netdev - RE: Question about linux kernel commit: "net/ipv6: move metrics from dst to rt6

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Message-ID: <AM6PR07MB5639E2AEF438DD017246DF13F2F00@AM6PR07MB5639.eurprd07.prod.outlook.com>
Date:   Wed, 10 Jul 2019 12:59:41 +0000
From:   Jan Szewczyk <jan.szewczyk@...csson.com>
To:     David Ahern <dsahern@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: Question about linux kernel commit: "net/ipv6: move metrics from
 dst to rt6_info"

Hi!
I digged up a little further and maybe it's not a problem with MTU itself. I checked every entry I get from RTM_GETROUTE netlink message and after triggering "too big packet" by pinging ipv6address I get exactly the same messages on 4.12 and 4.18, except that the one with that pinged ipv6address is missing on 4.18 at all. What is weird - it's visible when running "ip route get to ipv6address". Do you know why there is a mismatch there?

It's not easy for me to check this behavior on 4.19, because we have a pretty complex system here, but maybe I could try to reproduce it locally on some virtual box and check if it behaves the same.

Thanks for the tip about the testing tools, I'll try to use them.

BR,
Jan Szewczyk

-----Original Message-----
From: David Ahern <dsahern@...il.com> 
Sent: Wednesday, July 10, 2019 14:28
To: Jan Szewczyk <jan.szewczyk@...csson.com>; davem@...emloft.net
Cc: netdev@...r.kernel.org
Subject: Re: Question about linux kernel commit: "net/ipv6: move metrics from dst to rt6_info"

[ adding netdev so others can chime in ]

On 7/10/19 2:28 AM, Jan Szewczyk wrote:
> Hi guys!
> 
> We can see different behavior of one of our commands that supposed to 
> show pmtu information.
> 
> It's using netlink message RTM_GETROUTE to get the information and in 
> Linux kernel version 4.12 after sending big packet (and triggering 
> "packet too big") there is an entry with PMTU and expiration time.
> 
> In the version 4.18 unfortunately the entry looks different and there 
> is no PMTU information.

Can you try with 4.19.58 (latest stable release for 4.19)? Perhaps there was a bugfix that is missing from 4.18.

The kernel has 2 commands under tools/testing/selftests/net -- pmtu.sh and icmp_redirect.sh -- that verify exceptions are created and use 'ip ro get' to verify the mtu.

> 
> I can see that in your commit
> https://protect2.fireeye.com/url?k=5be21a17-07361dbb-5be25a8c-8667c4af
> e13e-f99413291ecbed59&q=1&u=https%3A%2F%2Fgithub.com%2Ftorvalds%2Flinu
> x%2Fcommit%2Fd4ead6b34b67fd711639324b6465a050bcb197d4,
> these lines disappeared from route.c:
> 
>  
> 
>      if (rt->rt6i_pmtu)
> 
>            metrics[RTAX_MTU - 1] = rt->rt6i_pmtu;
> 
>  
> 
> I'm very beginner in linux kernel code, can you help me and tell me if 
> that could cause this different behavior?
> 
>  
> 
>  
> 
> BR,
> 
> Jan Szewczyk
>