Syn flooding attacks

Jeffrey C. Ollie jeff at
Mon Oct 20 17:49:20 UTC 1997

Phil Howard wrote:
> The server can enlarge its table of pending connections and shorten it's
> timeout on them.  Currently I think this is on the order of 2 to 3 minutes
> and I think I can live with shortening it to 20 seconds, if I could get in
> the kernel to make that change (easy for Linux, FreeBSD, etc, but not for
> most commercial systems like Solaris, NT, etc).

On the latest Linux kernels (and perhaps on other free UNIXs) there is a
feature known as "SYN cookies".  Basically this feature lets servers
eliminate the table of half open connections by carefully crafting the
ACK so that the next packet from the originating host has enough
information to fully open the connection.  This was widely discussed on
nanog and other lists when SYN flooding attacks first became popular a
year or so ago.  Check the archives for more information or check out
the Linux TCP code in the latest kernels.

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