wg-quick's Ethernet unplug problem

[Tim Schumacher]

Hi,

there is this annoying problem with the way wg-quick sets up the routing, that when you unplug the Ethernet cable the routing rule (see ip rule) that checks the fwmark gets lost. So let's say you move to another location with your laptop and plug in an Ethernet cable again, it essentially disables your VPN. You're lucky if you have the popular kill switch in place blocking all your traffic instead of silently disabling your VPN.

PostUp = iptables -I OUTPUT ! -o %i -m mark ! --mark $(wg show %i fwmark) -m addrtype ! --dst-type LOCAL -j REJECT
PreDown = iptables -D OUTPUT ! -o %i -m mark ! --mark $(wg show %i fwmark) -m addrtype ! --dst-type LOCAL -j REJECT

I have no idea why the rule gets lost. It might be the kernel removing it for whatever reason. But it makes no sense to me because I don't see how the kernel would make a connection between that rule and the Ethernet link that goes down. Any ideas on the issue? Is this a known problem? Are there workarounds?

Greetings,
Tim