Re: Passt port forwards to guest when host is "offline"

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, May 11, 2026 at 12:52:45PM +0000, Niklas Beierl wrote:
> 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.

I came to the same conclusion...

>  Passt already seems to use that as a fallback for
> --address but that never triggers on my machine,

.. which is why this is the case...

> probably because when
> "offline" I still do have ip-addresses from docker bridges but no default
> gateway?

.. ah.  Right.  That could explain why "local mode" isn't triggering
for you.

Nevertheless link-local addresses still introduce their own
complications.  The basic question is whether the guest should be able
to observe a "link" (in the network sense) between itself and the
host. If it can, we can assign link-local addresses, and it simplifies
a number of other things.  However, historically the approach of
passt/pasta has been to give the illusion to the guest that it's
sitting in the place of the host, which implies that there not be
guest-visible link to the host.  We have real use cases that need that
to a pretty high level of fidelity.  The most obvious real-world
consequence is whether the guest can talk to link-local neighbours of
the host - or if the host has multiple links *which* link neighbours
the guest can talk to.

It's currently an active topic of investigation and debate whether we
need explicit different modes (passt-link visible vs. not visible), or
whether we can auto-select / auto-switch in some suitable way.

This might seem a bit pedantic in the case of RFC3927 addresses, which
are so rarely used.  However, the analagous case for IPv6 is pretty
routine, and we don't want to introduce inconsistencies if we don't
have to.

> > 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". :)

Right, that's one reason why this series is a good basis or improving
our behaviour here.

> > 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?

Ah, sorry, it's not an explicitly enabled feature.  Instead it is
fallback to a RFC3927 address which you observed.  Older versions
would simply fail to start if they couldn't pick an address.

> 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.

Statically configuring the gateway shouldn't preclude that.  We set
the guest's gateway to match the host to maintain that illusion of the
guest sitting in place of the host.  There are some cases where it
matters, but for most purposes it's cosmetic.  You can set the gateway
address to anything (e.g. another RFC3927 address) and the guest
should get connectivity, even though that doesn't match the host
configuration.  passt itself is the only thing that sees where the
guest directs the packets at the routing level, and it will make the
given address act as a working gateway, whether or not it exists in
the outside world.

> 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

That seems like a perfectly cromulent workaround, though statically
setting guest address and gateway to RFC3927 addresses might be a bit
more elegant.

-- 
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

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