Date: Thu, 23 May 1996 08:59:48 -0600 From: Warner Losh <imp@village.org> To: Paul Traina <pst@shockwave.com> Cc: Garrett Wollman <wollman@lcs.mit.edu>, Poul-Henning Kamp <phk@critter.tfs.com>, current@FreeBSD.ORG, blh@nol.net Subject: Re: freebsd + synfloods + ip spoofing Message-ID: <199605231459.IAA09701@rover.village.org> In-Reply-To: Your message of Wed, 22 May 1996 09:07:04 PDT
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: We're making tcp_iss random in tcp_init.c, but then manipulating it in : totally predictable ways. This is not random at all. The ISS needs to : be randomized on a PER tcp connection attempt. I realize that violates : RFC 793, but it has to be done. There was recently an RFC issued, as I'm sure you are aware, that suggests making the sequence number a random thing based on the MD-5 hash of the traditional TCP/IP 4-touple and some host private information (like output from /dev/random). Is that sufficient, or do sequence numbers need to be randomly jiggered more to prevent the attacks that are being whined, err ummm, talked about here? Per attempt is easy to ramdomize w/o violating 793 since you still have 2^31 bits of randomness that you can use (since the original request partitions the space in 2). Even if you picked a more conservative figure, you can make it fairly hard to guess the next ISS w/o significantly impacting the ability of TCP to detect and discard stale packets. Warner
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