How to set MTU during installation?


I am trying to install UCS on a KVM VM, but got stuck with this problem: the network interface must have MTU set to a fixed value 1400 (it’s on a Hetzner vSwitch). I searched this forum and found instructions how to set MTU on a running system. But the installation cannot finish without proper networking, so I need to set the MTU during the installation. Can it be done?


get into the network config.
for each “virtual” ethernetport there is a config file.
set a line in the config file:
having the wrong “MTU” on a vm (becasue of the additional tcp/ip wrapping) normally just slows the network down due to almost every packet, being split.
only a real problem for traffic & if you routers don’t allow re-assembly…

I solved this as follows:

  • import UCS KVM disk: UCS-KVM-Image.qcow2
  • boot the VM with a live CD (I used gparted live cd)
  • mount the ucs root fs under /mnt/rootfs
  • edit the following files:
 vi /mnt/rootfs/etc/network/interfaces # add:
      auto eth0
      iface eth0 inet static
        mtu 1400

    vi /mnt/rootfs/univention/templates/files/etc/network/interfaces.d/00-base
    # same block as above

    vi /mnt/rootfs/univention/templates/files/etc/network/interfaces.d/10-default
    # search for mtu, fix it as follows:
    # delete 2 orig. lines, then add
    print('\tmtu 1400')
  • reboot VM

Then the setup process finished successfully.