Source IP incorrect on multi homed systems

[Sebastian Hyrvall]

You should get into that debate. Proposing firewall workarounds is not a 
correct solution so please don't do it. It needs to be fixed. It's an 
immature VPN solution that always just proposed a workaround instead of 
fixing the problem. It seems to be designed by people that are good at 
software and cryptography but has no clue about networking stacks.

On 2023-02-19 23:32, David Kerr wrote:
> 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