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Thomas P
Asked: 2024-11-27 23:12:20 +0800 CST

libvirt: Add static route into isolated network

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I'm working with Ubuntu 24.04 Desktop and I created a VM connected to the default libvirt-NAT-Bridge and a second isolated network.

The bridged network is virbr0: 192.168.122.0/24 the host got 192.168.122.1 and the VM got static 192.168.122.128. The isolated network is 10.95.212.0/24.

I want to be able to ping/ssh into hosts in the isolated network that's why I set a static route (route add ...) on the host. It works, but it did not survive a reboot. I tried nmtui but the route vanishes on reboot.

I thought about virsh net-edit default, but I didn't found any documentation how to do it.

Where do I have to define the route that it survives the reboot?

kvm-virtualization

1 Answers

  1. Best Answer

    One option is to set up the static route on your host. It sounds like you tried to do that, but since you didn't provide us with details it's hard to tell why it didn't work.

    With my Ubuntu 24.04 system, I started with an interface configuration created like this:

    root@ubuntu:~# nmcli conn add type ethernet ifname enp1s0 \
  con-name wired ipv4.method auto autoconnect yes save yes

    This gets me an address on the default libvirt network (which on my system is 192.168.124.0/24):

    root@ubuntu:~# ip addr show enp1s0
2: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 52:54:00:eb:75:96 brd ff:ff:ff:ff:ff:ff
    inet 192.168.124.148/24 brd 192.168.124.255 scope global dynamic noprefixroute enp1s0
       valid_lft 3405sec preferred_lft 3405sec
    inet6 fe80::9714:efbc:6239:b9e/64 scope link noprefixroute
       valid_lft forever preferred_lft forever

    To add a static route to this configuration, I can modify it with the nmcli modify command:

    root@ubuntu:~# nmcli c mod wired +ipv4.routes "10.95.212.0/24 192.168.124.10"

    After a reboot, I have:

    root@ubuntu:~# ip route
default via 192.168.124.1 dev enp1s0 proto dhcp src 192.168.124.148 metric 100
10.95.212.0/24 via 192.168.124.10 dev enp1s0 proto static metric 100
192.168.124.0/24 dev enp1s0 proto kernel scope link src 192.168.124.148 metric 100

    According to the libvirt documentation, it should be possible to define static routes in the network definition so that they are provided to your hosts via DHCP. I experimented with this a bit, but it doesn't appear to work as documented.

    Fortunately, it is also possible to specify arbitrary dnsmasq options, which means we can add the necessary dnsmasq configuration like this:

    <network xmlns:dnsmasq='http://libvirt.org/schemas/network/dnsmasq/1.0' connections='1'>
  <name>default</name>
  <forward mode='nat'>
    <nat>
      <port start='1024' end='65535'/>
    </nat>
  </forward>
  <bridge name='virbr0' stp='on' delay='0'/>
  <mac address='52:54:00:9d:aa:d8'/>
  <ip address='192.168.124.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.124.10' end='192.168.124.200'/>
    </dhcp>
  </ip>

  <!-- HERE IS WHERE WE ADD THE ROUTE -->
  <dnsmasq:options>
    <dnsmasq:option value='dhcp-option=option:classless-static-route,10.95.212.0/24,192.168.124.10'/>
  </dnsmasq:options>
</network>

    With this in place, hosts that use DHCP for interface configuration will receive the route via a DHCP option. I've tested the above configuration and it works for hosts using NetworkManager and for hosts using systemd-networkd.

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