netdev - Re: IPv4 saddr do not match with selected output device in double default gateways scene

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Date:   Wed, 9 Mar 2022 19:57:03 -0700
From:   David Ahern <dsahern@...nel.org>
To:     "Ziyang Xuan (William)" <william.xuanziyang@...wei.com>,
        David Miller <davem@...emloft.net>, yoshfuji@...ux-ipv6.org,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>, ja@....bg
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: IPv4 saddr do not match with selected output device in double
 default gateways scene

On 3/7/22 11:41 PM, Ziyang Xuan (William) wrote:
> Create VLAN devices and add default gateways with following commands:
> 
> # ip link add link eth2 dev eth2.71 type vlan id 71
> # ip link add link eth2 dev eth2.72 type vlan id 72
> # ip addr add 192.168.71.41/24 dev eth2.71
> # ip addr add 192.168.72.41/24 dev eth2.72
> # ip link set eth2.71 up
> # ip link set eth2.72 up
> # route add -net default gw 192.168.71.1 dev eth2.71
> # route add -net default gw 192.168.72.1 dev eth2.72
> 

...

> We can find that IPv4 saddr "192.168.72.41" do not match with selected VLAN device "eth2.71".
> 
> I tracked the related processes, and found that user space program uses connect() firstly, then sends UDP packet.
> 

...

> Deep tracking, it because fa->fa_default has changed in fib_select_default() after first __ip_route_output_key() process,
> and a new fib_nh is selected in fib_select_default() within the second __ip_route_output_key() process but not update flowi4.
> So the phenomenon described at the beginning happens.
> 
> Does it a kernel bug or a user problem? If it is a kernel bug, is there any good solution?

That is a known problem with multipath routes.