netdev - Re: ip6-in-ip{4,6} ipsec tunnel issues with 1280 MTU

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Date:   Thu, 26 Apr 2018 13:51:17 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Ashwanth Goli <ashwanth@...eaurora.org>
Cc:     netdev@...r.kernel.org, maloney@...gle.com, edumazet@...gle.com,
        David Ahern <dsahern@...il.com>
Subject: Re: ip6-in-ip{4,6} ipsec tunnel issues with 1280 MTU

Hi,

[fixed CC list]

On Wed, 2018-04-25 at 21:43 +0530, Ashwanth Goli wrote:
> Hi Pablo,

Actually I'm Paolo, but yours is a recurring mistake ;)

> I am noticing an issue similar to the one reported by Alexis Perez 
> [Regression for ip6-in-ip4 IPsec tunnel in 4.14.16]
> 
> In my IPsec setup outer MTU is set to 1280, ip6_setup_cork sees an MTU 
> less than IPV6_MIN_MTU because of the tunnel headers. -EINVAL is being 
> returned as a result of the MTU check that got added with below patch.
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/commit/net/ipv6/ip6_output.c?h=v4.14.34&id=8278804e05f6bcfe3fdfea4a404020752ead15a6
> 
> Can we remove this MTU check since your recent patch [ipv6: the entire 
> IPv6 header chain must fit the first fragment] fixes a similar issue?

AFAICS, RFC 2473 implies we can have MTU below 1280 for tunnel devices
so we can probably relax the MTU check for such devices, but I think we
would still need it in the general case.

Cheers,

Paolo