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

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Message-ID: <cb0674df-8593-f14b-f680-ce278042c88c@gmail.com>
Date:   Wed, 10 Jul 2019 06:27:31 -0600
From:   David Ahern <dsahern@...il.com>
To:     Jan Szewczyk <jan.szewczyk@...csson.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"

[ 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://github.com/torvalds/linux/commit/d4ead6b34b67fd711639324b6465a050bcb197d4,
> 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
>