SYN attack. how does it *really* work

["Jonathan M. Bresler" <jmb () freefall freebsd org>]

Michael Dillon wrote:
> If it only takes 8 SYN packets to lock up a socket for 75 seconds then
> effective SYN flood attacks certainly *CAN* be launched from a dialup
> connection. And if the definition of an effective attack allows for
> intermittently shutting down a socket then effective attacks certainly
> *CAN be launched from places like Uruguay, Brazil, Indonesia and so forth.

        not 8, only 2 SYN packets into the same connection are needed
                (connection is a single src addr, src port, dest addr
                 dest port 4-tuple)
        not 75 seconds, ~11 minutes.

        the essence of the bug is:

        one timer t_timer[TCPT_KEEP] used for 2 purposes
                --to hold the 75 second half-open timer
                --to hold the 2 hour keepalive timer
        the first SYN packet sets the timer to 75 seconds
        the second trips the bug and resets the timer to 2 hours

        so where does the 11 minutes come from?

        the server (target) send SYN-ACK packets, and retransmits
        the SYN-ACK until it either gets a response or gives up
        when TCP_MAXRXTSHIFT is exceeded.  the latter take ~11 minutes.

        the fix is to qualify the settting of hte timer ala:

        if (TCPS_HAVEESTABLISHED(tp->t_state))
                tp->t_timer[TCPT_KEEP] = tcp_keepidle;

        and to set the timer a each location where the TCP/IP state
        machine transitions to TCPS_ESTABLISHED.

        each half-open socket consumes 264 bytes of memory (assuming
        perfect allocation ;)

        all BSD derived TCP/IP implementations are/may be susceptible
        to this bug.  that includes AIX, SVR4, and SunOS.

        stevens TCP/IP illustrated vol 3 p191 explains this much beter
        than i can
jmb
--
Jonathan M. Bresler           FreeBSD Postmaster             jmb () FreeBSD ORG
FreeBSD--4.4BSD Unix for PC clones, source included. http://www.freebsd.org/
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