libvirt-dnsmasqd running as part of libvirt

Open DNS server on external IP

Background

When you run libvirt / KVM on Ubuntu it seems the framework starts up a second local DNS server beside systemd-resolved. Since only one server can occupy localhost port 53 (systemd-resolved) libvirt-dnsmasq uses HOSTNAME / Port 53. So that will be whatever IP the DHCP server assigns to your PC. 

This IP is accessible from all other sources on the network as a matter of fact and running an open DNS server on it is probably not a good idea.

Solution

Check the virtual net status:

# virsh net-list

 Name                 State      Autostart     Persistent

----------------------------------------------------------

 default              active     yes           yes

Check to see if libvirt-dnsmasq is running

# lsof -i TCP:53

Remove the default virtual network for your virtual hosts.

# virsh net-destroy default

Prevent it from autostarting at all

# virsh net-autostart --network default --disable

If you need it again start it with

# virsh net-start default

Result

libvirt-dnsmasq is no longer listening on port 53 / DNS on whatever IP your interface(s) have been given on your network.

# lsof -i TCP:53

COMMAND   PID            USER   FD   TYPE DEVICE SIZE/OFF NODE NAME

systemd-r 635 systemd-resolve   13u  IPv4  25136      0t0  TCP localhost:domain (LISTEN)

# virsh net-list

 Name   State   Autostart   Persistent

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