Date: Sun, 15 Jan 2012 22:27:09 +0400 From: Andrey Zonov <andrey@zonov.org> To: Nikolay Denev <ndenev@gmail.com> Cc: freebsd-net@freebsd.org Subject: Re: ICMP attacks against TCP and PMTUD Message-ID: <4F131A7D.4020006@zonov.org> In-Reply-To: <EE6495BD-38D0-4EBE-9A94-7C40DC69F820@gmail.com> References: <EE6495BD-38D0-4EBE-9A94-7C40DC69F820@gmail.com>
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Hi, Could you please show the output of `vmstat -z | grep hostcache'? On 12.01.2012 21:55, Nikolay Denev wrote: > Hello, > > 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 : > > 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]. > > 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? > > I quickly glanced upon the source, but the TCP code is a bit too much for me :) > > Also if anybody has observed similar attack, how are you protecting yourself from it? > Simply blocking host-unreach need-frag would break PMTUD. > > 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. > > Regards, > Nikolay > > _______________________________________________ > 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" -- Andrey Zonov
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