DHCP host entries¶

It is often useful to always assign a specific IPv4 or IPv6 address to a particular VM. Open the network XML configuration in a text editor. For example, to edit the default network:

# virsh net-edit default

For IPv4, use the VM’s MAC address to add a DHCPv4 <host> element. Add as many <host> elements as required.

<network>
  <name>default</name>
  <bridge name="virbr0"/>
  <forward mode="nat"/>
  <ip address="192.168.122.1" netmask="255.255.255.0">
    <dhcp>
      <range start="192.168.122.2" end="192.168.122.254"/>
      <host mac="52:54:00:6f:78:f3" ip="192.168.122.222"/>
      <host mac="52:54:00:91:ed:23" ip="192.168.122.233"/>
    </dhcp>
  </ip>
</network>

For IPv6, MAC addresses are deprecated so use the VM’s DHCP Unique Identifier (DUID) instead. To find the DUID for a running VM, use virsh.

# virsh net-dhcp-leases routed

MAC address        Client ID or DUID
------------------------------------
52:54:00:8a:11:4c  00:01:00:01:1a:ff:2b:ff:52:54:00:8a:11:4c
52:54:00:9b:ae:41  00:01:00:01:1a:ff:2b:ff:52:54:00:9b:ae:41

Use the VM’s DUID to add a DHCPv6 <host> element. Add as many <host> elements as required.

<network>
  <name>default</name>
  <bridge name="virbr0"/>
  <forward mode="nat"/>
  <ip address="192.168.122.1" netmask="255.255.255.0">
    <dhcp>
      <range start="192.168.122.2" end="192.168.122.254"/>
    </dhcp>
  </ip>
  <ip family="ipv6" address="2001:db8::1" prefix="64">
    <dhcp>
      <range start="2001:db8::1000" end="2001:db8::1fff"/>
      <host id="00:01:00:01:1a:ff:2b:ff:52:54:00:8a:11:4c" ip="2001:db8::1222"/>
      <host id="00:01:00:01:1a:ff:2b:ff:52:54:00:9b:ae:41" ip="2001:db8::1333"/>
    </dhcp>
  </ip>
</network>

Restart the virtual network.

# virsh net-destroy default
# virsh net-start default

Without restarting the virtual network

It is a bit tiresome to restart the virtual network after every change. A much better approach is to use virsh net-update, which makes live changes to an active virtual network. (--parent-index specifies which element to select if there is more than one <ip> element.)

# virsh net-update default add-last ip-dhcp-host \
      '<host mac="52:54:00:6f:78:f3" ip="192.168.122.222"/>' \
      --live --config --parent-index 0

# virsh net-update default add-last ip-dhcp-host \
      '<host id="00:01:00:01:1a:ff:2b:ff:52:54:00:8a:11:4c" ip="2001:db8::1222"/>' \
      --live --config --parent-index 1

To modify a DHCP host entry on an active network:

# virsh net-update default modify ip-dhcp-host \
      '<host mac="52:54:00:6f:78:f3" ip="192.168.122.222"/>' \
      --live --config --parent-index 0

To delete a DHCP host entry on an active network:

# virsh net-update default delete ip-dhcp-host \
      '<host mac="52:54:00:6f:78:f3" ip="192.168.122.222"/>' \
      --live --config --parent-index 0