Unable to do nsswitch resolution on Avahi due to permissions issue

[Ken D'Ambrosio]

I set up Avahi on two 16.04 systems, but was unable to resolve between them via ping; note, however, that avahi-resolve worked fine:

root@clients-1:~# avahi-resolve -n -4 kentest.local
kentest.local 192.168.243.16 # This is a good thing

These, not so much good:
root@clients-1:~# ping kentest.local
ping: unknown host kentest.local

My nsswitch.conf file was thus:
hosts: files mdns4_minimal [NOTFOUND=return] dns

I did a strace on the ping, and saw this:
sun_path="/var/run/avahi-daemon/socket"}, 110) = -1 EACCES (Permission denied)

The socket's permissions were fine, so I looked at the directory the socket was in and saw:
root@clients-1:~# ls -ald /var/run/avahi-daemon/
drwx------ 2 avahi avahi 80 Mar 27 05:35 /var/run/avahi-daemon/

It seemed to me this should have been okay -- I mean, I was trying the ping as root, and presumably user avahi would be servicing the request if it wasn't user root. But "Permission denied" is "permission denied," so I changed directory permissions and tried again:

root@clients-1:~# chmod 777 /var/run/avahi-daemon/
root@clients-1:~# ping kentest.local
PING kentest.local (192.168.243.16) 56(84) bytes of data.
64 bytes from 192.168.243.16: icmp_seq=1 ttl=64 time=1.31 ms
64 bytes from 192.168.243.16: icmp_seq=2 ttl=64 time=0.742 ms

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: avahi-daemon 0.6.32~rc+dfsg-1ubuntu2
ProcVersionSignature: Ubuntu 4.4.0-66.87-generic 4.4.44
Uname: Linux 4.4.0-66-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
Date: Mon Mar 27 14:55:01 2017
Ec2AMI: ami-00000004
Ec2AMIManifest: FIXME
Ec2AvailabilityZone: nova
Ec2InstanceType: GB.small
Ec2Kernel: unavailable
Ec2Ramdisk: unavailable
ProcEnviron:
 TERM=xterm
 SHELL=/bin/bash
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 XDG_RUNTIME_DIR=<set>
SourcePackage: avahi
UpgradeStatus: No upgrade log present (probably fresh install)

[Ken D'Ambrosio (ken-jots)]

Note that this bug report should be filed against package avahi-daemon, and not avahi, since avahi, itself, isn't even a package, but the Launcpad form wouldn't let me submit it as such.