HOWTO
=====
dnsmasq comes with the possibility to run multiple systemd service instances on the same machine.
There is the main service which is enabled by default via `systemctl enable dnsmasq.service` and uses the configuration from `/etc/default/dnsmasq`.

Additional service instances can be enabled via `systemctl enable dnsmasq@<instance name>.service` that use the configuration from `/etc/default/dnsmasq.<instance name>`.
It is recommended to use a separate configuration file and directory for each instance.
Additionally make sure that all instances use either different ports and/or ip addresses to avoid binding collisions.

Example setup for an instance called "alt"
#1 File `/etc/dnsmasq.alt.conf` copied from `/etc/dnsmasq.conf`
#2 Directory `/etc/dnsmasq.alt.d`
#3 File `/etc/default/dnsmasq.alt` copied from `/etc/default/dnsmasq` with following adaptions:
   * The options DNSMASQ_OPTS and CONFIG_DIR point to the correct configuration file and directory.
     DNSMASQ_OPTS="... --conf-file=/etc/dnsmasq.alt.conf ..."
     CONFIG_DIR=/etc/dnsmasq.alt.d,.dpkg-dist,.dpkg-old,.dpkg-new
   * The option DNSMASQ_EXCEPT must contain "lo" to avoid that an instance becomes the machine's DNS resolver.
     DNSMASQ_EXCEPT="lo"
  * If the additional instance should bind to all IP addresses of a specific interface, e.g. "dnsalt01", then the following addition could be used:
    DNSMASQ_OPTS="... --bind-dynamic --interface=dnsalt01 ..."
    Additionally the main instance must be stopped from binding to interfaces that are used by other instances:
    DNSMASQ_OPTS="... --bind-dynamic --except-interface=dnsalt* ..."
  * If the additional instance should not use the machine's DNS resolver, normally that's the dnsmasq main instance, as upstream server, then the following addition could be used:
    IGNORE_RESOLVCONF=yes
#4 Enable additional instance via `systemctl enable dnsmasq@alt.service`
#5 Start additional instance without reboot via `systemctl start dnsmasq@alt.service`


TODO
====
#1 - Found shortcoming on 2019-03-10
Only the option DNSMASQ_EXCEPT="lo" avoids that an DNS instance will be set as the machine's DNS resolver.
This may interfere with the wish to run an additional instance on a different port on the localhost addresses.
My suggestion in the initial Debian report [1] was to specify an explicit variable for this.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=914305#5


#2 - Preferred configuration way
Should the variables DNSMASQ_INTERFACE and DNSMASQ_EXCEPT be used instead of --interface and --except-interface?  (while "lo" still has to be in DNSMASQ_EXCEPT as of now)