netdev - ipsec: icmp fragmentation-needed from ipsec-gateway is not encrypted

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Message-Id: <200709181257.58598.wolfgang.walter@studentenwerk.mhn.de>
Date:	Tue, 18 Sep 2007 12:57:58 +0200
From:	Wolfgang Walter <wolfgang.walter@...dentenwerk.mhn.de>
To:	netdev@...r.kernel.org
Cc:	herbert@...dor.apana.org.au, davem@...emloft.net
Subject: ipsec: icmp fragmentation-needed from ipsec-gateway is not encrypted

Hello,

I have the following problem:

router A has two interfaces eth0 and eth1.

router B has two interfaces eth0 and eth1.

The networks on A:eth1 and B:eth1 are connected over an ipsec-tunnel.

the mtu on A:eth1 is 1400 (all others are 1500).

both run 2.6.22.6

If I now ping a host HA on A:eth1 from host HB on B:eth1 with packet size 
greater 1400 the ping fails.

tcpdump on A:eth0 shows

an esp-tunnel-packet from B comes in
icmp echo-request packet from HB to HA comes in
        (the decrypted esp-packet)
an unecrypted icmp fragmentation-needed packet to HB from A (ip of eth1) sent 
out

It seems to me that this fragementation-needed packet generated by B is not 
handled by ipsec, is sent out unencrypted instead and this is the reason it 
does not reach HB.

I should not see the unecrypted packet going out at all? Because if I ping 
A:eth1 from HB then I don't see the unencrypted echo-reply packet (which has 
the same source-address as the fragmentation needed) but only the outgoing 
esp-packet (and the echo-reply reaches HB, by the way).

Regards,
-- 
Wolfgang Walter
Studentenwerk München
Anstalt des öffentlichen Rechts
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