On Sun, May 10, 2026 at 01:40:56PM +0000, Niklas Beierl wrote:

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.

Ok, this is a known problem - or, rather a known cluster of overlapping problems. There are some a fair few complications in working out exactly how best to address it though, which is why it's still there.

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>

So, this isn't really related to port forwarding at all. As long as the forward doesn't have a specific listening address (which your example doesn't), it should work fine whatever the guest address - even if the guest changes address. The problem with forwards is just due to not getting an address at all. The primary difficulty here is that if the host has no address, we have to conjure one out of thin air for the guest. But, if the host later goes online, we don't want whatever we picked for the guest to conflict with that. Finding a safe choice is quite tricky. Jon Maloy is currently working on a series of patches that reworks the management of guest addresses. The primary aim is to support multiple concurrent guest addresses. However, it will also provide a better foundation on which to handle the assignment of addresses when we can't take one from the host. In the meantime there's are some workarounds you could try: * What version of passt do you have? Newer versions have "local mode". It isn't perfect, but it has at least some ability to assign a guest address when the host doesn't have one. * If it's not important to you that the guest take the host address when the host *is* online, then passt has the ability to explicitly assign a guest address. You can use that via libvirt by adding lines like: <ip address='172.17.2.0' family='ipv4' prefix='24'/> <ip address='2001:db8:ac10:fd01::feed' family='ipv6'/> to the interface block. -- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson

On 11/05/2026 13:24, David Gibson wrote:

On Sun, May 10, 2026 at 01:40:56PM +0000, Niklas Beierl wrote:

...

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. Ok, this is a known problem - or, rather a known cluster of overlapping problems. There are some a fair few complications in working out exactly how best to address it though, which is why it's still there.

...

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> So, this isn't really related to port forwarding at all. As long as the forward doesn't have a specific listening address (which your example doesn't), it should work fine whatever the guest address - even if the guest changes address.

The problem with forwards is just due to not getting an address at all. The primary difficulty here is that if the host has no address, we have to conjure one out of thin air for the guest. But, if the host later goes online, we don't want whatever we picked for the guest to conflict with that. Finding a safe choice is quite tricky.

I'd venture to say that RFC3927 link-local addresses (169.254.0.0/16)  would be the sanest choice. Passt already seems to use that as a fallback for --address but that never triggers on my machine, probably because when "offline" I still do have ip-addresses from docker bridges but no default gateway?

Jon Maloy is currently working on a series of patches that reworks the management of guest addresses. The primary aim is to support multiple concurrent guest addresses. However, it will also provide a better foundation on which to handle the assignment of addresses when we can't take one from the host. Multiple ips would be pretty cool as you could use a link-local address as "fallback". :) In the meantime there's are some workarounds you could try:

* What version of passt do you have? Newer versions have "local mode". It isn't perfect, but it has at least some ability to assign a guest address when the host doesn't have one.

* If it's not important to you that the guest take the host address when the host *is* online, then passt has the ability to explicitly assign a guest address. You can use that via libvirt by adding lines like: <ip address='172.17.2.0' family='ipv4' prefix='24'/> <ip address='2001:db8:ac10:fd01::feed' family='ipv6'/> to the interface block.

Version: passt 2026_05_07.1afd4ed. What exactly is "local mode" ? I couldn't identify something relevant in the man page? I tried static IPs but it seemed to me that when I use that, I also need to statically configure the gateway, which makes the whole thing a hassle when I do have "internet" and want to use it from the guest. For the record, here is my workaround for the moment: I am adding a dummy (as in type dummy) interface with a private ipv4 range and a default route with the lowest possible precedence (high prio). This has the obvious caveat that I might encounter a conflict If I ever want to connect to a network with the chosen CIDR. ip link add dummy0 type dummy ip link set dummy0 up ip addr add 192.168.250.5/24 dev dummy0 ip route add default via 192.168.250.1 dev dummy0 metric 4294967295 This ensures that there always is at least a faked "local network" on my machine that provides an address for the guest To persist it with NetworkManager: nmcli connection add \   type dummy \   ifname dummy0 \   con-name dummy0 \   ipv4.method manual \   ipv4.addresses 192.168.250.5/24 \   ipv4.gateway 192.168.250.1 \   ipv4.route-metric 4294967295 \   ipv6.method disabled \   connection.autoconnect yes