FreeBSD-SA-14:19.tcp raised attention to the state of our stack
towards blind SYN/RST spoofed attack.

Originally our stack used in-window checks for incoming SYN/RST
as proposed by RFC793. Later, circa 2003 the RST attack was
mitigated using the technique described in P. Watson
"Slipping in the window" paper [1].

After that, the checks were only relaxed for the sake of
compatibility with some buggy TCP stacks. First, r192912
introduced the vulnerability, just fixed by aforementioned SA.
Second, r167310 had slightly relaxed the default RST checks,
instead of utilizing net.inet.tcp.insecure_rst sysctl.

In 2010 a new technique for mitigation of these attacks was
proposed in RFC5961 [2]. The idea is to send a "challenge ACK"
packet to the peer, to verify that packet arrived isn't spoofed.
If peer receives challenge ACK it should regenerate its RST or
SYN with correct sequence number. This should not only protect
against attacks, but also improve communication with broken
stacks, so authors of reverted r167310 and r192912 won't be
disappointed.

[1] http://bandwidthco.com/whitepapers/netforensics/tcpip/TCP Reset Attacks.pdf
[2] http://www.rfc-editor.org/rfc/rfc5961.txt

Changes made:

o Revert r167310.
o Implement "challenge ACK" protection as specificed in RFC5961

against RST attack. On by default.
- Carefully preserve r138098, which handles empty window edge
  case, not described by the RFC.
- Update net.inet.tcp.insecure_rst description.

o Implement "challenge ACK" protection as specificed in RFC5961

against SYN attack. On by default.
- Provide net.inet.tcp.insecure_syn sysctl, to turn off
  RFC5961 protection.

The changes were tested at Netflix. The tested box didn't show
any anomalies compared to control box, except slightly increased
number of TCP connection in LAST_ACK state.

Reviewed by: rrs
Sponsored by: Netflix
Sponsored by: Nginx, Inc.