Source IP for multihomed peer

[Toke Høiland-Jørgensen]

> 2) Is there any way to force the source ip of the connection from boxA 
> to always use address boxA1 ?

In theory this should be possible to enforce via policy routing. Just
tried this on a simple veth setup:

# ip a add 10.11.1.1/24 dev veth0
# ip a add 10.11.2.1/24 dev veth0
# ping 10.11.1.2 -c 1
12:09:22.385888 IP 10.11.1.1 > 10.11.1.2: ICMP echo request, id 15, seq 1, length 64
12:09:22.385903 IP 10.11.1.2 > 10.11.1.1: ICMP echo reply, id 15, seq 1, length 64

# ip r add 10.11.1.2 src 10.11.2.1 dev veth0
# ping 10.11.1.2 -c 1
12:09:53.251386 IP 10.11.2.1 > 10.11.1.2: ICMP echo request, id 16, seq 1, length 64
12:09:53.251403 IP 10.11.1.2 > 10.11.2.1: ICMP echo reply, id 16, seq 1, length 64

I think this ought to work for wireguard's source selection as well. If
you don't have a particular destination, you should be able to do
something similar based on sports with ip-rule using the wireguard
source port:

# ip rule add sport 1234 lookup 100
# ip route add table 100 default via 1.2.3.4 src 3.4.5.6

That last bit I didn't test, though...

-Toke