Date: Sun, 15 Jan 2012 20:59:32 +0200 From: Nikolay Denev <ndenev@gmail.com> To: Andrey Zonov <andrey@zonov.org> Cc: freebsd-net@freebsd.org Subject: Re: ICMP attacks against TCP and PMTUD Message-ID: <733BE6AF-33E0-4C16-A222-B5F5D0519194@gmail.com> In-Reply-To: <4F131A7D.4020006@zonov.org> References: <EE6495BD-38D0-4EBE-9A94-7C40DC69F820@gmail.com> <4F131A7D.4020006@zonov.org>
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On Jan 15, 2012, at 8:27 PM, Andrey Zonov wrote: > Hi, >=20 > Could you please show the output of `vmstat -z | grep hostcache'? >=20 > On 12.01.2012 21:55, Nikolay Denev wrote: >> Hello, >>=20 >> A web server that I administer running Nginx and FreeBSD-7.3-STABLE = was recently >> under a ICMP attack that generated a large amount of outgoing TCP = traffic. >> With some tcpdump and netflow analysis it was evident that the = attachers are using >> ICMP host-unreach need-frag messages to make the web server >> retransmit multiple times, giving a amplification factor of about = 1.6. >> Then I noticed RFC5927 ( http://www.faqs.org/rfcs/rfc5927.html ) and = specifically section 7.2 >> which discusses countermeasures against such attacks. The text reads = : >>=20 >> This section describes a modification to the PMTUD mechanism >> specified in [RFC1191] and [RFC1981] that has been incorporated in >> OpenBSD and NetBSD (since 2005) to improve TCP's resistance to the >> blind performance-degrading attack described in Section 7.1. The >> described counter-measure basically disregards ICMP messages when = a >> connection makes progress, without violating any of the = requirements >> stated in [RFC1191] and [RFC1981]. >>=20 >> The RFC is recent (dated from July 2010), and it mentions several = times Linux, Free,Open and NetBSD, >> but exactly in this paragraph it is mentioning only Net and = OpenBSD's, thus I'm asking if >> anyone has idea if these modifications were being put into FreeBSD? >>=20 >> I quickly glanced upon the source, but the TCP code is a bit too much = for me :) >>=20 >> Also if anybody has observed similar attack, how are you protecting = yourself from it? >> Simply blocking host-unreach need-frag would break PMTUD. >>=20 >> P.S.: I know 7.3 is pretty old, and I've planned upgrade to 8.2. I'm = also curious if 8.2 will behave differently. >>=20 >> Regards, >> Nikolay >>=20 >> _______________________________________________ >> freebsd-net@freebsd.org mailing list >> http://lists.freebsd.org/mailman/listinfo/freebsd-net >> To unsubscribe, send any mail to = "freebsd-net-unsubscribe@freebsd.org" >=20 > --=20 > Andrey Zonov % uptime=20 7:57PM up 608 days, 4:06, 1 user, load averages: 0.30, 0.21, 0.17 % vmstat -z|grep hostcache hostcache: 136, 15372, 15136, 236, 44946965, = 10972760 Hmm=85 probably I should increase this=85.
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