linux-kernel - Re: Inconsistency in packet drop due to MTU (eth vs veth)

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Date:   Thu, 19 Jan 2017 10:53:47 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Fredrik Markstrom <fredrik.markstrom@...il.com>
Cc:     netdev <netdev@...r.kernel.org>, Evan Jones <ej@...njones.ca>,
        "David S. Miller" <davem@...emloft.net>,
        Cong Wang <cwang@...pensource.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Vijay Pandurangan <vijayp@...ayp.ca>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Inconsistency in packet drop due to MTU (eth vs veth)

On Thu, 2017-01-19 at 17:41 +0100, Fredrik Markstrom wrote:
> Hello,
> 
> I've noticed an inconsistency between how physical ethernet and veth handles mtu.
> 
> If I setup two physical interfaces (directly connected) with different mtu:s, only the size of the outgoing packets are limited by the mtu. But with veth a packet is dropped if the mtu of the receiving interface is smaller then the packet size. 
> 
> This seems inconsistent to me, but maybe there is a reason for it ? 
> 
> Can someone confirm if it's a deliberate inconsistency or just a side effect of using dev_forward_skb() ?

It looks this was added in commit
38d408152a86598a50680a82fe3353b506630409
("veth: Allow setting the L3 MTU")

But what was really needed here was a way to change MRU :(