This is a significantly revised implementation of an earlier (year-old)
patch that no longer applies cleanly, with permission of the original
author (Adam Langley).  That patch was previously reviewed:

   http://thread.gmane.org/gmane.linux.network/102586

The principle difference is using a TCP option to carry the cookie nonce,
instead of a user configured offset in the data.  This is more flexible and
less subject to user configuration error.  Such a cookie option has been
suggested for many years, and is also useful without SYN data, allowing
several related concepts to use the same extension option.

   "Re: SYN floods (was: does history repeat itself?)", September 9, 1996.
   http://www.merit.net/mail.archives/nanog/1996-09/msg00235.html

   "Re: what a new TCP header might look like", May 12, 1998.
   ftp://ftp.isi.edu/end2end/end2end-interest-1998.mail

Data structures are carefully composed to require minimal additions.
For example, the struct tcp_options_received cookie_plus variable fits
between existing 16-bit and 8-bit variables, requiring no additional
space (taking alignment into consideration).  There are no additions to
tcp_request_sock, and only 1 pointer and 1 flag byte in tcp_sock.

Allocations have been rearranged to avoid requiring GFP_ATOMIC, with
only one unavoidable exception in tcp_create_openreq_child(), where the
tcp_sock itself is created GFP_ATOMIC.

These functions will also be used in subsequent patches that implement
additional features.
---
  include/linux/tcp.h      |   35 +++++++-
  include/net/tcp.h        |   72 ++++++++++++++--
  net/ipv4/syncookies.c    |    5 +-
  net/ipv4/tcp.c           |  133 +++++++++++++++++++++++++++-
  net/ipv4/tcp_input.c     |   82 +++++++++++++++---
  net/ipv4/tcp_ipv4.c      |   62 +++++++++++--
  net/ipv4/tcp_minisocks.c |   43 +++++++---
  net/ipv4/tcp_output.c    |  223 ++++++++++++++++++++++++++++++++++++++++++---
  net/ipv6/syncookies.c    |    5 +-
  net/ipv6/tcp_ipv6.c      |   47 +++++++++-
  10 files changed, 641 insertions(+), 66 deletions(-)