Passt port forwards to guest when host is "offline"

[Niklas Beierl <no.shoes@posteo.net>]

Hello,

I hope I am not violating any mailinglist etiquette. I am not too 
familiar with this way of communicating and I haven't found a guide for 
passts list. Please feel free to point me out.

I have the following Problem: I have a libvirt VM using passt as 
usermode networking backend. I set up a port-forward to access a service 
on the VM. In principle, this setup serves me well. There is just one 
edge-case that is very unergonomic:
When the VM boots while the host (laptop) is offline, the VM get's no IP 
either and the the port-forward doesn't work since passt has no IP to 
forward to.

35.2917: ERROR:   Flow 0 (INI): No rules to forward HOST TCP 
[127.0.0.1]:42336 -> [127.0.0.1]:3389

This unfortunately happens quite frequently because I travel a lot. I 
know this scenario doesn't really have an obvious, well defined 
solution, because passt is supposed to just make the guest believe it 
has the same ip as the host and in this case the host has no ip. A few 
things I have considered:

Assigning an additional, static IP to the guest and somehow forcing 
traffic for it to passt. Not sure if/how this would work.
Assigning an additional, static IP on some host interface, hoping passt 
will pick it up and advertise it via DHCP?
Am I missing a more elegant approach?

I was also thinking that it would perhaps be cool if passt could make 
port-forwards for localhost connections work irrespective of whether the 
host otherwise has network connectivity. Haven't thought it 100% 
through, but if 127.0.0.1/8 is src and dst of the connection, there is 
no broken return route on the guest or anything so in theory this should 
be possible I guess? I'd be happy to bounce ideas back and forth.

For reference: The interface definition:

     <interface type='user'>
       <mac address='52:54:00:16:63:9f'/>
       <portForward proto='tcp'>
         <range start='3389'/>
       </portForward>
       <model type='e1000e'/>
       <backend type='passt' logFile='/tmp/win11passt.log'/>
       <address type='pci' domain='0x0000' bus='0x01' slot='0x00' 
function='0x0'/>
     </interface>

Best

Niklas