18.12.8. References to Other Filters

Any filter may hold references to other filters. Individual filters may be referenced multiple times in a filter tree but references between filters must not introduce loops.

Example 18.7. An Example of a clean traffic filter

The following shows the XML of the clean-traffic network filter referencing several other filters. 
<filter name='clean-traffic'>
  <uuid>6ef53069-ba34-94a0-d33d-17751b9b8cb1</uuid>
  <filterref filter='no-mac-spoofing'/>
  <filterref filter='no-ip-spoofing'/>
  <filterref filter='allow-incoming-ipv4'/>
  <filterref filter='no-arp-spoofing'/>
  <filterref filter='no-other-l2-traffic'/>
  <filterref filter='qemu-announce-self'/>
</filter>
To reference another filter, the XML node filterref needs to be provided inside a filter node. This node must have the attribute filter whose value contains the name of the filter to be referenced.
New network filters can be defined at any time and may contain references to network filters that are not known to libvirt, yet. However, once a virtual machine is started or a network interface referencing a filter is to be hotplugged, all network filters in the filter tree must be available. Otherwise the virtual machine will not start or the network interface cannot be attached.