netdev - Re: IP routing sending local packet to gateway.

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Message-ID: <36d69eb1-fdb0-772e-4d3c-33ebead92b0a@gmail.com>
Date:   Wed, 1 Sep 2021 20:38:20 -0700
From:   David Ahern <dsahern@...il.com>
To:     David Laight <David.Laight@...LAB.COM>,
        "'netdev@...r.kernel.org'" <netdev@...r.kernel.org>
Subject: Re: IP routing sending local packet to gateway.

On 9/1/21 9:24 AM, David Laight wrote:
> I've found a script that gets run after the IP address and default route
> have been added that does:
> 
> 	SOURCE=192.168.1.88
> 	GATEWAY=192.168.1.1
> 
> 	ip rule add from "$SOURCE" lookup px0
> 	ip rule add to "$SOURCE" lookup px0
> 
> 	ip route add default via ${GATEWAY} dev px0 src ${SOURCE} table px0
> 
> The 'ip rule' are probably not related (or needed).
> I suspect they cause traffic to the local IP be transmitted on px0.
> (They may be from a strange setup we had where that might have been needed,
> but why something from 10 years ago appeared is beyond me - and our source control.)
> 
> Am I right in thinking that the 'table px0' bit is what causes 'Id 200'
> be created and that it would really need the normal 'use arp' route
> added as well?
> 

this is why the fib tracepoint exists. It shows what is happening at the
time of the fib lookup - inputs and lookup results (gw, device) - which
give the clue as to why the packet went the direction it did.