linux-kernel - Re: xfrm: is pmtu broken with ESP tunneling?

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Date:	Tue, 11 Feb 2014 03:32:58 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Ortwin Glück <odi@....ch>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: xfrm: is pmtu broken with ESP tunneling?

Hi!

On Mon, Feb 10, 2014 at 09:41:54AM +0100, Ortwin Glück wrote:
> I am using Openswan to configure an IPSec VPN (using the xfrm/netkey 
> backend). Large HTTP POST requests from the client seem to get stuck, 
> because the outgoing packets are 1530 bytes (before being wrapped into 
> ESP packets). The problem goes away by setting sysctl 
> net.ipv4.ip_no_pmtu_disc=1.

This setting will shrink the path mtu to min_pmtu when a frag needed icmp is
received. It sounds like we calculate the path mtu incorreclty in case of
fragmentation.

> May have something to do with it:
> The tunneled network is 10.6.6.6/32 and I am SNAT'ing some destinations 
> to that IP, so they get routed through the tunnel. Any other networks 
> are not to go through the tunnel.
> 
> iptables -t nat -A POSTROUTING -d "${R}" -j SNAT --to-source 10.6.6.6
> 
> It seems quite clear to me that xfrm is doing something wrong here.

Can you send a ip route get <ip> to the problematic target to see how
far off the calculated value is?

Thanks,

  Hannes

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