Date: Thu, 12 Apr 2001 12:22:44 -0600 From: Wes Peters <wes@softweyr.com> To: Mike Silbersack <silby@silby.com> Cc: Rob Simmons <rsimmons@wlcg.com>, Mark T Roberts <newsletter@marktroberts.com>, freebsd-security@FreeBSD.ORG Subject: Re: non-random IP IDs Message-ID: <3AD5F274.547D0350@softweyr.com> References: <Pine.BSF.4.31.0104121046150.3325-100000@achilles.silby.com>
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Mike Silbersack wrote: > > On Thu, 12 Apr 2001, Rob Simmons wrote: > > > On Thu, 12 Apr 2001, Mike Silbersack wrote: > > > > > Each IP packet sent has with it a 16-bit ID. The numbers must remain > > > unique over a short period of time so fragmentation can work properly. As > > > such, everything except recent openbsds simple increments the id by 1 for > > > each packet sent out. > > > > What is the behavior of OpenBSD for this? If its not important, why would > > they change it? > > They generate pseudo-random, nonrepeating ids. For the actual algorithm, > see: > > http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/netinet/ip_id.c?rev=1.2&content-type=text/x-cvsweb-markup&cvsroot=openbsd > > Although it's nice in theory, the amount of work required to generate the > ids seems too great to justify for each packet sent. (Note that I said > "seems", I'm not sure if anyone has done actual benchmarks to determine > the actual impact.) Just like TCP sequence numbers, non-predictable IP IDs are *supposed to* make it somewhat harder to insert bogus fragments into a packet stream. If you are a router, this won't make a bit of difference in your ability to frag a packet and stick whatever data you want into it; if you are not a router your ability to see a fragmented packet go by and inject other frags into it is almost non-existant anyhow, so I don't see much value in this. It's mostly just in fitting with the OpenBSD "deny them everything" approach. -- "Where am I, and what am I doing in this handbasket?" Wes Peters Softweyr LLC wes@softweyr.com http://softweyr.com/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-security" in the body of the message
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