ICMP attacks against TCP and PMTUD

[Nikolay Denev]

On Jan 15, 2012, at 8:27 PM, Andrey Zonov wrote:

> 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 at freebsd.org mailing list
>> http://lists.freebsd.org/mailman/listinfo/freebsd-net
>> To unsubscribe, send any mail to "freebsd-net-unsubscribe at freebsd.org"
> 
> -- 
> Andrey Zonov


% uptime 
 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… probably I should increase this….