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SYN attack. how does it *really* work

  • From: Jonathan M. Bresler
  • Date: Tue Sep 17 21:34:11 1996

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             [email protected]
FreeBSD--4.4BSD Unix for PC clones, source included. http://www.freebsd.org/
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