dnsmasq PXE filter

An inspection PXE DHCP stack is often implemented by the dnsmasq service. The dnsmasq PXE filter implementation relies on directly configuring the dnsmasq DHCP service to provide a caching PXE traffic filter of node MAC addresses.

How it works

The filter works by populating the dnsmasq DHCP hosts directory with a configuration file per MAC address. Each file is either enabling or disabling, thru the ignore directive, the DHCP service for a particular MAC address:

$ cat /etc/dnsmasq.d/de-ad-be-ef-de-ad

The filename is used to keep track of all MAC addresses in the cache, avoiding file parsing. The content of the file determines the MAC address access policy.

Thanks to the inotify facility, dnsmasq is notified once a new file is created or an existing file is modified in the DHCP hosts directory. Thus, to allow a MAC address, the filter removes the ignore directive:

$ cat /etc/dnsmasq.d/de-ad-be-ef-de-ad

The hosts directory content establishes a cached MAC addresses filter that is kept synchronized with the ironic port list.


The dnsmasq inotify facility implementation doesn’t react to a file being removed or truncated.


The inotify facility was introduced to dnsmasq in the version 2.73. This filter driver has been checked by ironic-inspector CI with dnsmasq versions >=2.76.

To enable the dnsmasq PXE filter, update the PXE filter driver name in the ironic-inspector configuration file:

driver = dnsmasq

The DHCP hosts directory can be specified to override the default /var/lib/ironic-inspector/dhcp-hostsdir:

dhcp_hostsdir = /etc/ironic-inspector/dhcp-hostsdir

The filter design relies on the hosts directory being in exclusive ironic-inspector control. The hosts directory should be considered a private cache directory of ionic-inspector that dnsmasq polls configuration updates from, through the inotify facility. The directory has to be writable by ironic-inspector and readable by dnsmasq.

It is also possible to override the default (empty) dnsmasq start and stop commands to, for instance, directly control the dnsmasq service:

dnsmasq_start_command = dnsmasq --conf-file /etc/ironic-inspector/dnsmasq.conf
dnsmasq_stop_command = kill $(cat /var/run/dnsmasq.pid)


The commands support shell expansion. The default empty start command means the dnsmasq service won’t be started upon the filter initialization. Conversely, the default empty stop command means the service won’t be stopped upon an (error) exit.


These commands are executed through the rootwrap facility, so overriding may require a filter file to be created in the rootwrap.d directory. A sample configuration to use with the systemctl facility might be:

sudo cat > /etc/ironic-inspector/rootwrap.d/ironic-inspector-dnsmasq-systemctl.filters <<EOF
# ironic_inspector/pxe_filter/dnsmasq.py
systemctl: CommandFilter, systemctl, root, restart, dnsmasq
systemctl: CommandFilter, systemctl, root, stop, dnsmasq


The initial synchronization will put some load on the dnsmasq service starting based on the amount of ports ironic keeps. The start-up can take up to a minute of full CPU load for huge amounts of MACs (tens of thousands). Subsequent filter synchronizations will only cause the dnsmasq to parse the modified files. Typically those are the bare metal nodes being added or phased out from the compute service, meaning dozens of file updates per sync call.

The ironic-inspector takes over the control of the DHCP hosts directory to implement its filter cache. Files are generated dynamically so should not be edited by hand. To minimize the interference between the deployment and introspection, ironic-inspector has to start the dnsmasq service only after the initial synchronization. Conversely, the dnsmasq service is stopped upon (unexpected) ironic-inspector exit.

To avoid accumulating stale DHCP host files over time, the driver cleans up the DHCP hosts directory before the initial synchronization during the start-up.

Although the filter driver tries its best to always stop the dnsmasq service, it is recommended that the operator configures the dnsmasq service in such a way that it terminates upon ironic-inspector (unexpected) exit to prevent a stale deny list from being used by the dnsmasq service.