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Re: ICMP PTB spoofing attacks

Mark Andrews <marka@isc.org> Sat, 06 February 2016 00:35 UTC

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To: Fernando Gont <fgont@si6networks.com>
From: Mark Andrews <marka@isc.org>
References: <2134F8430051B64F815C691A62D983183395EF42@XCH-BLV-105.nw.nos.boeing.com> <56B4E39A.5040608@si6networks.com> <2134F8430051B64F815C691A62D983183395F017@XCH-BLV-105.nw.nos.boeing.com> <56B4EA46.3080102@si6networks.com> <2134F8430051B64F815C691A62D983183395F0CE@XCH-BLV-105.nw.nos.boeing.com> <56B50285.9010204@si6networks.com> <2134F8430051B64F815C691A62D983183395F2DD@XCH-BLV-105.nw.nos.boeing.com> <56B50889.8080201@si6networks.com>
Subject: Re: ICMP PTB spoofing attacks
In-reply-to: Your message of "Fri, 05 Feb 2016 17:39:37 -0300." <56B50889.8080201@si6networks.com>
Date: Sat, 06 Feb 2016 11:35:40 +1100
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Cc: 6man WG <ipv6@ietf.org>, Bob Hinden <bob.hinden@gmail.com>, "Fred Baker (fred)" <fred@cisco.com>
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In message <56B50889.8080201@si6networks.com>, Fernando Gont writes:
> On 02/05/2016 05:25 PM, Templin, Fred L wrote:
> >>>
> >>> By "accurate" I mean:
> >>>
> >>> - the router that sends the ICMP PTB is telling the truth that there is a restricting link
> >>> - the MTU value that the router writes in the PTB is in fact the true MTU of the
> >>>    restricting link
> >>
> >> Well, most of these protocols run on "good faith", right? -- If I cannot
> >> trust the router to tell the truth, why should I trust it to e.g.
> >> forward my packets?
> > 
> > A router's trustworthiness for forwarding packets and for telling the truth
> > in any ICMP PTB message it sends are mutually exclusive - the first question
> > is one of availability and the second is one of integrity.
> 
> I think that, at the end of the day, its about trust. Can I trust the
> device, or not?
> 
> I my goal as an attacker is to DoS your communications, I don't care
> whether I achieve that by simply throwing your packets away, or by
> sending you some packet that will cause yourself to shutdown and not talk.
> 
> I should say, however, that in the IPv6 case, the worst case scenario is
> a 1280-byte MTU link...

Which is what we force the nameservers we ship to use (for both TCP
and UDP) as PMTUD takes too long for DNS even when it works.

> > Plus, it is not necessarily just the on-path routers - any off-path "router"
> > could also inject spurious PTBs.
> 
> For off-path attackers, you validate ICMP PTBs. If you don't, well...
> prepare for trouble.  And, yes, if EHs are in use, you may not be able
> to verify them.

Only if all the original headers + PTB header > 1280 which should
be a vanishing small pecentage of times.

> -- 
> Fernando Gont
> SI6 Networks
> e-mail: fgont@si6networks.com
> PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492
> 
> 
> 
> 
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-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka@isc.org

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