Automatic hostname entries for libvirt domains

Fri 04 October 2013 by Lars Kellogg-Stedman Tags libvirt virtualization

Have you ever wished that you could use libvirt domain names as hostnames? So that you could do something like this:

$ virt-install -n anewhost ...
$ ssh clouduser@anewhost

Since this is something that would certainly make my life convenient, I put together a small script called virt-hosts that makes this possible. You can find virt-hosts in my virt-utils GitHub repository:

  • https://raw.github.com/larsks/virt-utils/master/virt-hosts

Run by itself, with no options, virt-hosts will scan through your running domains for interfaces on the libvirt default network, look up those MAC addresses up in the corresponding default.leases file, and then generate a hosts file on stdout like this:

$ virt-hosts
192.168.122.221 compute-tmp0-net0.default.virt compute-tmp0.default.virt
192.168.122.101 centos-0-net0.default.virt centos-0.default.virt
192.168.122.214 controller-tmp-net0.default.virt controller-tmp.default.virt

Each address will be assigned the name <domain_name>-<interface_name>.<network_name>.virt. The first interface on the network will also be given the alias <domain_name>.<network_name>.virt, so a host with multiple interfaces on the same network would look like this:

$ virt-hosts
192.168.122.221 host0-net0.default.virt host0.default.virt
192.168.122.110 host0-net1.default.virt

Of course, this is only half the solution: having generated a hosts file we need to put it somewhere where your system can find it.

An aside: incron

Both of the following solutions rely on incron, a tool that uses the Linux inotify subsystem to trigger scripts in reaction to events on file and directories. In this case, we'll be using incron to monitor the dnsmasq default.leases file and firing off a script when it changes.

You could accomplish the same thing using the inotifywait program from the inotify-tools package and a small wrapper script, or you could hook up something to the libvirt events framework.

Using /etc/hosts

If you want to update your /etc/hosts file, you can place the following into a script called update-virt-hosts (somewhere in root's PATH) and run that via incron:

#!/bin/sh

sed -i '/^# BEGIN VIRT HOSTS/,/^# END VIRT HOSTS/ d' /etc/hosts
cat <<EOF >>/etc/hosts
# BEGIN VIRT HOSTS
$(virt-hosts)
# END VIRT HOSTS
EOF

Make sure you have incron installed, and add the following to /etc/incron.d/virt-hosts:

/var/lib/libvirt/dnsmasq/default.leases IN_MODIFY update-virt-hosts

This will cause incron to run your update-virt-hosts script whenever it sees an IN_MODIFY event on the default.leases file.

Using NetworkManager + dnsmasq

I am running NetworkManager with the dnsmasq dns plugin. I created the file /etc/NetworkManager/dnsmasq.d/virthosts containing:

addn-hosts=/var/lib/libvirt/dnsmasq/default.addnhosts

This will cause the dnsmasq process started by NetworkManager to use that file as an additional hosts file. I then installed the incron package and dropped the following in /etc/incron.d/virt-hosts:

/var/lib/libvirt/dnsmasq/default.leases IN_MODIFY /usr/local/bin/virt-hosts -ur

This has incron listen for changes to the default.leases file, and whenever it receives the IN_MODIFY event it runs virt-hosts with the -u (aka --update) and -r (aka --reload-dnsmasq) flags. Thef former causes virt-hosts to send output to /var/lib/libvirt/dnsmasq/default.addnhosts instead of stdout, and the latter does a killall -HUP dnsmasq after installing the new hosts file.


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