PMTU problem with >2.6.38 and IPIP tunnels

PMTU problem with >2.6.38 and IPIP tunnels

[Dan Siemon]

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I'm running into some MTU related problems with kernels newer than
2.6.38. I've tried bisecting and it appears the problem was introduced
between 2.6.39rc2 and rc3 but I'm not 100% certain because the problem
does not always present itself immediately. I am certain this does not
occur with 2.6.38.8.

I tunnel all of my home network traffic over an IPIP tunnel. My Internet
access is DSL based so there's a PPPoE connection as well. Both ends of
the IPIP tunnel are Linux.

When the gateway is first booted with the problem kernel everything is
fine. Using 'show ip route cache' I see MTU entries with the correct
values. After a short period the MTU on all the routes starts dropping
in what often appears to be 20 byte increments.

show ip route cache | egrep -i mtu
cache <src-direct>  expires 435sec ipid 0x28de mtu 712 iif eth1
cache <src-direct>  expires 542sec ipid 0x66e8 mtu 672 iif eth1

Until eventually all routes show a MTU of 552.

I'd be happy to run tests if anyone has an idea of what is causing this.

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