libvirt: Change dhcp-setup without restarting

Question

I'm using libvirt and kvm to virtualize my server. I configured libvirt to start a dhcp-server on the bridge-network-interface to give the vm's their ips. Every VM's gets always the same ip based on its mac, this is configured like this:

<dhcp>
 <range start='10.1.1.2' end='10.1.1.254' />
 <host mac='54:52:00:21:01:ba' name='virstvm' ip='10.1.1.10' />
 <host mac='00:16:36:2d:71:f9' name='secvm' ip='10.1.1.20' />
</dhcp>

The problem: Whenever I add a new host to the dhcp section I have to restart libvirt-bin which restarts all my vm's. This cant be the solution because it means a downtime every time I add a new Server. Is there a solution?

Answer 1

The added net-update command in virsh should allow an dhcp-host update without restarting the virtual network (I have not tested it yet).

man virsh:

net-update network command section xml [--parent-index index] [[--live] [--config] | [--current]]

Update the given section of an existing network definition, with the changes optionally taking effect immediately, without needing to destroy and re-start the network.

command is one of "add-first", "add-last", "add" (a synonym for add-last), "delete", or "modify".

section is one of "bridge", "domain", "ip", "ip-dhcp-host", "ip-dhcp-range", "forward", "forward-interface", "forward-pf", "portgroup", "dns-host", "dns-txt", or "dns-srv", each section being named by a concatenation of the xml element hierarchy leading to the element being changed. For example, "ip-dhcp-host" will change a <host> element that is contained inside a <dhcp> element inside an <ip> element of the network.

xml is either the text of a complete xml element of the type being changed (e.g. "<host mac="00:11:22:33:44:55' ip='1.2.3.4'/>", or the name of a file that contains a complete xml element. Disambiguation is done by looking at the first character of the provided text - if the first character is "<", it is xml text, if the first character is not "<", it is the name of a file that contains the xml text to be used.

The --parent-index option is used to specify which of several parent elements the requested element is in (0-based). For example, a dhcp <host> element could be in any one of multiple <ip> elements in the network; if a parent-index isn't provided, the "most appropriate" <ip> element will be selected (usually the only one that already has a <dhcp> element), but if --parent-index is given, that particular instance of <ip> will get the modification.

If --live is specified, affect a running network. If --config is specified, affect the next startup of a persistent network. If --current is specified, affect the current network state. Both --live and --config flags may be given, but --current is exclusive. Not specifying any flag is the same as specifying --current.

Answer 2

The following procedure worked for me with libvirt Version: 0.7.5-5ubuntu27 on a Ubuntu 10.04.4 LTS host:

  virsh net-dumpxml default > default.xml

  $EDITOR default.xml

  virsh net-destroy default
  virsh net-define default.xml
  virsh net-start default

This also works for other networks beside the default network. Just replace the word default in the example with the name of the network.

Answer 3

@pefu's solution did it for CentOS 6.4 too!

Using net-edit in virsh did not update the XML file at /var/lib/libvirt/network/default.xml nor did it change the state of the running network config.

Re-running net-edit showed the edited version even after a reboot but it just was not loaded!

Exporting it to real XML file, editing and reloading it works!

Answer 4

I think libvirt uses dnsmaq for its dhcp server. Maybe you could just restart dnsmasq and all would be well?

Answer 5

After a diskussion on the Mailinglist I found out: this currently is not possible at all!