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which was filed against the linux-image-2.6-686 package:
#587789: linux-image-2.6-686: netfilters clamp-mss-to-pmtu sets bad MSS when
non was set before
It has been closed by Ben Hutchings b...@decadent.org.uk.
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Betreff:
Re: Bug#587789: linux-image-2.6-686: netfilters clamp-mss-to-pmtu sets
bad MSS when non was set before
Von:
Ben Hutchings b...@decadent.org.uk
Datum:
Fri, 02 Jul 2010 23:28:52 +0100
An:
587789-d...@bugs.debian.org
An:
587789-d...@bugs.debian.org
On Thu, 2010-07-01 at 18:43 +0200, Daniel Gibson wrote:
Package: linux-image-2.6-686
Version: 2.6.26+17+lenny1
Severity: important
Hi,
Netfilters clamp-mss-to-pmtu (as used in iptables -A FORWARD -p tcp --tcp-flags
SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu) sets MSS in packets that had no MSS set
before.
The documentation says that TCPMSS sets the MSS option, unconditionally,
so this behaviour is correct.Never increase MSS, even when setting it, as
doing so
The code explicitly says Never increase MSS, even when setting it, as
doing so results in problems for hosts that rely on MSS being set
correctly. So the MSS option is *not* set unconditionally by
--clamp-mss-pmtu. The documentation doesn't explicitly say that, though.
To set the MSS to a fixed value --set-mss should be used, I guess.
(Explicitly set MSS option to specified value)
No MSS set (at a TCP packet) implies the default MSS of 536 as specified
by RFC 879. So TCPMSS should in that case either set a MSS of 536 (e.g.
before the oldmss newmss check, so if PMTU returned a even lower MSS
it is set to that lower value) or at least leave the MSS untouched.
The documentation says This target is used to overcome criminally
braindead ISPs or servers which block ICMP Fragmentation Needed or
ICMPv6 Packet Too Big packets.
So, if a host (implicitly, in conformance to the RFC) expects packets of
default size (MSS 536) and the braindead ISP (or server-/
firewall-admin), blocks the (according to the RFC optional) ICMP
Fragmentation Needed packets, the PMTU will not return the correct MTU
(of 536) but the MTU of your DSL connection or whatever.
TCPMSS is meant to fix exactly this case (blocked ICPM packets), but the
author apparently only thought of the case that the server sets a MSS
that's to big for the client, so the client must fix that by setting a
lower MSS - but not the case that a server expects a smaller MSS (that
happens to be the standard MSS) without explicitly saying so.
Regards,
- Daniel
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