diff --git a/share/man/man7/security.7 b/share/man/man7/security.7
index b4aeb5728313..bb7e120a1d46 100644
--- a/share/man/man7/security.7
+++ b/share/man/man7/security.7
@@ -1,1116 +1,1116 @@
 .\" Copyright (C) 1998 Matthew Dillon. All rights reserved.
 .\" Copyright (c) 2019 The FreeBSD Foundation, Inc.
 .\"
 .\" Parts of this documentation were written by
 .\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship
 .\" from the FreeBSD Foundation.
 .\"
 .\" Redistribution and use in source and binary forms, with or without
 .\" modification, are permitted provided that the following conditions
 .\" are met:
 .\" 1. Redistributions of source code must retain the above copyright
 .\"    notice, this list of conditions and the following disclaimer.
 .\" 2. Redistributions in binary form must reproduce the above copyright
 .\"    notice, this list of conditions and the following disclaimer in the
 .\"    documentation and/or other materials provided with the distribution.
 .\"
 .\" THIS SOFTWARE IS PROVIDED BY AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 .\" ARE DISCLAIMED.  IN NO EVENT SHALL AUTHOR OR CONTRIBUTORS BE LIABLE
 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
 .\" $FreeBSD$
 .\"
 .Dd February 28, 2021
 .Dt SECURITY 7
 .Os
 .Sh NAME
 .Nm security
 .Nd introduction to security under FreeBSD
 .Sh DESCRIPTION
 Security is a function that begins and ends with the system administrator.
 While all
 .Bx
 multi-user systems have some inherent security, the job of building and
 maintaining additional security mechanisms to keep users
 .Dq 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.
 .Ux
 systems,
 in general, are capable of running a huge number of simultaneous processes
 and many of these processes operate as servers \(em 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 ever bigger issue.
 .Pp
 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.
 .Pp
 System security also pertains to dealing with various forms of attacks,
 including attacks that attempt to crash or otherwise make a system unusable
 but do not attempt to break root.
 Security concerns can be split up into
 several categories:
 .Bl -enum -offset indent
 .It
 Denial of Service attacks (DoS)
 .It
 User account compromises
 .It
 Root compromise through accessible servers
 .It
 Root compromise via user accounts
 .It
 Backdoor creation
 .El
 .Pp
 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 advantages 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 fill up your Internet
 pipe.
 .Pp
 A user account compromise is even more common than a DoS attack.
 Many
 sysadmins still run standard
 .Xr telnetd 8
 and
 .Xr ftpd 8
 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 log in to a system)
 will have his or her password sniffed.
 The attentive system administrator will analyze
 his remote access logs looking for suspicious source addresses
 even for successful logins.
 .Pp
 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.
 .Pp
 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 an 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 clean up after himself, so most attackers do install
 backdoors.
 This gives you a convenient way to detect the attacker.
 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 used to
 break in originally.
 .Pp
 Security remedies should always be implemented with a multi-layered
 .Dq onion peel
 approach and can be categorized as follows:
 .Bl -enum -offset indent
 .It
 Securing root and staff accounts
 .It
 Securing root \(em root-run servers and SUID/SGID binaries
 .It
 Securing user accounts
 .It
 Securing the password file
 .It
 Securing the kernel core, raw devices, and file systems
 .It
 Quick detection of inappropriate changes made to the system
 .It
 Paranoia
 .El
 .Sh SECURING THE ROOT ACCOUNT AND SECURING STAFF ACCOUNTS
 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
 .Em 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 a
 .Xr su 1
 utility.
 For example, make sure that your PTYs are specified as being
 .Dq Li insecure
 in the
 .Pa /etc/ttys
 file
 so that direct root logins via
 .Xr telnet 1
 are disallowed.
 If using
 other login services such as
 .Xr sshd 8 ,
 make sure that direct root logins are
 disabled there as well.
 Consider every access method \(em services such as
 .Xr ftp 1
 often fall through the cracks.
 Direct root logins should only be allowed
 via the system console.
 .Pp
 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
 .Dq Li wheel
 group (in
 .Pa /etc/group ) .
 The staff members placed in the
 .Li wheel
 group are allowed to
 .Xr su 1
 to root.
 You should never give staff
 members native
 .Li wheel
 access by putting them in the
 .Li wheel
 group in their password entry.
 Staff accounts should be placed in a
 .Dq Li staff
 group, and then added to the
 .Li wheel
 group via the
 .Pa /etc/group
 file.
 Only those staff members who actually need to have root access
 should be placed in the
 .Li wheel
 group.
 It is also possible, when using an
 authentication method such as Kerberos, to use Kerberos's
 .Pa .k5login
 file in the root account to allow a
 .Xr ksu 1
 to root without having to place anyone at all in the
 .Li wheel
 group.
 This
 may be the better solution since the
 .Li 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
 .Li wheel
 mechanism
 is better than having nothing at all, it is not necessarily the safest
 option.
 .Pp
 An indirect way to secure the root account is to secure your staff accounts
 by using an alternative login access method and *'ing out the crypted password
 for the staff accounts.
 This way an intruder may be able to steal the password
 file but will not be able to break into any staff accounts or root, even if
 root has a crypted password associated with it (assuming, of course, that
 you have limited root access to the console).
 Staff members
 get into their staff accounts through a secure login mechanism such as
 .Xr kerberos 8
 or
 .Xr ssh 1
 using a private/public
 key pair.
 When you use something like Kerberos you generally must secure
 the machines which run the Kerberos servers and your desktop workstation.
 When you use a public/private key pair with SSH, you must generally secure
 the machine you are logging in
 .Em from
 (typically your workstation),
 but you can
 also add an additional layer of protection to the key pair by password
 protecting the keypair when you create it with
 .Xr ssh-keygen 1 .
 Being able
 to star-out the passwords for staff accounts also guarantees that staff
 members can only log in through secure access methods that you have set up.
 You can
 thus force all staff members to use secure, encrypted connections for
 all their sessions which closes an important hole used by many intruders: that
 of sniffing the network from an unrelated, less secure machine.
 .Pp
 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.
 .Pp
 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
 affect all the machines the staff member may have an account on.
 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).
 .Sh SECURING ROOT \(em ROOT-RUN SERVERS AND SUID/SGID BINARIES
 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
 .Xr imapd 8
 or
 .Xr popper 8 Pq Pa ports/mail/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
 .Xr talkd 8 ,
 .Xr comsat 8 ,
 and
 .Xr fingerd 8
 daemons can be run in special user
 .Dq sandboxes .
 A sandbox is not perfect unless you go to 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 log in via
 .Xr sshd 8
 and never log in via
 .Xr telnetd 8
 then turn off those services!
 .Pp
 .Fx
 now defaults to running
 .Xr talkd 8 ,
 .Xr comsat 8 ,
 and
 .Xr fingerd 8
 in a sandbox.
 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.
 .Pp
 There are a number of other servers that typically do not run in sandboxes:
 .Xr sendmail 8 ,
 .Xr popper 8 ,
 .Xr imapd 8 ,
 .Xr ftpd 8 ,
 and others.
 There are alternatives to
 some of these, but installing them may require more work than you are willing
 to put
 (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.
 .Pp
 The other big potential root hole in a system are the SUID-root and SGID
 binaries installed on the system.
 Most of these binaries, such as
 .Xr su 1 ,
 reside in
 .Pa /bin , /sbin , /usr/bin ,
 or
 .Pa /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
 .Xr xterm 1 Pq Pa ports/x11/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
 .Pq Dq Li "chmod 000"
 any SUID binaries that nobody uses.
 A server with no display generally does not need an
 .Xr xterm 1
 binary.
 SGID binaries can be almost as dangerous.
 If an intruder can break an SGID-kmem binary the
 intruder might be able to read
 .Pa /dev/kmem
 and thus read the crypted password
 file, potentially compromising any passworded account.
 Alternatively an
 intruder who breaks group
 .Dq Li kmem
 can monitor keystrokes sent through PTYs,
 including PTYs used by users who log in through secure methods.
 An intruder
 that breaks the
 .Dq Li 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.
 .Sh SECURING USER ACCOUNTS
 User accounts are usually the most difficult to secure.
 While you can impose
 draconian access restrictions on your staff and *-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.
 .Sh 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
 crypted password file
 .Pq Pa /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.
 .Pp
 Your security scripts should always check for and report changes to
 the password file
 (see
 .Sx CHECKING FILE INTEGRITY
 below).
 .Sh SECURING THE KERNEL CORE, RAW DEVICES, AND FILE SYSTEMS
 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
 .Fx
 it is called
 the
 .Xr bpf 4
 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 should not have the
 .Xr bpf 4
 device compiled in.
 .Pp
 But even if you turn off the
 .Xr bpf 4
 device, you still have
 .Pa /dev/mem
 and
 .Pa /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,
 .Xr kldload 8 .
 An enterprising intruder can use a KLD module to install
 his own
 .Xr bpf 4
 device or other sniffing device on a running kernel.
 To avoid these problems you have to run
 the kernel at a higher security level, at least level 1.
 The security level can be set with a
 .Xr sysctl 8
 on the
 .Va kern.securelevel
 variable.
 Once you have
 set the security level to 1, write access to raw devices will be denied and
 special
 .Xr chflags 1
 flags, such as
 .Cm schg ,
 will be enforced.
 You must also ensure
 that the
 .Cm schg
 flag is set on critical startup binaries, directories, and
 script files \(em everything that gets run
 up to the point where the security level is set.
 This might be overdoing it, and upgrading the system is much more
 difficult when you operate at a higher security level.
 You may compromise and
 run the system at a higher security level but not set the
 .Cm schg
 flag for every
 system file and directory under the sun.
 Another possibility is to simply
 mount
 .Pa /
 and
 .Pa /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.
 .Pp
 The kernel runs with five different security levels.
 Any super-user process can raise the level, but no process
 can lower it.
 The security levels are:
 .Bl -tag -width flag
 .It Ic -1
 Permanently insecure mode \- always run the system in insecure mode.
 This is the default initial value.
 .It Ic 0
 Insecure mode \- immutable and append-only flags may be turned off.
 All devices may be read or written subject to their permissions.
 .It Ic 1
 Secure mode \- the system immutable and system append-only flags may not
 be turned off;
 disks for mounted file systems,
 .Pa /dev/mem
 and
 .Pa /dev/kmem
 may not be opened for writing;
 .Pa /dev/io
 (if your platform has it) may not be opened at all;
 kernel modules (see
 .Xr kld 4 )
 may not be loaded or unloaded.
 The kernel debugger may not be entered using the
 .Va debug.kdb.enter
 sysctl.
 A panic or trap cannot be forced using the
 .Va debug.kdb.panic ,
 .Va debug.kdb.panic_str
 and other sysctl's.
 .It Ic 2
 Highly secure mode \- same as secure mode, plus disks may not be
 opened for writing (except by
 .Xr mount 2 )
 whether mounted or not.
 This level precludes tampering with file systems by unmounting them,
 but also inhibits running
 .Xr newfs 8
 while the system is multi-user.
 .Pp
 In addition, kernel time changes are restricted to less than or equal to one
 second.
 Attempts to change the time by more than this will log the message
 .Dq Time adjustment clamped to +1 second .
 .It Ic 3
 Network secure mode \- same as highly secure mode, plus
 IP packet filter rules (see
 .Xr ipfw 8 ,
 .Xr ipfirewall 4
 and
 .Xr pfctl 8 )
 cannot be changed and
 .Xr dummynet 4
 or
 .Xr pf 4
 configuration cannot be adjusted.
 .El
 .Pp
 The security level can be configured with variables documented in
 .Xr rc.conf 5 .
 .Sh CHECKING FILE INTEGRITY: BINARIES, CONFIG 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
 .Xr chflags 1
 to set the
 .Cm schg
 bit on most of the files in
 .Pa /
 and
 .Pa /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 \(em 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 layer a chance to catch him in
 the act.
 .Pp
 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 keypairs to allow the limit-access
 box to SSH to the other machines.
 Except for its network traffic, NFS is
 the least visible method \(em allowing you to monitor the file systems 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.
 .Pp
 Once you give a limit-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
 .Xr find 1
 and
 .Xr md5 1 .
 It is best to physically
 .Xr md5 1
 the client-box files boxes at least once a
 day, and to test control files such as those found in
 .Pa /etc
 and
 .Pa /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
 .Pa /
 and
 .Pa /usr .
 .Pp
 When using SSH rather than NFS, writing the security script is much more
 difficult.
 You essentially have to
 .Xr scp 1
 the scripts to the client box in order to run them, making them visible, and
 for safety you also need to
 .Xr scp 1
 the binaries (such as
 .Xr find 1 )
 that those scripts use.
 The
 .Xr sshd 8
 daemon on the client box may already be compromised.
 All in all,
 using SSH may be necessary when running over unsecure links, but it is also a
 lot harder to deal with.
 .Pp
 A good security script will also check for changes to user and staff members
 access configuration files:
 .Pa .rhosts , .shosts , .ssh/authorized_keys
 and so forth, files that might fall outside the purview of the MD5 check.
 .Pp
 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 on those partitions is a good
 idea.
 The
 .Cm nosuid
 option
 (see
 .Xr mount 8 )
 is what you want to look into.
 I would 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.
 .Pp
 Process accounting
 (see
 .Xr accton 8 )
 is a relatively low-overhead feature of
 the operating system which I recommend using 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.
 .Pp
 Finally, security scripts should process the log files and the logs themselves
 should be generated in as secure a manner as possible \(em 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.
 .Sh PARANOIA
 A little paranoia never hurts.
 As a rule, a sysadmin can add any number
 of security features as long as they do not affect convenience, and
 can add security features that do affect convenience with some added
 thought.
 Even more importantly, a security administrator should mix it up
 a bit \(em if you use recommendations such as those given by this manual
 page verbatim, you give away your methodologies to the prospective
 attacker who also has access to this manual page.
 .Sh SPECIAL SECTION ON DoS ATTACKS
 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.
 .Bl -enum -offset indent
 .It
 Limiting server forks
 .It
 Limiting springboard attacks (ICMP response attacks, ping broadcast, etc.)
 .It
 Kernel Route Cache
 .El
 .Pp
 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.
 The
 .Xr inetd 8
 server
 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
 .Xr inetd 8
 manual page carefully and pay specific attention
 to the
 .Fl c , C ,
 and
 .Fl R
 options.
 Note that spoofed-IP attacks will circumvent
 the
 .Fl C
 option to
 .Xr inetd 8 ,
 so typically a combination of options must be used.
 Some standalone servers have self-fork-limitation parameters.
 .Pp
 The
 .Xr sendmail 8
 daemon has its
 .Fl OMaxDaemonChildren
 option which tends to work much
 better than trying to use
 .Xr sendmail 8 Ns 's
 load limiting options due to the
 load lag.
 You should specify a
 .Va MaxDaemonChildren
 parameter when you start
 .Xr sendmail 8
 high enough to handle your expected load but not so high that the
 computer cannot handle that number of
 .Nm sendmail Ns 's
 without falling on its face.
 It is also prudent to run
 .Xr sendmail 8
 in
 .Dq queued
 mode
 .Pq Fl ODeliveryMode=queued
 and to run the daemon
 .Pq Dq Nm sendmail Fl bd
 separate from the queue-runs
 .Pq Dq Nm sendmail Fl q15m .
 If you still want real-time delivery you can run the queue
 at a much lower interval, such as
 .Fl q1m ,
 but be sure to specify a reasonable
 .Va MaxDaemonChildren
 option for that
 .Xr sendmail 8
 to prevent cascade failures.
 .Pp
 The
 .Xr syslogd 8
 daemon can be attacked directly and it is strongly recommended that you use
 the
 .Fl s
 option whenever possible, and the
 .Fl a
 option otherwise.
 .Pp
 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.
 .Pp
 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.,
 .So
 firewall everything
 .Em except
 ports A, B, C, D, and M-Z
 .Sc .
 This
 way you can firewall off all of your low ports except for certain specific
 services such as
 .Xr talkd 8 ,
 .Xr sendmail 8 ,
 and other internet-accessible services.
 If you try to configure the firewall the other
 way \(em as an inclusive or permissive firewall, there is a good chance that you
 will forget to
 .Dq 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
 .Fx
 allows you to
 control the range of port numbers used for dynamic binding via the various
 .Va net.inet.ip.portrange
 sysctl's
 .Pq Dq Li "sysctl net.inet.ip.portrange" ,
 which can also
 ease the complexity of your firewall's configuration.
 I usually use a normal
 first/last range of 4000 to 5000, and a hiport range of 49152 to 65535, then
 block everything under 4000 off in my firewall
 (except for certain specific
 internet-accessible ports, of course).
 .Pp
 Another common DoS attack is called a springboard attack \(em to attack a server
 in a manner that causes the server to generate responses which then overload
 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
 .Vt mbuf Ns 's ,
 especially if the server cannot drain the
 ICMP responses it generates fast enough.
 The
 .Fx
 kernel has a new kernel
 compile option called
 .Dv ICMP_BANDLIM
 which limits the effectiveness of these
 sorts of attacks.
 The last major class of springboard attacks is related to
 certain internal
 .Xr inetd 8
 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
 .Xr inetd 8 Ns -internal
 test services.
 .Sh ACCESS ISSUES WITH KERBEROS AND SSH
 There are a few issues with both Kerberos and SSH that need to be addressed
 if you intend to use them.
 Kerberos5 is an excellent authentication
 protocol but the kerberized
 .Xr telnet 1
 suck rocks.
 There are bugs that make them unsuitable for dealing with binary streams.
 Also, by default
 Kerberos does not encrypt a session unless you use the
 .Fl x
 option.
 SSH encrypts everything by default.
 .Pp
 SSH works quite well in every respect except when it is set up to
 forward encryption keys.
 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
 .Xr ssh 1
 to an
 unsecure machine, your keys become exposed.
 The actual keys themselves are
 not exposed, but
 .Xr ssh 1
 installs a forwarding port for the duration of your
 login and if an attacker has broken root on the unsecure machine he can utilize
 that port to use your keys to gain access to any other machine that your
 keys unlock.
 .Pp
 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).
 We also recommend that you either turn off
 key-forwarding in the SSH configuration, or that you make use of the
 .Va from Ns = Ns Ar IP/DOMAIN
 option that SSH allows in its
 .Pa authorized_keys
 file to make the key only usable to entities logging in from specific
 machines.
 .Sh KNOBS AND TWEAKS
 .Fx
 provides several knobs and tweak handles that make some introspection
 information access more restricted.
 Some people consider this as improving system security, so the knobs are
 briefly listed there, together with controls which enable some mitigations
 of the hardware state leaks.
 .Pp
 Hardware mitigation sysctl knobs described below have been moved under
 .Pa machdep.mitigations ,
 with backwards-compatibility shims to accept the existing names.
 A future change will rationalize the sense of the individual sysctls
 (so that enabled / true always indicates that the mitigation is active).
 For that reason the previous names remain the canonical way to set the
 mitigations, and are documented here.
 Backwards compatibility shims for the interim sysctls under
 .Pa machdep.mitigations
 will not be added.
 .Bl -tag -width security.bsd.unprivileged_proc_debug
 .It Dv security.bsd.see_other_uids
 Controls visibility of processes owned by different uid.
 The knob directly affects the
 .Dv kern.proc
 sysctls filtering of data, which results in restricted output from
 utilities like
 .Xr ps 1 .
 .It Dv security.bsd.see_other_gids
 Same, for processes owned by different gid.
 .It Dv security.bsd.see_jail_proc
 Same, for processes belonging to a jail.
 .It Dv security.bsd.conservative_signals
 When enabled, unprivileged users are only allowed to send job control
 and usual termination signals like
 .Dv SIGKILL ,
 .Dv SIGINT ,
 and
 .Dv SIGTERM ,
 to the processes executing programs with changed uids.
 .It Dv security.bsd.unprivileged_proc_debug
 Controls availability of the process debugging facilities to non-root users.
 See also
 .Xr proccontrol 1
 mode
 .Dv trace .
 .It Dv vm.pmap.pti
 Tunable, amd64-only.
 Enables mode of operation of virtual memory system where usermode page
 tables are sanitized to prevent so-called Meltdown information leak on
 some Intel CPUs.
 By default, the system detects whether the CPU needs the workaround,
 and enables it automatically.
 See also
 .Xr proccontrol 1
 mode
 .Dv kpti .
 .It Dv machdep.mitigations.flush_rsb_ctxsw
 amd64.
 Controls Return Stack Buffer flush on context switch, to prevent
 cross-process ret2spec attacks.
 Only needed, and only enabled by default, if the machine
 supports SMEP, otherwise IBRS would do necessary flushing on kernel
 entry anyway.
 .It Dv hw.mds_disable
 amd64 and i386.
 Controls Microarchitectural Data Sampling hardware information leak
 mitigation.
 .It Dv hw.spec_store_bypass_disable
 amd64 and i386.
 Controls Speculative Store Bypass hardware information leak mitigation.
 .It Dv hw.ibrs_disable
 amd64 and i386.
 Controls Indirect Branch Restricted Speculation hardware information leak
 mitigation.
-.It Dv machdep.syscall_ret_l1d_flush
+.It Dv machdep.syscall_ret_flush_l1d
 amd64.
 Controls force-flush of L1D cache on return from syscalls which report
 errors other than
 .Ev EEXIST ,
 .Ev EAGAIN ,
 .Ev EXDEV ,
 .Ev ENOENT ,
 .Ev ENOTCONN ,
 and
 .Ev EINPROGRESS .
 This is mostly a paranoid setting added to prevent hypothetical exploitation
 of unknown gadgets for unknown hardware issues.
 The error codes exclusion list is composed of the most common errors which
 typically occurs on normal system operation.
 .It Dv machdep.nmi_flush_l1d_sw
 amd64.
 Controls force-flush of L1D cache on NMI;
 this provides software assist for bhyve mitigation of L1 terminal fault
 hardware information leak.
 .It Dv hw.vmm.vmx.l1d_flush
 amd64.
 Controls the mitigation of L1 Terminal Fault in bhyve hypervisor.
 .It Dv vm.pmap.allow_2m_x_ept
 amd64.
 Allows the use of superpages for executable mappings under the EPT
 page table format used by hypervisors on Intel CPUs to map the guest
 physical address space to machine physical memory.
 May be disabled to work around a CPU Erratum called
 Machine Check Error Avoidance on Page Size Change.
 .It Dv machdep.mitigations.rngds.enable
 amd64 and i386.
 Controls mitigation of Special Register Buffer Data Sampling versus
 optimization of the MCU access.
 When set to zero, the mitigation is disabled, and the RDSEED and RDRAND
 instructions do not incur serialization overhead for shared buffer accesses,
 and do not serialize off-core memory accessses.
 .It Dv kern.elf32.aslr.enable
 Controls system-global Address Space Layout Randomization (ASLR) for
 normal non-PIE (Position Independent Executable) 32bit binaries.
 See also
 .Xr proccontrol 1
 mode
 .Dv aslr ,
 also affected by the per-image control note flag.
 .It Dv kern.elf32.aslr.pie_enable
 Controls system-global Address Space Layout Randomization for
 position-independent (PIE) 32bit binaries.
 .It Dv kern.elf32.aslr.honor_sbrk
 Makes ASLR less aggressive and more compatible with old binaries
 relying on the sbrk area.
 .It Dv kern.elf32.aslr.stack_gap
 If ASLR is enabled for a binary, a non-zero value creates a randomized
 stack gap between strings and the end of the aux vector.
 The value is the maximum percentage of main stack to waste on the gap.
 Cannot be greater than 50, i.e., at most half of the stack.
 .It Dv kern.elf64.aslr.enable
 64bit binaries ASLR control.
 .It Dv kern.elf64.aslr.pie_enable
 64bit PIE binaries ASLR control.
 .It Dv kern.elf64.aslr.honor_sbrk
 64bit binaries ASLR sbrk compatibility control.
 .It Dv kern.elf64.aslr.stack_gap
 Controls stack gap for 64bit binaries.
 .It Dv kern.elf32.nxstack
 Enables non-executable stack for 32bit processes.
 Enabled by default if supported by hardware and corresponding binary.
 .It Dv kern.elf64.nxstack
 Enables non-executable stack for 64bit processes.
 .It Dv kern.elf32.allow_wx
 Enables mapping of simultaneously writable and executable pages for
 32bit processes.
 .It Dv kern.elf64.allow_wx
 Enables mapping of simultaneously writable and executable pages for
 64bit processes.
 .El
 .Sh SEE ALSO
 .Xr chflags 1 ,
 .Xr find 1 ,
 .Xr md5 1 ,
 .Xr netstat 1 ,
 .Xr openssl 1 ,
 .Xr proccontrol 1 ,
 .Xr ps 1 ,
 .Xr ssh 1 ,
 .Xr xdm 1 Pq Pa ports/x11/xorg-clients ,
 .Xr group 5 ,
 .Xr ttys 5 ,
 .Xr accton 8 ,
 .Xr init 8 ,
 .Xr sshd 8 ,
 .Xr sysctl 8 ,
 .Xr syslogd 8 ,
 .Xr vipw 8
 .Sh HISTORY
 The
 .Nm
 manual page was originally written by
 .An Matthew Dillon
 and first appeared
 in
 .Fx 3.1 ,
 December 1998.