Source IP incorrect on multi homed systems

Mike O'Connor mike at
Sun Feb 19 00:45:55 UTC 2023

Generally all OSs will if sending from a local process will use the 
address of the outgoing interface for the packet.

If the packet is forwarded and no NAT is used the address will be routed 
via the interface suggested by the routing table.

So local routing can be a real pain, policy based routing is an option. 
The other option could be to setup an 'output' NAT to an address which 
is multi-homed.

I have a system running which is multi-homed with out issue other than 
the actual routing machine. This machine is BGP connected to three 

There is no NAT setup and because I also add the wireguard link 
addresses to the BGP sessions.


On 19/2/2023 6:44 am, Nico Schottelius wrote:
> Dear group,
> I was wondering how wireguard [Linux kernel] or wireguard-go [FreeBSD]
> are supposed to decide which IP address to use for replying?
> I have seen both on FreeBSD and Linux that wireguard seems to use the IP
> address of the outgoing interface, i.e. the one with the route returning
> to the sender. However in multi homed situations, this can be wrong,
> let's take this example:
>        19:57:24.607526 net1  In  IP > UDP, length 148
>        19:57:24.608358 net2  Out IP > UDP, length 92
> The initiator sends from to the receiver
> Wireguard then replies with the source IP of instead of
> As the node is multi homed, the packet might leave through any of its
> uplinks and thus return with a random (unexpected) IP address and will
> not pass NAT rules on firewalls and finally be dropped. F.i. in above
> example the firewall drops the packet from, because there
> is no session entry for that.
> I have observed this behaviour both on Linux 6.1.11 as well as
> wireguard-go 0.0.20220316_8,1 on FreeBSD and in both cases the
> connection will break depending on which active interface is taken as
> exit.
> I would argue that wireguard should by default invert the IP
> addresses, i.e. switch dst=src, src=dst and then reply with that,
> instead of adapting an interface specific address, or is there a good
> reason for the current behaviour?
> Best regards,
> Nico
> --
> Sustainable and modern Infrastructures by

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