/etc/avahi/services/apt-cacher-ng.service is in apt-cacher-ng 2-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | <?xml version="1.0" standalone='no'?>
<!--
From: Lisandro Damián Nicanor Pérez Meyer <lisandro@debian.org>
X-Source: http://blog.surgut.co.uk/2013/03/avahi-apt-cacher-ng-sbuild.html
 See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=704790 for details.
-->
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
 <name replace-wildcards="yes">apt-cacher-ng proxy on %h</name>
 <service protocol="ipv6">
  <type>_apt_proxy._tcp</type>
  <port>3142</port>
 </service>
 <service protocol="ipv4">
  <type>_apt_proxy._tcp</type>
  <port>3142</port>
 </service>
</service-group>
 |