Altering MTU in ICMPv6 packet too big message

Altering MTU in ICMPv6 packet too big message

[Linus Lüssing]

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Hi there,

we're having a buggy, closed source IPv6 router here, which keeps
announcing a wrong MTU via ICMPv6 packet too big messages. Is
there a way to modify the 1500 to a 1480 with a rule in the mangle
table on the host that receives those broken messages? I think
despite this one value, also the checksum would have to be
recalculated.

Cheers, Linus

PS: the tcpmss-target extension is not enough, as I'm still having
trouble with UDP packets.

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Re: Altering MTU in ICMPv6 packet too big message

[Amos Jeffries]

On Wed, 24 Feb 2010 00:42:28 +0100, Linus Lüssing <linus.luessing@web.de>
wrote:
> Hi there,
> 
> we're having a buggy, closed source IPv6 router here, which keeps
> announcing a wrong MTU via ICMPv6 packet too big messages. Is
> there a way to modify the 1500 to a 1480 with a rule in the mangle
> table on the host that receives those broken messages? I think
> despite this one value, also the checksum would have to be
> recalculated.
> 
> Cheers, Linus
> 
> PS: the tcpmss-target extension is not enough, as I'm still having
> trouble with UDP packets.

If someone more experienced does not have a cleaner solution... I've found
setting fixed MTU on the NIC of each box plugged into such hardware to work
nicely for all protocols. Generating the ICMP message at a box that works.

AYJ
Amos