Src addr code review (Was: Source IP incorrect on multi homed systems)

Nico Schottelius nico.schottelius at
Mon Feb 20 10:40:28 UTC 2023

Hello 曹煜,

on github it seems your patch was applied / the issue was closed - is
that the correct current status?

Best regards,


曹煜 <cao88yu at> writes:

> Hi all,
> I've hacked that source code myself months ago, and it works well on
> my use case (I have 4 dual stack pppoe wan set on my openwrt router,
> and seted a wireguard sever on it), my hack will pickup the dst_addr
> from incoming handshake packet in kernel sk_buff, and then use that
> addr as src_addr to reply.
> I'm not good at source code, and I know that my hack may be ugly, but
> it works, hope this patch can help:
> Daniel Gröber <dxld at> 于2023年2月20日周一 06:42写道:
>> Hi,
>> I though it might be useful to do some quick and dirty code review instead
>> of speculating wildly to figure out where these source IP selection
>> problems could be coming from ;)
>> From previous code deep dives I know the udp_tunnel_xmit_skb function is
>> where tunnel packets get handed off to the kernel. So in
>> net/wireguard/socket.c:send4 we have:
>>         udp_tunnel_xmit_skb(rt, sock, skb, fl.saddr, fl.daddr, ds,
>>                             ip4_dst_hoplimit(&rt->dst), 0, fl.fl4_sport,
>>                             fl.fl4_dport, false, false);
>> Where fl.saddr is the source address that's supposedly wrong (sometimes? I
>> guess?) Where does that come from?
>> Let's look at the code (heavily culled):
>>         struct flowi4 fl = {
>>                 .saddr = endpoint->src4.s_addr,
>>         };
>>         if (cache)
>>                 rt = dst_cache_get_ip4(cache, &fl.saddr);
>>         if (!rt) {
>>                 if (unlikely(!inet_confirm_addr(sock_net(sock), NULL, 0,
>>                                                 fl.saddr, RT_SCOPE_HOST)))
>>                         fl.saddr = 0;
>>                 if (unlikely(endpoint->src_if4 && ((IS_ERR(rt) &&
>>                              PTR_ERR(rt) == -EINVAL) || (!IS_ERR(rt) &&
>>                              rt->>ifindex != endpoint->src_if4))))
>>                         fl.saddr = 0;
>> Well it's initialized from endpoint->src4.s_addr, overwritten with zero in
>> some cases, which I believe lets the kernel do it's regular source addr
>> selection, and populated from something called dst_cache at some callsites.
>> @Nico could it perhaps simply be that you're hitting one of these zero'ing
>> cases and that's why it's using regular kernel src addr selection instead
>> of the cached endpoint src4 address?
>> The first case !inet_confirm_addr(..., RT_SCOPE_HOST) ought to confirm that
>> the saddr is actually still a local address. Makes sens if the address we
>> remembered was removed from the interface we can't use it anymore.
>> The second case looks like it's checking if the (sometimes cached) src_if4
>> interface index is still what the route we're about to use points to.
>> If neither of those seem likely we can keep reading :)
>> --Daniel

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