I am encountering an issue with virt-customize when the host kernel, on which virt-customize runs, has CONFIG_IPV6_SIT built-in. Below is the error output from the command:

virt-customize -a noble-server-cloudimg-amd64.qcow2 --install isc-dhcp-client
[   0.0] Examining the guest ...
[   2.9] Setting a random seed
virt-customize: warning: random seed could not be set for this type of guest
[   2.9] Installing packages: isc-dhcp-client
Reading package lists...
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/noble/InRelease  Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/noble-updates/InRelease  Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/noble-backports/InRelease  Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/noble-security/InRelease  Temporary failure resolving 'security.ubuntu.com'
W: Some index files failed to download. They have been ignored, or old ones used instead.
Reading package lists...
Building dependency tree...
Reading state information...
Package grub2 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or is only available from another source
However the following packages replace it:
  grub-efi-amd64 grub-pc-bin grub-pc grub-ieee1275-bin grub-ieee1275
  grub-efi-ia32-bin grub-efi-ia32 grub-efi-amd64-bin

E: Unable to locate package isc-dhcp-client
E: Unable to locate package isc-dhcp-client-ddns
E: Package 'grub2' has no installation candidate
...
If reporting bugs, run virt-customize with debugging enabled and include the complete output:
  virt-customize -v -x [...]

Using the -v verbose option, I discovered that the IP address assigned in the QEMU command line was not reflected inside the guest. Here is the relevant ip a output from the log:

+ ip a 1: lo: mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 brd 127.255.255.255 scope host lo valid_lft forever preferred_lft forever inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: eth0: mtu 1500 qdisc pfifo_fast state UP group default qlen 1000 link/ether 52:54:00:12:34:56 brd ff:ff:ff:ff:ff:ff inet6 fe80::5054:ff:fe12:3456/64 scope link tentative valid_lft forever preferred_lft forever 3: sit0@NONE: mtu 1480 qdisc noqueue state UNKNOWN group default qlen 1000 link/sit 0.0.0.0 brd 0.0.0.0 inet6 ::127.0.0.1/96 scope host valid_lft forever preferred_lft forever

Further debugging revealed that when the host kernel has SIT-related configurations enabled (i.e., CONFIG_IPV6_SIT), the supermin appliance, which libguestfs uses, inherits the same kernel and initrd. Consequently, the guest environment booted by virt-customize includes the sit0 device.

When the sit0 device is present, the primary NIC (eth0) does not receive the static IP address assigned by libguestfs in the QEMU command line:

/usr/bin/qemu-system-x86_64 \ -global virtio-blk-pci.scsi=off \ ... -netdev user,id=usernet,net=169.254.2.15/16 \ -device virtio-net-pci,netdev=usernet \ ..

I verified this issue on another machine where the host kernel had no SIT-related configurations enabled. On that machine, the same virt-customize command with the same Noble image worked correctly. I have attached verbose logs for both working and non-working configurations. A diff of these logs shows that the sit0 device causes the IP assignment issue with the eth0 interface.