Date: Fri, 04 Jan 2002 15:35:35 -0800 From: Terry Lambert <tlambert2@mindspring.com> To: "Louis A. Mamakos" <louie@TransSys.COM> Cc: Guido van Rooij <guido@gvr.org>, William Carrel <william.carrel@infospace.com>, freebsd-hackers@FreeBSD.ORG Subject: Re: path_mtu_discovery Message-ID: <3C363C47.1AE94141@mindspring.com> References: <26E71536-013D-11D6-8ED3-003065D5E9A4@infospace.com> <3C36149B.B9C02DCF@mindspring.com> <20020104223235.GA64301@gvr.gvr.org> <200201042302.g04N2A789953@whizzo.transsys.com>
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"Louis A. Mamakos" wrote: > One possibility is that the code in icmp_input() processing the > PMTU discovery-induced ICMP message could verify that the returned > header in fact is associated with a connection on the host and > maybe even has sane sequence numbers (for TCP segments). This would > make it more difficult to just spray these packets at host and > drop the MTU on routes. Of course, now you've let the dirty little secret out of the bag: the MTU is on the *route*, which means on the next hop, so a spoof that got through would frag basically all traffic out of the victim machine down to 296 bytes... A client machine could do much worse, of course, fragging the inverse traceroute until the fragging was successful, after sending the SYN, in response to the server's "SYN-ACK"... The obvious "fix" for that is to not let the MTU be dropped if the last sent packet's size is smaller than the drop-to (can't use a "max successful" because of multiple routes and/or route assymetry). This would prevent doing it on the "SYN-ACK", or other small packet (maybe disallowing it entirely, until the first data packet has been sent). But the obvious "fix" for the obvious "fix" is to establish a connection, and then trigger the largest packet you can to be sent from the server (e.g. request a big HTTP document, which most initial pages in fact are). It's always an arms race... -- Terry To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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