Set the MTU in the interface inside the sandbox

[cmainas]

In some cases we have experienced an issue where the MTU inside a Linux VM was different than the MTU of the tap device and the virtual ethernet device in the host. In particular, the CNI was setting a specific MTU value for the virtual ethernet network device inside the network namespace. Then in urunc we correctly create a new tap device using the MTU value obtained from the virtual ethernet network device. However, the Linux VM was not getting the MTU information from anywhere and was using the default value (e.g. 1500).

This mismatch can cause various problems in network connectivity. Therefore, we need to make sure that the network devices inside the sandbox always have the same MTU as the host network devices. For guests that use virtio-net (e.g. Linux) this can be done by setting the MTU value in the monitor. Unfortunately, not all monitors support this. In more details:

• In Qemu, we can set the mtu or host_mtu (depending on the version) option in -netdev cli option (See https://www.qemu.org/docs/master/system/qemu-manpage.html#hxtool-5)
• In Cloud-hypervisor we can also set it through cli options (SEE https://github.com/cloud-hypervisor/cloud-hypervisor/blob/0686045290204c59661f46f987e2cd60c5f40ee7/release-notes.md?plain=1#L184)
• In Firecracker this does not seem possible
• In Solo5 newer versions should auto-detect from the tap device (See https://github.com/Solo5/solo5/blob/a333cbb20152ecfad9bfd4957aa7f8b237408600/CHANGES.md?plain=1#L27)

[cmainas]

For Qemu and cloud hypervisor #564 sets the MTU as the one of the network interface that the CNI created inside the network namespace and for firecracker there is firecracker-microvm/firecracker#5828. For Solo5 support for auto-detection is already there.