Source IP incorrect on multi homed systems

[David Kerr]

Without getting into the debate of whether wireguard is acting
correctly or not, I think there is a possible workaround.

1. In the iptables mangle table PREROUTING, match the incoming
interface and destination address and --set-xmark a firewall MARK
unique to this interface/destination
2. Create a new ip route table that sets the default route to go out
on the interface with the source address you want (same as destination
address in iptables)
3. Create a new ip rule that sends all packets with firewall mark set
in iptables to the routing table you just created

Repeat above for each interface/address you need to mangle, with a
unique firewall mark and routing table for each.

It may be necessary to use CONNMARK in PREROUTING and OUTPUT to
--restore_mark.  I can't remember if this is needed or not, its been a
while since I configured iptables with this.

This should ensure that any packet that comes into an
interface/address is replied to from the same interface/address.

David


On Sun, Feb 19, 2023 at 9:44 AM Christoph Loesch <wireguard-mail at chil.at> wrote:
>
> Hi,
>
> I don't think no one wants to fix it, there are several users having this issue. I rather guess no one could find a suitable solution to fix it.
>
> @Nico: did you try to delete the affected route and add it again with the correct source IP ?
>
> as I mentioned it in https://lists.zx2c4.com/pipermail/wireguard/2021-November/007324.html
>
> ip route del <NET>
> ip route add <NET> dev <ALIAS_DEV> src <SRC_IP>
>
> This way I was able to (at least temporary) fix this issue on multi homed systems.
>
> Kind regards,
> Christoph
>
> Am 19.02.2023 um 13:13 schrieb Nico Schottelius:
> > Hey Sebastian,
> >
> > Sebastian Hyrwall <sh at keff.org> writes:
> >
> >> It is kinda. It's been mentioned multiple times over the years but no one seems to want to fix it. Atleast you should be able to specify bind/src ip in the
> >> config. I gave up WG because of it. Wasn't accepted by my projects security policy since src ip could not be configured.
> >>
> >> There is an unofficial patch however,
> >>
> >> https://github.com/torvalds/linux/commit/5fa98082093344c86345f9f63305cae9d5f9f281
> > the binding is somewhat related to this issue and I was looking for that
> > feature some time ago, too. While it is correlated and I would really
> > appreciate binding support, I am not sure whether the linked patch does
> > actually fix the problem I am seeing in multi homed devices.
> >
> > As long as wireguard does not reply with the same IP address it was
> > contacted with, packets will get dropped on stateful firewalls, because
> > the returning packet does not match the state session database.
> >
> > Best regards,
> >
> > Nico
> >
> > --
> > Sustainable and modern Infrastructures by ungleich.ch