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

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Date:   Thu, 1 Aug 2019 22:16:00 -0600
From:   David Ahern <dsahern@...il.com>
To:     Hangbin Liu <liuhangbin@...il.com>
Cc:     netdev@...r.kernel.org, Stefano Brivio <sbrivio@...hat.com>,
        Marcelo Ricardo Leitner <mleitner@...hat.com>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH net] ipv4/route: do not check saddr dev if iif is
 LOOPBACK_IFINDEX

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.