netdev - Re: [PATCH net] ipv4/route: do not check saddr dev if iif is LOOPBACK

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Date:   Sun, 11 Aug 2019 20:49:18 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     dsahern@...il.com
Cc:     liuhangbin@...il.com, netdev@...r.kernel.org, sbrivio@...hat.com,
        mleitner@...hat.com
Subject: Re: [PATCH net] ipv4/route: do not check saddr dev if iif is
 LOOPBACK_IFINDEX

From: David Ahern <dsahern@...il.com>
Date: Thu, 1 Aug 2019 22:16:00 -0600

> On 8/1/19 10:13 PM, Hangbin Liu wrote:
>> On Thu, Aug 01, 2019 at 01:51:25PM -0600, David Ahern wrote:
>>> On 8/1/19 2:29 AM, Hangbin Liu wrote:
>>>> Jianlin reported a bug that for IPv4, ip route get from src_addr would fail
>>>> if src_addr is not an address on local system.
>>>>
>>>> \# ip route get 1.1.1.1 from 2.2.2.2
>>>> RTNETLINK answers: Invalid argument
>>>
>>> so this is a forwarding lookup in which case iif should be set. Based on
>> 
>> with out setting iif in userspace, the kernel set iif to lo by default.
> 
> right, it presumes locally generated traffic.
>> 
>>> the above 'route get' inet_rtm_getroute is doing a lookup as if it is
>>> locally generated traffic.
>> 
>> yeah... but what about the IPv6 part. That cause a different behavior in
>> userspace.
> 
> just one of many, many annoying differences between v4 and v6. We could
> try to catalog it.

I think we just have to accept this difference because this change would
change behavior for all route lookups, not just those done by ip route get.