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

[曹煜]

Hi Nico,
That issue was closed by myself, but the patch didn't get applied
cause the issue was came from wireguard itself, and the maintener told
me that I should send my patch to wireguard upstream (but I just gave
up for sending it to wireguard team).

Nico Schottelius <nico.schottelius at ungleich.ch> 于2023年2月20日周一 18:41写道：
>
>
> Hello 曹煜,
>
> on github it seems your patch was applied / the issue was closed - is
> that the correct current status?
>
> Best regards,
>
> Nico
>
> 曹煜 <cao88yu at gmail.com> 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:
> > https://github.com/openwrt/packages/issues/9538#issuecomment-1150592803
> >
> > Daniel Gröber <dxld at darkboxed.org> 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->dst.dev->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
> >>
> >>
> >>
>
>
> --
> Sustainable and modern Infrastructures by ungleich.ch