Re: Port Forwarding between host-guest via passt using QEMU-libvirt

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 6 Nov 2025 12:08:07 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> That information might supply some clues, but it's pretty likely we'll
> need debugging or packet capture output from passt to work this out.
> Unfortunately, that's a bit trickier than it should be because libvirt
> doesn't (yet) have the ability to pass the necessary options to passt.

By the way, this feature request for libvirt is currently tracked at
(public RHEL ticket, but applies to libvirt in general):

  https://issues.redhat.com/browse/RHEL-52281

Meanwhile, one thing you can do is to add a wrapper at
/usr/local/bin/passt (don't forget to chmod 755 it) with these lines:

---
#!/bin/sh

/usr/bin/passt --debug --log-file /tmp/passt.log --pcap /tmp/passt.pcap $@
---

that's the way I currently debug stuff with libvirt, at least.

Note that passt can take guest-side packet captures (you would find it
at /tmp/passt.pcap, which you can open with Wireshark / tshark later),
which is usually convenient for cases like these.

-- 
Stefano