connect to Kerberos + enabled servers, though if that does not work and obtaining a + ticket does the problem is likely with the server and not with + the client or the KDC. + + When testing an application like telnet, + try using a packet sniffer (such as &man.tcpdump.1;) + to confirm that your password is not sent in the clear. Try + using telnet with the -x + option, which encrypts the entire data stream (similar to + ssh). + + The core Kerberos client applications + (traditionally named kinit, + klist, kdestroy and + kpasswd) are installed in + the base &os; install. Note that &os; versions prior to 5.0 + renamed them to k5init, + k5list, k5destroy, + k5passwd, and kstash + (though it's typically only used once). + + Various non-core Kerberos client + applications are also installed by default. This is where the + minimal nature of the base Heimdal installation is + felt: telnet is the only + Kerberos enabled service. + + The Heimdal port adds some of the missing client applications: + Kerberos enabled versions of + ftp, rsh, + rcp, rlogin, and a few + other less common programs. The MIT port also + contains a full suite of Kerberos + client applications. + + + + + User configuration files: <filename>.k5login</filename> and <filename>.k5users</filename> + + Users within a realm typically have their + Kerberos principal (such as + tillman@EXAMPLE.ORG) mapped to a local + user account (such as a local account named + tillman). Client applications such as + telnet usually do not require a user name + or a principal. + + Occasionally, however, you want to grant access to a local + user account to someone who does not have a matching + Kerberos principal. For example, + tillman@EXAMPLE.ORG may need access to the + local user account webdevelopers. Other + principals may also need access to that local account. + + The .k5login and + .k5users files, placed in a users home + directory, can be used similar to a powerful combination of + .hosts and .rhosts, + solving this problem. For example, if a + .k5login with the following + contents: + + tillman@example.org + jdoe@example.org + + Were to be placed into the home directory of the local user + webdevelopers then both principals listed + would have access to that account without requiring a shared + password. + + Reading the man pages for these commands is recommended. + Note that the ksu man page covers + .k5users. + + + + + <application>Kerberos</application> Tips, Tricks, and Troubleshooting + + + + When using either the Heimdal or MIT + Kerberos ports ensure that your + PATH environment variable lists the + Kerberos versions of the client + applications before the system versions. + + + + Is your time in sync? Are you sure? If the time is not in + sync (typically within five minutes) authentication will + fail. + + + + MIT and Heimdal inter-operate nicely. + Except for kadmin, the protocol for + which is not standardized. + + + + If you change your hostname, you also need to change your + host/ principal and update your keytab. + This also applies to special keytab entries like the + www/ principal used for Apache's + www/mod_auth_kerb. + + + + All hosts in your realm must be resolvable (both forwards + and reverse) in DNS (or + /etc/hosts as a minimum). CNAMEs + will work, but the A and PTR records must be correct and in + place. The error message isn't very intuitive: + Kerberos5 refuses authentication because Read req + failed: Key table entry not found. + + + + Some operating systems that may being acting as clients + to your KDC do not set the permissions + for ksu to be setuid + root. This means that + ksu does not work, which is a good + security idea but annoying. This is not a + KDC error. + + + + With MIT + Kerberos, if you want to allow a + principal to have a ticket life longer than the default ten + hours, you must use modify_principal in + kadmin to change the maxlife of both the + principal in question and the krbtgt + principal. Then the principal can use the + -l option with kinit + to request a ticket with a longer lifetime. + + + + If you run a packet sniffer on your + KDC to add in troubleshooting and then + run kinit from a workstation, you will + notice that your TGT is sent + immediately upon running kinit — + even before you type your password! The explanation is + that the Kerberos server freely + transmits a TGT (Ticket Granting + Ticket) to any unauthorized request; however, every + TGT is encrypted in a key derived from + the user's password. Therefore, when a user types their + password it is not being sent to the KDC, + it's being used to decrypt the TGT that + kinit already obtained. If the decryption + process results in a valid ticket with a valid time stamp, + the user has valid Kerberos + credentials. These credentials include a session key for + establishing secure communications with the + Kerberos server in the future, as + well as the actual ticket-granting ticket, which is actually + encrypted with the Kerberos + server's own key. This second layer of encryption is + unknown to the user, but it's what allows the + Kerberos server to verify + the authenticity of each TGT. + + + + You have to keep the time in sync between all the + computers in your realm. NTP is + perfect for this. For more information on + NTP, see + . + + + + If you want to use long ticket lifetimes (a week, for + example) and you are using OpenSSH + to connect to the machine where your ticket is stored, make + sure that Kerberos + is set to no + in your sshd_config or else your tickets + will be deleted when you log out. + + + + Remember that host principals can have a longer ticket + lifetime as well. If your user principal has a lifetime of a + week but the host you are connecting to has a lifetime of nine + hours, you will have an expired host principal in your cache + and the ticket cache will not work as expected. + + + + When setting up a krb5.dict file to + prevent specific bad passwords from being used (the manual page + for kadmind covers this briefly), remember + that it only applies to principals that have a password policy + assigned to them. The krb5.dict files + format is simple: one string per line. Creating a symbolic + link to /usr/share/dict/words might be + useful. + + + + + + + Differences with the <acronym>MIT</acronym> port + + The major difference between the MIT + and Heimdal installs relates to the kadmin + program which has a different (but equivalent) set of commands + and uses a different protocol. This has a large implications + if your KDC is MIT as you + will not be able to use the Heimdal kadmin + program to administer your KDC remotely + (or vice versa, for that matter). + + The client applications may also take slightly different + command line options to accomplish the same tasks. Following + the instructions on the MIT + Kerberos web site + () + is recommended. Be careful of path issues: the + MIT port installs into + /usr/local/ by default, and the + normal system applications may be run instead + of MIT if your PATH + environment variable lists the system directories first. + + With the MIT + security/krb5 port + that is provided by &os;, be sure to read the + /usr/local/share/doc/krb5/README.FreeBSD + file installed by the port if you want to understand why logins + via telnetd and klogind + behave somewhat oddly. Most importantly, correcting the + incorrect permissions on cache file behavior + requires that the login.krb5 binary be used + for authentication so that it can properly change ownership for + the forwarded credentials. + + + + + Mitigating limitations found in <application>Kerberos</application> + + + <application>Kerberos</application> is an all-or-nothing approach + + Every service enabled on the network must be modified to + work with Kerberos (or be otherwise + secured against network attacks) or else the users credentials + could be stolen and re-used. An example of this would be + Kerberos enabling all remote shells + (via rsh and telnet, for + example) but not converting the POP3 mail + server which sends passwords in plaintext. + + + + + <application>Kerberos</application> is intended for single-user workstations + + In a multi-user environment, + Kerberos is less secure. + This is because it stores the tickets in the + /tmp directory, which is readable by all + users. If a user is sharing a computer with several other + people simultaneously (i.e. multi-user), it is possible that + the user's tickets can be stolen (copied) by another + user. + + This can be overcome with the -c + filename command-line option or (preferably) the + KRB5CCNAME environment variable, but this + is rarely done. In principal, storing the ticket in the users + home directory and using simple file permissions can mitigate + this problem. + + + + + The KDC is a single point of failure + + By design, the KDC must be as secure as + the master password database is contained on it. The + KDC should have absolutely no other + services running on it and should be physically secured. The + danger is high because Kerberos + stores all passwords encrypted with the same key (the + master key), which in turn is stored as a file + on the KDC. + + As a side note, a compromised master key is not quite as + bad as one might normally fear. The master key is only used + to encrypt the Kerberos database + and as a seed for the random number generator. As long as + access to your KDC is secure, an attacker + cannot do much with the master key. + + Additionally, if the KDC is unavailable + (perhaps due to a denial of service attack or network problems) + the network services are unusable as authentication can not be + performed, a recipe for a denial-of-service attack. This can + alleviated with multiple KDCs (a single + master and one or more slaves) and with careful implementation + of secondary or fall-back authentication + (PAM is excellent for this). + + + + + <application>Kerberos</application> Shortcomings + + Kerberos allows users, hosts + and services to authenticate between themselves. It does not + have a mechanism to authenticate the KDC + to the users, hosts or services. This means that a trojanned + kinit (for example) could record all user + names and passwords. Something like + security/tripwire or + other file system integrity checking tools can alleviate + this. + + + + + + Resources and further information + + + + + The Kerberos FAQ + + + + Designing + an Authentication System: a Dialogue in Four Scenes + + + + RFC 1510, + The Kerberos Network Authentication Service + (V5) + + + + MIT + Kerberos home page + + + + Heimdal + Kerberos home page + + + + + + Gary Palmer Contributed by Alex Nash Firewalls firewall security firewalls Firewalls are an area of increasing interest for people who are connected to the Internet, and are even finding applications on private networks to provide enhanced security. This section will hopefully explain what firewalls are, how to use them, and how to use the facilities provided in the FreeBSD kernel to implement them. People often think that having a firewall between your internal network and the Big Bad Internet will solve all your security problems. It may help, but a poorly set up firewall system is more of a security risk than not having one at all. A firewall can add another layer of security to your systems, but it cannot stop a really determined cracker from penetrating your internal network. If you let internal security lapse because you believe your firewall to be impenetrable, you have just made the crackers job that much easier. What Is a Firewall? There are currently two distinct types of firewalls in common use on the Internet today. The first type is more properly called a packet filtering router. This type of firewall utilizes a multi-homed machine and a set of rules to determine whether to forward or block individual packets. A multi-homed machine is simply a device with multiple network interfaces. The second type, known as a proxy server, relies on daemons to provide authentication and to forward packets, possibly on a multi-homed machine which has kernel packet forwarding disabled. Sometimes sites combine the two types of firewalls, so that only a certain machine (known as a bastion host) is allowed to send packets through a packet filtering router onto an internal network. Proxy services are run on the bastion host, which are generally more secure than normal authentication mechanisms. FreeBSD comes with a kernel packet filter (known as IPFW), which is what the rest of this section will concentrate on. Proxy servers can be built on FreeBSD from third party software, but there is such a variety of proxy servers available that it would be impossible to cover them in this section. Packet Filtering Routers A router is a machine which forwards packets between two or more networks. A packet filtering router is programmed to compare each packet to a list of rules before deciding if it should be forwarded or not. Most modern IP routing software includes packet filtering functionality that defaults to forwarding all packets. To enable the filters, you need to define a set of rules. To decide whether a packet should be passed on, the firewall looks through its set of rules for a rule which matches the contents of the packet's headers. Once a match is found, the rule action is obeyed. The rule action could be to drop the packet, to forward the packet, or even to send an ICMP message back to the originator. Only the first match counts, as the rules are searched in order. Hence, the list of rules can be referred to as a rule chain. The packet-matching criteria varies depending on the software used, but typically you can specify rules which depend on the source IP address of the packet, the destination IP address, the source port number, the destination port number (for protocols which support ports), or even the packet type (UDP, TCP, ICMP, etc). Proxy Servers Proxy servers are machines which have had the normal system daemons (telnetd, ftpd, etc) replaced with special servers. These servers are called proxy servers, as they normally only allow onward connections to be made. This enables you to run (for example) a proxy telnet server on your firewall host, and people can telnet in to your firewall from the outside, go through some authentication mechanism, and then gain access to the internal network (alternatively, proxy servers can be used for signals coming from the internal network and heading out). Proxy servers are normally more secure than normal servers, and often have a wider variety of authentication mechanisms available, including one-shot password systems so that even if someone manages to discover what password you used, they will not be able to use it to gain access to your systems as the password expires immediately after the first use. As they do not actually give users access to the host machine, it becomes a lot more difficult for someone to install backdoors around your security system. Proxy servers often have ways of restricting access further, so that only certain hosts can gain access to the servers. Most will also allow the administrator to specify which users can talk to which destination machines. Again, what facilities are available depends largely on what proxy software you choose. What Does IPFW Allow Me to Do? ipfw IPFW, the software supplied with FreeBSD, is a packet filtering and accounting system which resides in the kernel, and has a user-land control utility, &man.ipfw.8;. Together, they allow you to define and query the rules used by the kernel in its routing decisions. There are two related parts to IPFW. The firewall section performs packet filtering. There is also an IP accounting section which tracks usage of the router, based on rules similar to those used in the firewall section. This allows the administrator to monitor how much traffic the router is getting from a certain machine, or how much WWW traffic it is forwarding, for example. As a result of the way that IPFW is designed, you can use IPFW on non-router machines to perform packet filtering on incoming and outgoing connections. This is a special case of the more general use of IPFW, and the same commands and techniques should be used in this situation. Enabling IPFW on FreeBSD ipfw enabling As the main part of the IPFW system lives in the kernel, you will need to add one or more options to your kernel configuration file, depending on what facilities you want, and recompile your kernel. See "Reconfiguring your Kernel" () for more details on how to recompile your kernel. IPFW defaults to a policy of deny ip from any to any. If you do not add other rules during startup to allow access, you will lock yourself out of the server upon rebooting into a firewall-enabled kernel. We suggest that you set firewall_type=open in your /etc/rc.conf file when first enabling this feature, then refining the firewall rules in /etc/rc.firewall after you have tested that the new kernel feature works properly. To be on the safe side, you may wish to consider performing the initial firewall configuration from the local console rather than via ssh. Another option is to build a kernel using both the IPFIREWALL and IPFIREWALL_DEFAULT_TO_ACCEPT options. This will change the default rule of IPFW to allow ip from any to any and avoid the possibility of a lockout. There are currently four kernel configuration options relevant to IPFW: options IPFIREWALL Compiles into the kernel the code for packet filtering. options IPFIREWALL_VERBOSE Enables code to allow logging of packets through &man.syslogd.8;. Without this option, even if you specify that packets should be logged in the filter rules, nothing will happen. options IPFIREWALL_VERBOSE_LIMIT=10 Limits the number of packets logged through &man.syslogd.8; on a per entry basis. You may wish to use this option in hostile environments in which you want to log firewall activity, but do not want to be open to a denial of service attack via syslog flooding. When a chain entry reaches the packet limit specified, logging is turned off for that particular entry. To resume logging, you will need to reset the associated counter using the &man.ipfw.8; utility: &prompt.root; ipfw zero 4500 Where 4500 is the chain entry you wish to continue logging. options IPFIREWALL_DEFAULT_TO_ACCEPT This changes the default rule action from deny to allow. This avoids the possibility of locking yourself out if you happen to boot a kernel with IPFIREWALL support but have not configured your firewall yet. It is also very useful if you often use &man.ipfw.8; as a filter for specific problems as they arise. Use with care though, as this opens up the firewall and changes the way it works. Previous versions of FreeBSD contained an IPFIREWALL_ACCT option. This is now obsolete as the firewall code automatically includes accounting facilities. Configuring IPFW ipfw configuring The configuration of the IPFW software is done through the &man.ipfw.8; utility. The syntax for this command looks quite complicated, but it is relatively simple once you understand its structure. There are currently four different command categories used by the utility: addition/deletion, listing, flushing, and clearing. Addition/deletion is used to build the rules that control how packets are accepted, rejected, and logged. Listing is used to examine the contents of your rule set (otherwise known as the chain) and packet counters (accounting). Flushing is used to remove all entries from the chain. Clearing is used to zero out one or more accounting entries. Altering the IPFW Rules The syntax for this form of the command is: ipfw -N command index action log protocol addresses options There is one valid flag when using this form of the command: -N Resolve addresses and service names in output. The command given can be shortened to the shortest unique form. The valid commands are: add Add an entry to the firewall/accounting rule list delete Delete an entry from the firewall/accounting rule list Previous versions of IPFW used separate firewall and accounting entries. The present version provides packet accounting with each firewall entry. If an index value is supplied, it is used to place the entry at a specific point in the chain. Otherwise, the entry is placed at the end of the chain at an index 100 greater than the last chain entry (this does not include the default policy, rule 65535, deny). The log option causes matching rules to be output to the system console if the kernel was compiled with IPFIREWALL_VERBOSE. Valid actions are: reject Drop the packet, and send an ICMP host or port unreachable (as appropriate) packet to the source. allow Pass the packet on as normal. (aliases: pass, permit, and accept) deny Drop the packet. The source is not notified via an ICMP message (thus it appears that the packet never arrived at the destination). count Update packet counters but do not allow/deny the packet based on this rule. The search continues with the next chain entry. Each action will be recognized by the shortest unambiguous prefix. The protocols which can be specified are: all Matches any IP packet icmp Matches ICMP packets tcp Matches TCP packets udp Matches UDP packets The address specification is: from address/maskport to address/maskport via interface You can only specify port in conjunction with protocols which support ports (UDP and TCP). The is optional and may specify the IP address or domain name of a local IP interface, or an interface name (e.g. ed0) to match only packets coming through this interface. Interface unit numbers can be specified with an optional wildcard. For example, ppp* would match all kernel PPP interfaces. The syntax used to specify an address/mask is: address or address/mask-bits or address:mask-pattern A valid hostname may be specified in place of the IP address. is a decimal number representing how many bits in the address mask should be set. e.g. specifying 192.216.222.1/24 will create a mask which will allow any address in a class C subnet (in this case, 192.216.222) to be matched. is an IP address which will be logically AND'ed with the address given. The keyword any may be used to specify any IP address. The port numbers to be blocked are specified as: port,port,port… to specify either a single port or a list of ports, or port-port to specify a range of ports. You may also combine a single range with a list, but the range must always be specified first. The options available are: frag Matches if the packet is not the first fragment of the datagram. in Matches if the packet is on the way in. out Matches if the packet is on the way out. ipoptions spec Matches if the IP header contains the comma separated list of options specified in spec. The supported IP options are: ssrr (strict source route), lsrr (loose source route), rr (record packet route), and ts (time stamp). The absence of a particular option may be specified with a leading !. established Matches if the packet is part of an already established TCP connection (i.e. it has the RST or ACK bits set). You can optimize the performance of the firewall by placing established rules early in the chain. setup Matches if the packet is an attempt to establish a TCP connection (the SYN bit is set but the ACK bit is not). tcpflags flags Matches if the TCP header contains the comma separated list of flags. The supported flags are fin, syn, rst, psh, ack, and urg. The absence of a particular flag may be indicated by a leading !. icmptypes types Matches if the ICMP type is present in the list types. The list may be specified as any combination of ranges and/or individual types separated by commas. Commonly used ICMP types are: 0 echo reply (ping reply), 3 destination unreachable, 5 redirect, 8 echo request (ping request), and 11 time exceeded (used to indicate TTL expiration as with &man.traceroute.8;). Listing the IPFW Rules The syntax for this form of the command is: ipfw -a -c -d -e -t -N -S list There are seven valid flags when using this form of the command: -a While listing, show counter values. This option is the only way to see accounting counters. -c List rules in compact form. -d Show dynamic rules in addition to static rules. -e If was specified, also show expired dynamic rules. -t Display the last match times for each chain entry. The time listing is incompatible with the input syntax used by the &man.ipfw.8; utility. -N Attempt to resolve given addresses and service names. -S Show the set each rule belongs to. If this flag is not specified, disabled rules will not be listed. Flushing the IPFW Rules The syntax for flushing the chain is: ipfw flush This causes all entries in the firewall chain to be removed except the fixed default policy enforced by the kernel (index 65535). Use caution when flushing rules; the default deny policy will leave your system cut off from the network until allow entries are added to the chain. Clearing the IPFW Packet Counters The syntax for clearing one or more packet counters is: ipfw zero index When used without an index argument, all packet counters are cleared. If an index is supplied, the clearing operation only affects a specific chain entry. Example Commands for <application>ipfw</application> This command will deny all packets from the host evil.crackers.org to the telnet port of the host nice.people.org: &prompt.root; ipfw add deny tcp from evil.crackers.org to nice.people.org 23 The next example denies and logs any TCP traffic from the entire crackers.org network (a class C) to the nice.people.org machine (any port). &prompt.root; ipfw add deny log tcp from evil.crackers.org/24 to nice.people.org If you do not want people sending X sessions to your internal network (a subnet of a class C), the following command will do the necessary filtering: &prompt.root; ipfw add deny tcp from any to my.org/28 6000 setup To see the accounting records: &prompt.root; ipfw -a list or in the short form &prompt.root; ipfw -a l You can also see the last time a chain entry was matched with: &prompt.root; ipfw -at l Building a Packet Filtering Firewall The following suggestions are just that: suggestions. The requirements of each firewall are different and we cannot tell you how to build a firewall to meet your particular requirements. When initially setting up your firewall, unless you have a test bench setup where you can configure your firewall host in a controlled environment, it is strongly recommend you use the logging version of the commands and enable logging in the kernel. This will allow you to quickly identify problem areas and cure them without too much disruption. Even after the initial setup phase is complete, I recommend using the logging for `deny' as it allows tracing of possible attacks and also modification of the firewall rules if your requirements alter. If you use the logging versions of the accept command, be aware that it can generate large amounts of log data. One log entry will be generated for every packet that passes through the firewall, so large FTP/http transfers, etc, will really slow the system down. It also increases the latencies on those packets as it requires more work to be done by the kernel before the packet can be passed on. syslogd will also start using up a lot more processor time as it logs all the extra data to disk, and it could quite easily fill the partition /var/log is located on. You should enable your firewall from /etc/rc.conf.local or /etc/rc.conf. The associated manual page explains which knobs to fiddle and lists some preset firewall configurations. If you do not use a preset configuration, ipfw list will output the current ruleset into a file that you can pass to rc.conf. If you do not use /etc/rc.conf.local or /etc/rc.conf to enable your firewall, it is important to make sure your firewall is enabled before any IP interfaces are configured. The next problem is what your firewall should actually do! This is largely dependent on what access to your network you want to allow from the outside, and how much access to the outside world you want to allow from the inside. Some general rules are: Block all incoming access to ports below 1024 for TCP. This is where most of the security sensitive services are, like finger, SMTP (mail) and telnet. Block all incoming UDP traffic. There are very few useful services that travel over UDP, and what useful traffic there is, is normally a security threat (e.g. Suns RPC and NFS protocols). This has its disadvantages also, since UDP is a connectionless protocol, denying incoming UDP traffic also blocks the replies to outgoing UDP traffic. This can cause a problem for people (on the inside) using external archie (prospero) servers. If you want to allow access to archie, you will have to allow packets coming from ports 191 and 1525 to any internal UDP port through the firewall. ntp is another service you may consider allowing through, which comes from port 123. Block traffic to port 6000 from the outside. Port 6000 is the port used for access to X11 servers, and can be a security threat (especially if people are in the habit of doing xhost + on their workstations). X11 can actually use a range of ports starting at 6000, the upper limit being how many X displays you can run on the machine. The upper limit as defined by RFC 1700 (Assigned Numbers) is 6063. Check what ports any internal servers use (e.g. SQL servers, etc). It is probably a good idea to block those as well, as they normally fall outside the 1-1024 range specified above. Another checklist for firewall configuration is available from CERT at As stated above, these are only guidelines. You will have to decide what filter rules you want to use on your firewall yourself. We cannot accept ANY responsibility if someone breaks into your network, even if you follow the advice given above. IPFW Overhead and Optimization Many people want to know how much overhead IPFW adds to a system. The answer to this depends mostly on your rule set and processor speed. For most applications dealing with Ethernet and small rule sets, the answer is negligible. For those of you that need actual measurements to satisfy your curiosity, read on. The following measurements were made using 2.2.5-STABLE on a 486-66. (While IPFW has changed slightly in later releases of FreeBSD, it still performs with similar speed.) IPFW was modified to measure the time spent within the ip_fw_chk routine, displaying the results to the console every 1000 packets. Two rule sets, each with 1000 rules, were tested. The first set was designed to demonstrate a worst case scenario by repeating the rule: &prompt.root; ipfw add deny tcp from any to any 55555 This demonstrates a worst case scenario by causing most of IPFW's packet check routine to be executed before finally deciding that the packet does not match the rule (by virtue of the port number). Following the 999th iteration of this rule was an allow ip from any to any. The second set of rules were designed to abort the rule check quickly: &prompt.root; ipfw add deny ip from 1.2.3.4 to 1.2.3.4 The non-matching source IP address for the above rule causes these rules to be skipped very quickly. As before, the 1000th rule was an allow ip from any to any. The per-packet processing overhead in the former case was approximately 2.703 ms/packet, or roughly 2.7 microseconds per rule. Thus the theoretical packet processing limit with these rules is around 370 packets per second. Assuming 10 Mbps Ethernet and a ~1500 byte packet size, we would only be able to achieve 55.5% bandwidth utilization. For the latter case each packet was processed in approximately 1.172 ms, or roughly 1.2 microseconds per rule. The theoretical packet processing limit here would be about 853 packets per second, which could consume 10 Mbps Ethernet bandwidth. The excessive number of rules tested and the nature of those rules do not provide a real-world scenario -- they were used only to generate the timing information presented here. Here are a few things to keep in mind when building an efficient rule set: Place an established rule early on to handle the majority of TCP traffic. Do not put any allow tcp statements before this rule. Place heavily triggered rules earlier in the rule set than those rarely used (without changing the permissiveness of the firewall, of course). You can see which rules are used most often by examining the packet counting statistics with ipfw -a l. OpenSSL security OpenSSL OpenSSL As of FreeBSD 4.0, the OpenSSL toolkit is a part of the base system. OpenSSL provides a general-purpose cryptography library, as well as the Secure Sockets Layer v2/v3 (SSLv2/SSLv3) and Transport Layer Security v1 (TLSv1) network security protocols. However, one of the algorithms (specifically IDEA) included in OpenSSL is protected by patents in the USA and elsewhere, and is not available for unrestricted use. IDEA is included in the OpenSSL sources in FreeBSD, but it is not built by default. If you wish to use it, and you comply with the license terms, enable the MAKE_IDEA switch in /etc/make.conf and rebuild your sources using make world. Today, the RSA algorithm is free for use in USA and other countries. In the past it was protected by a patent. OpenSSL install Source Code Installations OpenSSL is part of the src-crypto and src-secure CVSup collections. See the Obtaining FreeBSD section for more information about obtaining and updating FreeBSD source code. Nik Clayton
nik@FreeBSD.org
 Written by VPN over IPsec Creating a VPN between two networks, separated by the Internet, using FreeBSD gateways. Hiten M. Pandya
hmp@FreeBSD.org
 Written by Understanding IPsec This section will guide you through the process of setting up IPsec, and to use it in an environment which consists of FreeBSD and µsoft.windows; 2000/XP machines, to make them communicate securely. In order to set up IPsec, it is necessary that you are familiar with the concepts of building a custom kernel (see ). IPsec is a protocol which sits on top of the Internet Protocol (IP) layer. It allows two or more hosts to communicate in a secure manner (hence the name). The FreeBSD IPsec network stack is based on the KAME implementation, which has support for both protocol families, IPv4 and IPv6. FreeBSD 5.X contains a hardware accelerated IPsec stack, known as Fast IPsec, that was obtained from OpenBSD. It employs cryptographic hardware (whenever possible) via the &man.crypto.4; subsystem to optimize the performance of IPsec. This subsystem is new, and does not support all the features that are available in the KAME version of IPsec. However, in order to enable hardware-accelerated IPsec, the following kernel option has to be added to your kernel configuration file: options FAST_IPSEC # new IPsec (cannot define w/ IPSEC) Note, that it is not currently possible to use the Fast IPsec subsystem in lue with the KAME implementation of IPsec. Consult the &man.fast.ipsec.4; manual page for more information. IPsec consists of two sub-protocols: Encapsulated Security Payload (ESP), protects the IP packet data from third party interference, by encrypting the contents using symmetric cryptography algorithms (like Blowfish, 3DES). Authentication Header (AH), protects the IP packet header from third party interference and spoofing, by computing a cryptographic checksum and hashing the IP packet header fields with a secure hashing function. This is then followed by an additional header that contains the hash, to allow the information in the packet to be authenticated. ESP and AH can either be used together or seperately, depending on the environment. IPsec can either be used to directly encrypt the traffic between two hosts (known as Transport Mode); or to build virtual tunnels between two subnets, which could be used for secure communication between two corporate networks (known as Tunnel Mode). The latter is more commonly known as a Virtual Private Network (VPN). The &man.ipsec.4; manual page should be consulted for detailed information on the IPsec subsystem in FreeBSD. To add IPsec support to your kernel, add the following options to your kernel configuration file: options IPSEC #IP security options IPSEC_ESP #IP security (crypto; define w/ IPSEC) If IPsec debugging support is desired, the following kernel option should also be added: options IPSEC_DEBUG #debug for IP security The Problem There's no standard for what constitutes a VPN. VPNs can be implemented using a number of different technologies, each of which have their own strengths and weaknesses. This article presents a number of scenarios, and strategies for implementing a VPN for each scenario. Scenario #1: Two networks, connected to the Internet, to behave as one This is the scenario that caused me to first investigating VPNs. The premise is as follows: You have at least two sites Both sites are using IP internally Both sites are connected to the Internet, through a gateway that is running FreeBSD. The gateway on each network has at least one public IP address. The internal addresses of the two networks can be public or private IP addresses, it doesn't matter. You can be running NAT on the gateway machine if necessary. The internal IP addresses of the two networks do not collide. While I expect it is theoretically possible to use a combination of VPN technology and NAT to get this to work, I expect it to be a configuration nightmare. If you find that you are trying to connect two networks, both of which, internally, use the same private IP address range (e.g., both of them use 192.168.1.x), then one of the networks will have to be renumbered. The network topology might look something like this: Network #1 [ Internal Hosts ] Private Net, 192.168.1.2-254 [ Win9x/NT/2K ] [ UNIX ] | | .---[fxp1]---. Private IP, 192.168.1.1 | FreeBSD | `---[fxp0]---' Public IP, A.B.C.D | | -=-=- Internet -=-=- | | .---[fxp0]---. Public IP, W.X.Y.Z | FreeBSD | `---[fxp1]---' Private IP, 192.168.2.1 | | Network #2 [ Internal Hosts ] [ Win9x/NT/2K ] Private Net, 192.168.2.2-254 [ UNIX ] Notice the two public IP addresses. I'll use the letters to refer to them in the rest of this article. Anywhere you see those letters in this article, replace them with your own public IP addresses. Note also that that internally, the two gateway machines have .1 IP addresses, and that the two networks have different private IP address (192.168.1.x and 192.168.2.x respectively). All the machines on the private networks have been configured to use the .1 machine as their default gateway. The intention is that, from a network point of view, each network should view the machines on the other network as though they were directly attached the same router -- albeit a slightly slow router with an occasional tendency to drop packets. This means that (for example), machine 192.168.1.20 should be able to run ping 192.168.2.34 and have it work, transparently. &windows; machines should be able to see the machines on the other network, browse file shares, and so on, in exactly the same way that they can browse machines on the local network. And the whole thing has to be secure. This means that traffic between the two networks has to be encrypted. Creating a VPN between these two networks is a multi-step process. The stages are as follows: Create a virtual network link between the two networks, across the Internet. Test it, using tools like &man.ping.8;, to make sure it works. Apply security policies to ensure that traffic between the two networks is transparently encrypted and decrypted as necessary. Test this, using tools like &man.tcpdump.1;, to ensure that traffic is encrypted. Configure additional software on the FreeBSD gateways, to allow &windows; machines to see one another across the VPN. Step 1: Creating and testing a <quote>virtual</quote> network link Suppose that you were logged in to the gateway machine on network #1 (with public IP address A.B.C.D, private IP address 192.168.1.1), and you ran ping 192.168.2.1, which is the private address of the machine with IP address W.X.Y.Z. What needs to happen in order for this to work? The gateway machine needs to know how to reach 192.168.2.1. In other words, it needs to have a route to 192.168.2.1. Private IP addresses, such as those in the 192.168.x range are not supposed to appear on the Internet at large. Instead, each packet you send to 192.168.2.1 will need to be wrapped up inside another packet. This packet will need to appear to be from A.B.C.D, and it will have to be sent to W.X.Y.Z. This process is called encapsulation. Once this packet arrives at W.X.Y.Z it will need to unencapsulated, and delivered to 192.168.2.1. You can think of this as requiring a tunnel between the two networks. The two tunnel mouths are the IP addresses A.B.C.D and W.X.Y.Z, and the tunnel must be told the addresses of the private IP addresses that will be allowed to pass through it. The tunnel is used to transfer traffic with private IP addresses across the public Internet. This tunnel is created by using the generic interface, or gif devices on FreeBSD. As you can imagine, the gif interface on each gateway host must be configured with four IP addresses; two for the public IP addresses, and two for the private IP addresses. Support for the gif device must be compiled in to the FreeBSD kernel on both machines. You can do this by adding the line: pseudo-device gif to the kernel configuration files on both machines, and then compile, install, and reboot as normal. Configuring the tunnel is a two step process. First the tunnel must be told what the outside (or public) IP addresses are, using &man.gifconfig.8;. Then the private IP addresses must be configured using &man.ifconfig.8;. On the gateway machine on network #1 you would run the following two commands to configure the tunnel. gifconfig gif0 A.B.C.D W.X.Y.Z ifconfig gif0 inet 192.168.1.1 192.168.2.1 netmask 0xffffffff On the other gateway machine you run the same commands, but with the order of the IP addresses reversed. gifconfig gif0 W.X.Y.Z A.B.C.D ifconfig gif0 inet 192.168.2.1 192.168.1.1 netmask 0xffffffff You can then run: gifconfig gif0 to see the configuration. For example, on the network #1 gateway, you would see this: &prompt.root; gifconfig gif0 gif0: flags=8011<UP,POINTTOPOINT,MULTICAST> mtu 1280 inet 192.168.1.1 --> 192.168.2.1 netmask 0xffffffff physical address inet A.B.C.D --> W.X.Y.Z As you can see, a tunnel has been created between the physical addresses A.B.C.D and W.X.Y.Z, and the traffic allowed through the tunnel is that between 192.168.1.1 and 192.168.2.1. This will also have added an entry to the routing table on both machines, which you can examine with the command netstat -rn. This output is from the gateway host on network #1. &prompt.root; netstat -rn Routing tables Internet: Destination Gateway Flags Refs Use Netif Expire ... 192.168.2.1 192.168.1.1 UH 0 0 gif0 ... As the Flags value indicates, this is a host route, which means that each gateway knows how to reach the other gateway, but they do not know how to reach the rest of their respective networks. That problem will be fixed shortly. It is likely that you are running a firewall on both machines. This will need to be circumvented for your VPN traffic. You might want to allow all traffic between both networks, or you might want to include firewall rules that protect both ends of the VPN from one another. It greatly simplifies testing if you configure the firewall to allow all traffic through the VPN. You can always tighten things up later. If you are using &man.ipfw.8; on the gateway machines then a command like ipfw add 1 allow ip from any to any via gif0 will allow all traffic between the two end points of the VPN, without affecting your other firewall rules. Obviously you will need to run this command on both gateway hosts. This is sufficient to allow each gateway machine to ping the other. On 192.168.1.1, you should be able to run ping 192.168.2.1 and get a response, and you should be able to do the same thing on the other gateway machine. However, you will not be able to reach internal machines on either network yet. This is because of the routing -- although the gateway machines know how to reach one another, they do not know how to reach the network behind each one. To solve this problem you must add a static route on each gateway machine. The command to do this on the first gateway would be: route add 192.168.2.0 192.168.2.1 netmask 0xffffff00 This says In order to reach the hosts on the network 192.168.2.0, send the packets to the host 192.168.2.1. You will need to run a similar command on the other gateway, but with the 192.168.1.x addresses instead. IP traffic from hosts on one network will now be able to reach hosts on the other network. That has now created two thirds of a VPN between the two networks, in as much as it's virtual and it's a network. It's not private yet. You can test this using &man.ping.8; and &man.tcpdump.1;. Log in to the gateway host and run tcpdump dst host 192.168.2.1 In another log in session on the same host run ping 192.168.2.1 You will see output that looks something like this: 16:10:24.018080 192.168.1.1 > 192.168.2.1: icmp: echo request 16:10:24.018109 192.168.1.1 > 192.168.2.1: icmp: echo reply 16:10:25.018814 192.168.1.1 > 192.168.2.1: icmp: echo request 16:10:25.018847 192.168.1.1 > 192.168.2.1: icmp: echo reply 16:10:26.028896 192.168.1.1 > 192.168.2.1: icmp: echo request 16:10:26.029112 192.168.1.1 > 192.168.2.1: icmp: echo reply As you can see, the ICMP messages are going back and forth unencrypted. If you had used the parameter to &man.tcpdump.1; to grab more bytes of data from the packets you would see more information. Obviously this is unacceptable. The next section will discuss securing the link between the two networks so that it all traffic is automatically encrypted. Summary: Configure both kernels with pseudo-device gif. Edit /etc/rc.conf on gateway host #1 and add the following lines (replacing IP addresses as necessary). gifconfig_gif0="A.B.C.D W.X.Y.Z" ifconfig_gif0="inet 192.168.1.1 192.168.2.1 netmask 0xffffffff" static_routes="vpn" route_vpn="192.168.2.0 192.168.2.1 netmask 0xffffff00" Edit your firewall script (/etc/rc.firewall, or similar) on both hosts, and add ipfw add 1 allow ip from any to any via gif0 Make similar changes to /etc/rc.conf on gateway host #2, reversing the order of IP addresses. Step 2: Securing the link To secure the link we will be using IPsec. IPsec provides a mechanism for two hosts to agree on an encryption key, and to then use this key in order to encrypt data between the two hosts. The are two areas of configuration to be considered here. There must be a mechanism for two hosts to agree on the encryption mechanism to use. Once two hosts have agreed on this mechanism there is said to be a security association between them. There must be a mechanism for specifying which traffic should be encrypted. Obviously, you don't want to encrypt all your outgoing traffic -- you only want to encrypt the traffic that is part of the VPN. The rules that you put in place to determine what traffic will be encrypted are called security policies. Security associations and security policies are both maintained by the kernel, and can be modified by userland programs. However, before you can do this you must configure the kernel to support IPsec and the Encapsulated Security Payload (ESP) protocol. This is done by configuring a kernel with: options IPSEC options IPSEC_ESP and recompiling, reinstalling, and rebooting. As before you will need to do this to the kernels on both of the gateway hosts. You have two choices when it comes to setting up security associations. You can configure them by hand between two hosts, which entails choosing the encryption algorithm, encryption keys, and so forth, or you can use daemons that implement the Internet Key Exchange protocol (IKE) to do this for you. I recommend the latter. Apart from anything else, it's easier to set up. Editing and displaying security policies is carried out using &man.setkey.8;. By analogy, setkey is to the kernel's security policy tables as &man.route.8; is to the kernel's routing tables. setkey can also display the current security associations, and to continue the analogy further, is akin to netstat -r in that respect. There are a number of choices for daemons to manage security associations with FreeBSD. This article will describe how to use one of these, racoon. racoon is in the FreeBSD ports collection, in the security/ category, and is installed in the usual way. racoon must be run on both gateway hosts. On each host it is configured with the IP address of the other end of the VPN, and a secret key (which you choose, and must be the same on both gateways). The two daemons then contact one another, confirm that they are who they say they are (by using the secret key that you configured). The daemons then generate a new secret key, and use this to encrypt the traffic over the VPN. They periodically change this secret, so that even if an attacker were to crack one of the keys (which is as theoretically close to unfeasible as it gets) it won't do them much good -- by the time they've cracked the key the two daemons have chosen another one. racoon's configuration is stored in ${PREFIX}/etc/racoon. You should find a configuration file there, which should not need to be changed too much. The other component of racoon's configuration, which you will need to change, is the pre-shared key. The default racoon configuration expects to find this in the file ${PREFIX}/etc/racoon/psk.txt. It is important to note that the pre-shared key is not the key that will be used to encrypt your traffic across the VPN link, it is simply a token that allows the key management daemons to trust one another. psk.txt contains a line for each remote site you are dealing with. In this example, where there are two sites, each psk.txt file will contain one line (because each end of the VPN is only dealing with one other end). On gateway host #1 this line should look like this: W.X.Y.Z secret That is, the public IP address of the remote end, whitespace, and a text string that provides the secret. Obviously, you shouldn't use secret as your key -- the normal rules for choosing a password apply. On gateway host #2 the line would look like this A.B.C.D secret That is, the public IP address of the remote end, and the same secret key. psk.txt must be mode 0600 (i.e., only read/write to root) before racoon will run. You must run racoon on both gateway machines. You will also need to add some firewall rules to allow the IKE traffic, which is carried over UDP to the ISAKMP (Internet Security Association Key Management Protocol) port. Again, this should be fairly early in your firewall ruleset. ipfw add 1 allow udp from A.B.C.D to W.X.Y.Z isakmp ipfw add 1 allow udp from W.X.Y.Z to A.B.C.D isakmp Once racoon is running you can try pinging one gateway host from the other. The connection is still not encrypted, but racoon will then set up the security associations between the two hosts -- this might take a moment, and you may see this as a short delay before the ping commands start responding. Once the security association has been set up you can view it using &man.setkey.8;. Run setkey -D on either host to view the security association information. That's one half of the problem. They other half is setting your security policies. To create a sensible security policy, let's review what's been set up so far. This discussions hold for both ends of the link. Each IP packet that you send out has a header that contains data about the packet. The header includes the IP addresses of both the source and destination. As we already know, private IP addresses, such as the 192.168.x.y range are not supposed to appear on the public Internet. Instead, they must first be encapsulated inside another packet. This packet must have the public source and destination IP addresses substituted for the private addresses. So if your outgoing packet started looking like this: .----------------------. | Src: 192.168.1.1 | | Dst: 192.168.2.1 | | <other header info> | +----------------------+ | <packet data> | `----------------------' Then it will be encapsulated inside another packet, looking something like this: .--------------------------. | Src: A.B.C.D | | Dst: W.X.Y.Z | | <other header info> | +--------------------------+ | .----------------------. | | | Src: 192.168.1.1 | | | | Dst: 192.168.2.1 | | | | <other header info> | | | +----------------------+ | | | <packet data> | | | `----------------------' | `--------------------------' This encapsulation is carried out by the gif device. As you can see, the packet now has real IP addresses on the outside, and our original packet has been wrapped up as data inside the packet that will be put out on the Internet. Obviously, we want all traffic between the VPNs to be encrypted. You might try putting this in to words, as: If a packet leaves from A.B.C.D, and it is destined for W.X.Y.Z, then encrypt it, using the necessary security associations. If a packet arrives from W.X.Y.Z, and it is destined for A.B.C.D, then decrypt it, using the necessary security associations. That's close, but not quite right. If you did this, all traffic to and from W.X.Y.Z, even traffic that was not part of the VPN, would be encrypted. That's not quite what you want. The correct policy is as follows If a packet leaves from A.B.C.D, and that packet is encapsulating another packet, and it is destined for W.X.Y.Z, then encrypt it, using the necessary security associations. If a packet arrives from W.X.Y.Z, and that packet is encapsulating another packet, and it is destined for A.B.C.D, then encrypt it, using the necessary security associations. A subtle change, but a necessary one. Security policies are also set using &man.setkey.8;. &man.setkey.8; features a configuration language for defining the policy. You can either enter configuration instructions via stdin, or you can use the option to specify a filename that contains configuration instructions. The configuration on gateway host #1 (which has the public IP address A.B.C.D) to force all outbound traffic to W.X.Y.Z to be encrypted is: spdadd A.B.C.D/32 W.X.Y.Z/32 ipencap -P out ipsec esp/tunnel/A.B.C.D-W.X.Y.Z/require; Put these commands in a file (e.g., /etc/ipsec.conf) and then run &prompt.root; setkey -f /etc/ipsec.conf tells &man.setkey.8; that we want to add a rule to the secure policy database. The rest of this line specifies which packets will match this policy. A.B.C.D/32 and W.X.Y.Z/32 are the IP addresses and netmasks that identify the network or hosts that this policy will apply to. In this case, we want it to apply to traffic between these two hosts. tells the kernel that this policy should only apply to packets that encapsulate other packets. says that this policy applies to outgoing packets, and says that the packet will be secured. The second line specifies how this packet will be encrypted. is the protocol that will be used, while indicates that the packet will be further encapsulated in an IPsec packet. The repeated use of A.B.C.D and W.X.Y.Z is used to select the security association to use, and the final mandates that packets must be encrypted if they match this rule. This rule only matches outgoing packets. You will need a similar rule to match incoming packets. spdadd W.X.Y.Z/32 A.B.C.D/32 ipencap -P in ipsec esp/tunnel/W.X.Y.Z-A.B.C.D/require; Note the instead of in this case, and the necessary reversal of the IP addresses. The other gateway host (which has the public IP address W.X.Y.Z) will need similar rules. spdadd W.X.Y.Z/32 A.B.C.D/32 ipencap -P out ipsec esp/tunnel/W.X.Y.Z-A.B.C.D/require; spdadd A.B.C.D/32 W.X.Y.Z/32 ipencap -P in ipsec esp/tunnel/A.B.C.D-W.X.Y.Z/require; Finally, you need to add firewall rules to allow ESP and IPENCAP packets back and forth. These rules will need to be added to both hosts. ipfw add 1 allow esp from A.B.C.D to W.X.Y.Z ipfw add 1 allow esp from W.X.Y.Z to A.B.C.D ipfw add 1 allow ipencap from A.B.C.D to W.X.Y.Z ipfw add 1 allow ipencap from W.X.Y.Z to A.B.C.D Because the rules are symmetric you can use the same rules on each gateway host. Outgoing packets will now look something like this: .------------------------------. --------------------------. | Src: A.B.C.D | | | Dst: W.X.Y.Z | | | <other header info> | | Encrypted +------------------------------+ | packet. | .--------------------------. | -------------. | contents | | Src: A.B.C.D | | | | are | | Dst: W.X.Y.Z | | | | completely | | <other header info> | | | |- secure | +--------------------------+ | | Encap'd | from third | | .----------------------. | | -. | packet | party | | | Src: 192.168.1.1 | | | | Original |- with real | snooping | | | Dst: 192.168.2.1 | | | | packet, | IP addr | | | | <other header info> | | | |- private | | | | +----------------------+ | | | IP addr | | | | | <packet data> | | | | | | | | `----------------------' | | -' | | | `--------------------------' | -------------' | `------------------------------' --------------------------' When they are received by the far end of the VPN they will first be decrypted (using the security associations that have been negotiated by racoon). Then they will enter the gif interface, which will unwrap the second layer, until you are left with the innermost packet, which can then travel in to the inner network. You can check the security using the same &man.ping.8; test from earlier. First, log in to the A.B.C.D gateway machine, and run: tcpdump dst host 192.168.2.1 In another log in session on the same host run ping 192.168.2.1 This time you should see output like the following: XXX tcpdump output Now, as you can see, &man.tcpdump.1; shows the ESP packets. If you try and examine them with the option you will see (apparently) gibberish, because of the encryption. Congratulations. You have just set up a VPN between two remote sites. Summary Configure both kernels with: options IPSEC options IPSEC_ESP Install security/racoon. Edit ${PREFIX}/etc/racoon/psk.txt on both gateway hosts, adding an entry for the remote host's IP address and a secret key that they both know. Make sure this file is mode 0600. Add the following lines to /etc/rc.conf on each host: ipsec_enable="YES" ipsec_file="/etc/ipsec.conf" Create an /etc/ipsec.conf on each host that contains the necessary spdadd lines. On gateway host #1 this would be: spdadd A.B.C.D/32 W.X.Y.Z/32 ipencap -P out ipsec esp/tunnel/A.B.C.D-W.X.Y.Z/require; spdadd W.X.Y.Z/32 A.B.C.D/32 ipencap -P in ipsec esp/tunnel/W.X.Y.Z-A.B.C.D/require; On gateway host #2 this would be: spdadd W.X.Y.Z/32 A.B.C.D/32 ipencap -P out ipsec esp/tunnel/W.X.Y.Z-A.B.C.D/require; spdadd A.B.C.D/32 W.X.Y.Z/32 ipencap -P in ipsec esp/tunnel/A.B.C.D-W.X.Y.Z/require; Add firewall rules to allow IKE, ESP, and IPENCAP traffic to both hosts: ipfw add 1 allow udp from A.B.C.D to W.X.Y.Z isakmp ipfw add 1 allow udp from W.X.Y.Z to A.B.C.D isakmp ipfw add 1 allow esp from A.B.C.D to W.X.Y.Z ipfw add 1 allow esp from W.X.Y.Z to A.B.C.D ipfw add 1 allow ipencap from A.B.C.D to W.X.Y.Z ipfw add 1 allow ipencap from W.X.Y.Z to A.B.C.D The previous two steps should suffice to get the VPN up and running. Machines on each network will be able to refer to one another using IP addresses, and all traffic across the link will be automatically and securely encrypted. Chern Lee Contributed by OpenSSH OpenSSH security OpenSSH OpenSSH is a set of network connectivity tools used to access remote machines securely. It can be used as a direct replacement for rlogin, rsh, rcp, and telnet. Additionally, any other TCP/IP connections can be tunneled/forwarded securely through SSH. OpenSSH encrypts all traffic to effectively eliminate eavesdropping, connection hijacking, and other network-level attacks. OpenSSH is maintained by the OpenBSD project, and is based upon SSH v1.2.12 with all the recent bug fixes and updates. It is compatible with both SSH protocols 1 and 2. OpenSSH has been in the base system since FreeBSD 4.0. Advantages of Using OpenSSH Normally, when using &man.telnet.1; or &man.rlogin.1;, data is sent over the network in an clear, un-encrypted form. Network sniffers anywhere in between the client and server can steal your user/password information or data transferred in your session. OpenSSH offers a variety of authentication and encryption methods to prevent this from happening. Enabling sshd OpenSSH enabling Be sure to make the following addition to your rc.conf file: sshd_enable="YES" This will load &man.sshd.8;, the daemon program for OpenSSH, the next time your system initializes. Alternatively, you can simply run directly the sshd daemon by typing sshd on the command line. SSH Client OpenSSH client The &man.ssh.1; utility works similarly to &man.rlogin.1;. &prompt.root; ssh user@example.com Host key not found from the list of known hosts. Are you sure you want to continue connecting (yes/no)? yes Host 'example.com' added to the list of known hosts. user@example.com's password: ******* The login will continue just as it would have if a session was created using rlogin or telnet. SSH utilizes a key fingerprint system for verifying the authenticity of the server when the client connects. The user is prompted to enter yes only when connecting for the first time. Future attempts to login are all verified against the saved fingerprint key. The SSH client will alert you if the saved fingerprint differs from the received fingerprint on future login attempts. The fingerprints are saved in ~/.ssh/known_hosts, or ~/.ssh/known_hosts2 for SSH v2 fingerprints. By default, OpenSSH servers are configured to accept both SSH v1 and SSH v2 connections. The client, however, can choose between the two. Version 2 is known to be more robust and secure than its predecessor. The &man.ssh.1; command can be forced to use either protocol by passing it the or argument for v1 and v2, respectively. Secure Copy OpenSSH secure copy scp The &man.scp.1; command works similarly to &man.rcp.1;; it copies a file to or from a remote machine, except in a secure fashion. &prompt.root; scp user@example.com:/COPYRIGHT COPYRIGHT user@example.com's password: ******* COPYRIGHT 100% |*****************************| 4735 00:00 &prompt.root; Since the fingerprint was already saved for this host in the previous example, it is verified when using &man.scp.1; here. The arguments passed to &man.scp.1; are similar to &man.cp.1;, with the file or files in the first argument, and the destination in the second. Since the file is fetched over the network, through SSH, one or more of the file arguments takes on the form . Configuration OpenSSH configuration The system-wide configuration files for both the OpenSSH daemon and client reside within the /etc/ssh directory. ssh_config configures the client settings, while sshd_config configures the daemon. Additionally, the (/usr/sbin/sshd by default), and rc.conf options can provide more levels of configuration. ssh-keygen Instead of using passwords, &man.ssh-keygen.1; can be used to generate RSA keys to authenticate a user: &prompt.user; ssh-keygen -t rsa1 Initializing random number generator... Generating p: .++ (distance 66) Generating q: ..............................++ (distance 498) Computing the keys... Key generation complete. Enter file in which to save the key (/home/user/.ssh/identity): Enter passphrase: Enter the same passphrase again: Your identification has been saved in /home/user/.ssh/identity. ... &man.ssh-keygen.1; will create a public and private key pair for use in authentication. The private key is stored in ~/.ssh/identity, whereas the public key is stored in ~/.ssh/identity.pub. The public key must be placed in ~/.ssh/authorized_keys of the remote machine in order for the setup to work. This will allow connection to the remote machine based upon RSA authentication instead of passwords. The option will create RSA keys for use by SSH protocol version 1. If you want to use RSA keys with the SSH protocol version 2, you have to use the command ssh-keygen -t rsa. If a passphrase is used in &man.ssh-keygen.1;, the user will be prompted for a password each time in order to use the private key. A SSH protocol version 2 DSA key can be created for the same purpose by using the ssh-keygen -t dsa command. This will create a public/private DSA key for use in SSH protocol version 2 sessions only. The public key is stored in ~/.ssh/id_dsa.pub, while the private key is in ~/.ssh/id_dsa. DSA public keys are also placed in ~/.ssh/authorized_keys on the remote machine. &man.ssh-agent.1; and &man.ssh-add.1; are utilities used in managing multiple passworded private keys. The various options and files can be different according to the OpenSSH version you have on your system, to avoid problems you should consult the &man.ssh-keygen.1; manual page. SSH Tunneling OpenSSH tunneling OpenSSH has the ability to create a tunnel to encapsulate another protocol in an encrypted session. The following command tells &man.ssh.1; to create a tunnel for telnet: &prompt.user; ssh -2 -N -f -L 5023:localhost:23 user@foo.example.com &prompt.user; The ssh command is used with the following options: Forces ssh to use version 2 of the protocol. (Do not use if you are working with older SSH servers) Indicates no command, or tunnel only. If omitted, ssh would initiate a normal session. Forces ssh to run in the background. Indicates a local tunnel in localport:remotehost:remoteport fashion. The remote SSH server. An SSH tunnel works by creating a listen socket on localhost on the specified port. It then forwards any connection received on the local host/port via the SSH connection to the specified remote host and port. In the example, port 5023 on localhost is being forwarded to port 23 on localhost of the remote machine. Since 23 is telnet, this would create a secure telnet session through an SSH tunnel. This can be used to wrap any number of insecure TCP protocols such as SMTP, POP3, FTP, etc. Using SSH to Create a Secure Tunnel for SMTP &prompt.user; ssh -2 -N -f -L 5025:localhost:25 user@mailserver.example.com user@mailserver.example.com's password: ***** &prompt.user; telnet localhost 5025 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. 220 mailserver.example.com ESMTP This can be used in conjunction with an &man.ssh-keygen.1; and additional user accounts to create a more seamless/hassle-free SSH tunneling environment. Keys can be used in place of typing a password, and the tunnels can be run as a separate user. Practical SSH Tunneling Examples Secure Access of a POP3 Server At work, there is an SSH server that accepts connections from the outside. On the same office network resides a mail server running a POP3 server. The network, or network path between your home and office may or may not be completely trustable. Because of this, you need to check your e-mail in a secure manner. The solution is to create an SSH connection to your office's SSH server, and tunnel through to the mail server. &prompt.user; ssh -2 -N -f -L 2110:mail.example.com:110 user@ssh-server.example.com user@ssh-server.example.com's password: ****** When the tunnel is up and running, you can point your mail client to send POP3 requests to localhost port 2110. A connection here will be forwarded securely across the tunnel to mail.example.com. Bypassing a Draconian Firewall Some network administrators impose extremely draconian firewall rules, filtering not only incoming connections, but outgoing connections. You may be only given access to contact remote machines on ports 22 and 80 for SSH and web surfing. You may wish to access another (perhaps non-work related) service, such as an Ogg Vorbis server to stream music. If this Ogg Vorbis server is streaming on some other port than 22 or 80, you will not be able to access it. The solution is to create an SSH connection to a machine outside of your network's firewall, and use it to tunnel to the Ogg Vorbis server. &prompt.user; ssh -2 -N -f -L 8888:music.example.com:8000 user@unfirewalled-system.example.org user@unfirewalled-system.example.org's password: ******* Your streaming client can now be pointed to localhost port 8888, which will be forwarded over to music.example.com port 8000, successfully evading the firewall. Further Reading OpenSSH &man.ssh.1; &man.scp.1; &man.ssh-keygen.1; &man.ssh-agent.1; &man.ssh-add.1; &man.sshd.8; &man.sftp-server.8; Robert Watson Sponsored by DARPA and Network Associates Laboratories. Contributed by MAC Mandatory Access Control (MAC) FreeBSD 5.0 includes a new kernel security framework, the TrustedBSD MAC Framework. The MAC Framework permits compile-time, boot-time, and run-time extension of the kernel access control policy, and can be used to load support for Mandatory Access Control (MAC), and custom security modules such as hardening modules. The MAC Framework is currently considered to be an experimental feature, and should not yet be used in production environments without careful consideration. It is anticipated that the MAC Framework will be appropriate for more widespread production use by FreeBSD 5.2. When configured into a kernel, the MAC Framework permits security modules to augment the existing kernel access control model, restricting access to system services and objects. For example, the &man.mac.bsdextended.4; module augments file system access control, permitting administrators to provide a firewall-like ruleset constraining access to file system objects based on user ids and group membership. Some modules require little or no configuration, such as &man.mac.seeotheruids.4, whereas others perform ubiquitous object labeling, such as &man.mac.biba.4; and &man.mac.mls.4;, and require extensive configuration. To enable the MAC Framework in your system kernel, you must add the following entry to your kernel configuration: options MAC Security policy modules shipped with the base system may be loaded using &man.kldload.8; or in the boot &man.loader.8; They may also be compiled directly into the kernel using the following options, if the use of modules is not desired. Different MAC policies may be configured in different ways; frequently, MAC policy modules export configuration parameters using the &man.sysctl.8; MIB using the security.mac namespace. Policies relying on file system or other labels may require a configuration step that involves assigning initial labels to system objects or creating a policy configuration file. For information on how to configure and use each policy module, see its man page. A variety of tools are available to configure the MAC Framework and labels maintained by various policies. Extensions have been made to the login and credential management mechanisms (&man.setusercontext.3;) to support initial user labeling using &man.login.conf.5;. In addition, modifications have been made to &man.su.1;, &man.ps.1;, &man.ls.1;, and &man.ifconfig.8; to inspect and set labels on processes, files, and interfaces. In addition, several new tools have been added to manage labels on objects, including &man.getfmac.8;, &man.setfmac.8;, and &man.setfsmac.8; to manage labels on files, and &man.getpmac.8; and &man.setpmac.8;. What follows is a list of policy modules shipped with FreeBSD 5.0. Biba Integrity Policy (mac_biba) Biba Integrity Policy Vendor: TrustedBSD Project Module name: mac_biba.ko Kernel option: MAC_BIBA TCB The Biba Integrity Policy (&man.mac.biba.4;) provides for hierarchical and non-hierarchical labeling of all system objects with integrity data, and the strict enforcement of an information flow policy to prevent corruption of high integrity subjects and data by low-integrity subjects. Integrity is enforced by preventing high integrity subjects (generally processes) from reading low integrity objects (often files), and preventing low integrity subjects from writing to high integrity objects. This security policy is frequently used in commercial trusted systems to provide strong protection for the Trusted Code Base (TCB). Because it provides ubiquitous labeling, the Biba integrity policy must be compiled into the kernel or loaded at boot. File System Firewall Policy (mac_bsdextended) File System Firewall Policy Vendor: TrustedBSD Project Module name: mac_bsdextended.ko Kernel option: MAC_BSDEXTENDED The File System Firewall Policy (&man.mac.bsdextended.4;) provides an extension to the BSD file system permission model, permitting the administrator to define a set of firewall-like rules for limiting access to file system objects owned by other users and groups. Managed using &man.ugidfw.8;, rules may limit access to files and directories based on the uid and gids of the process attempting the access, and the owner and group of the target of the access attempt. All rules are restrictive, so they may be placed in any order. This policy requires no prior configuration or labeling, and may be appropriate in multi-user environments where mandatory limits on inter-user data exchange are required. Caution should be exercised in limiting access to files owned by the super-user or other system user ids, as many useful programs and directories are owned by these users. As with a network firewall, improper application of file system firewall rules may render the system unusable. New tools to manage the rule set may be easily written using the &man.libugidfw.3; library. Interface Silencing Policy (mac_ifoff) Interface Silencing Policy Vendor: TrustedBSD Project Module name: mac_ifoff.ko Kernel option: MAC_IFOFF The interface silencing policy (&man.mac.ifoff.4;) prohibits the use of network interfaces during the boot until explicitly enabled, preventing spurious stack output stack response to incoming packets. This is appropriate for use in environments where the monitoring of packets is required, but no traffic may be generated. Low-Watermark Mandatory Access Control (LOMAC) (mac_lomac) MAC Low-Watermark LOMAC Vendor: Network Associates Laboratories Module name: mac_lomac.ko Kernel option: MAC_LOMAC Similar to the Biba Integrity Policy, the LOMAC policy (&man.mac.lomac.4;) relies on the ubiquitous labeling of all system objects with integrity labels. Unlike Biba, LOMAC permits high integrity subjects to read from low integrity objects, but then downgrades the label on the subject to prevent future writes to high integrity objects. This policy may provide for greater compatibility, as well as require less initial configuration than Biba. However, as with Biba, it ubiquitously labels objects and must therefore be compiled into the kernel or loaded at boot. Multi-Level Security Policy (MLS) (mac_mls) Multi-Level Security Policy MAC Multi-Level Vendor: TrustedBSD Project Module name: mac_mls.ko Kernel option: MAC_MLS Multi-Level Security (MLS) (&man.mac.mls.4;) provides for hierarchical and non-hierarchical labeling of all system objects with sensitivity data, and the strict enforcement of an information flow policy to prevent the leakage of confidential data to untrusted parties. The logical conjugate of the Biba Integrity Policy, MLS is frequently shipped in commercial trusted operating systems to protect data secrecy in multi-user environments. Hierarchal labels provide support for the notion of clearances and classifications in traditional parlance; non-hierarchical labels provide support for need-to-know. As with Biba, ubiquitous labeling of objects occurs, and it must therefore be compiled into the kernel or loaded at boot. As with Biba, extensive initial configuration may be required. MAC Stub Policy (mac_none) MAC Stub Policy Vendor: TrustedBSD Project Module name: mac_none.ko Kernel option: MAC_NONE The None policy (&man.mac.none.4;) provides a stub sample policy for developers, implementing all entry points, but not changing the system access control policy. Running this on a production system would not be highly beneficial. Process Partition Policy (mac_partition) Process Partition Policy Vendor: TrustedBSD Project Module name: mac_partition.ko Kernel option: MAC_PARTITION The Partition policy (&man.mac.partition.4;) provides for a simple process visibility limitation, assigning labels to processes identifying what numeric system partition they are present in. If none, all other processes are visible using standard monitoring tools; if a partition identifier is present, then only other processes in the same partition are visible. This policy may be compiled into the kernel, loaded at boot, or loaded at run-time. See Other Uids Policy (mac_seeotheruids) See Other Uids Policy Vendor: TrustedBSD Project Module name: mac_seeotheruids.ko Kernel option: MAC_SEEOTHERUIDS The See Other Uids policy (&man.mac.seeotheruids.4;) implements a similar process visibility model to mac_partition, except that it relies on process credentials to control visibility of processes, rather than partition labels. This policy may be configured to exempt certain users and groups, including permitting system operators to view all processes without special privilege. This policy may be compiled into the kernel, loaded at boot, or loaded at run-time. MAC Framework Test Policy (mac_test) MAC Framework Test Policy Vendor: TrustedBSD Project Module name: mac_test.ko Kernel option: MAC_TEST The Test policy (&man.mac.test.4;) provides a regression test environment for the MAC Framework, and will cause a fail-stop in the event that internal MAC Framework assertions about proper data labeling fail. This module can be used to detect failures to properly label system objects in the kernel implementation. This policy may be compiled into the kernel, loaded at boot, or loaded at run-time. Tom Rhodes Contributed by ACL File System Access Control Lists In conjunction with file system enhancements like snapshots, FreeBSD 5.0 and later offers the security of File System Access Control Lists (ACLs). Access Control Lists extend the standard &unix; permission model in a highly compatible (&posix;.1e) way. This feature permits an administrator to make use of and take advantage of a more sophisticated security model. To enable ACL support for UFS file systems, the following: options UFS_ACL must be compiled into the kernel. If this option has not been compiled in, a warning message will be displayed when attempting to mount a file system supporting ACLs. This option is included in the GENERIC kernel. ACLs rely on extended attributes being enabled on the file system. Extended attributes are natively supported in the next generation &unix; file system, UFS2. A higher level of administrative overhead is required to configure extended attributes on UFS1 than on UFS2. The performance of extended attributes on UFS2 is also substantially higher. As a result, UFS2 is generally recommended in preference to UFS1 for use with access control lists. ACLs are enabled by the mount-time administrative flag, , which may be added to /etc/fstab. The mount-time flag can also be automatically set in a persistent manner using &man.tunefs.8; to modify a superblock ACLs flag in the file system header. In general, it is preferred to use the superblock flag for several reasons: The mount-time ACLs flag cannot be changed by a remount (&man.mount.8; ), only by means of a complete &man.umount.8; and fresh &man.mount.8;. This means that ACLs cannot be enabled on the root file system after boot. It also means that you cannot change the disposition of a file system once it is in use. Setting the superblock flag will cause the file system to always be mounted with ACLs enabled even if there is not an fstab entry or if the devices re-order. This prevents accidental mounting of the file system without ACLs enabled, which can result in ACLs being improperly enforced, and hence security problems. We may change the ACLs behavior to allow the flag to be enabled without a complete fresh &man.mount.8;, but we consider it desirable to discourage accidental mounting without ACLs enabled, because you can shoot your feet quite nastily if you enable ACLs, then disable them, then re-enable them without flushing the extended attributes. In general, once you have enabled ACLs on a file system, they should not be disabled, as the resulting file protections may not be compatible with those intended by the users of the system, and re-enabling ACLs may re-attach the previous ACLs to files that have since had their permissions changed, resulting in other unpredictable behavior. File systems with ACLs enabled will show a + (plus) sign in their permission settings when viewed. For example: drwx------ 2 robert robert 512 Dec 27 11:54 private drwxrwx---+ 2 robert robert 512 Dec 23 10:57 directory1 drwxrwx---+ 2 robert robert 512 Dec 22 10:20 directory2 drwxrwx---+ 2 robert robert 512 Dec 27 11:57 directory3 drwxr-xr-x 2 robert robert 512 Nov 10 11:54 public_html Here we see that the directory1, directory2, and directory3 directories are all taking advantage of ACLs. The public_html directory is not.