[Libguestfs] Re: appliance not getting ipv4 assigned when host kernel has CONFIG_IPV6_SIT builtin

On Fri, Jun 20, 2025 at 05:12:27PM +0530, Aithal, Srikanth wrote: > Hello all, > > 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 [ ... > 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: You would not expect the IP inside the appliance to be the same as outside. We're using two tools called libslirp & passt to create a user-level network.

> + 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 \ In this case it would use libslirp. I'm pretty sure slirp doesn't support IPv6 at all which is the problem here. But also ...

> 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. > > I have the following queries and would appreciate any responses: > > 1. Why does the user network IP not get applied when the sit0 device is > present in the appliance?

> 2. Is there a way to append parameters to the libguestfs QEMU command line to > blacklist the sit module in such cases? > 3. Can we override the kernel and initrd used by libguestfs via the > virt-customize command line? > libguestfs: command: run: passt --help > sh: 1: passt: not found If you use passt (which supports IPv6) it might work better, or at least differently ...