NST Avahi (mDNS) FAQ

Avahi/mDNS General

What Services Need to Be Running?

The avahi-daemon.service needs to be running. You can check its status via:

 systemctl status avahi-daemon.service

You can start it via:

 sudo systemctl start avahi-daemon.service

You can enable it so it is started at boot time via:

 sudo systemctl enable avahi-daemon.service

How Do I Resolve Avahi (mDNS) addresses when using systemd-resolved?

The systemd-resolved service can be used to handle the "resolve" entry in /etc/nsswitch.conf (this is typically just before dns is invoked).

 hosts:      files myhostname mdns4_minimal [NOTFOUND=return] resolve [!UNAVAIL=return] dns

If you choose to enabled systemd-resolved, you can leave /etc/nsswitch.conf alone. However, if you'd like to reduce it, you can simplify the hosts entry to:

 hosts:      myhostname resolve

To enable and start the systemd-resolved service:

 sudo systemctl enable --now systemd-resolved

The default configuration for systemd-resolved has the MulticastDNS option disabled. So, you won't be able to resolve Avahi (mDNS) entries by default. To enable, uncomment or add the following line to /etc/systemd/resolved.conf:

 MulticastDNS=yes

Then restart the systemd-resolved service:

 sudo systemctl restart systemd-resolved

Finally, you can use the systemd-resolve command to configure how systemd-resolved should resolve mDNS addresses, in particular what interfaces it should resolve (resolvectl can probably be used as well):

 sudo systemd-resolve --set-mdns=yes --interface=enp0s3

You can then use the systemd-resolve command to check that addresses can be resolved to host names and that host names can be resolved to addresses. Verify DNS, file and mDNS entries:

 systemctl-resolve localhost
 systemctl-resolve nas.local
 systemctl-resolve 192.168.1.201
 systemctl-resolve www.google.com

You can also use PHP or Python to make sure that the gethostbyname and gethostbyaddr functions are also able to resolve names and addresses. Here is a PHP example:

 echo "<?php echo(gethostbyname('www.google.com')) ?>" | php
 echo "<?php echo(gethostbyname('pollomocha.local')) ?>" | php
 echo "<?php echo(gethostbyaddr('192.168.1.201')) ?>" | php

How Do I Resolve Using Avahi (mDNS)?

You can use the avahi-resolve command to verify that a host name resolves to an IPv4 or IPv6 address.

[root@nst-test ~]# avahi-resolve -4 -n nst-test.local
nst-test.local	192.168.1.221
[root@nst-test ~]# avahi-resolve -6 -n nst-test.local
nst-test.local	2600:1700:1850:e820:56a0:f48:d01f:86bd
[root@nst-test ~]# 

How Do I Add Avahi Resolving to nsswitch.conf?

You need to have the following packages installed:

 sudo dnf install avahi-daemon nss-mdns

You need to have the avahi-daemon service running:

 sudo systemctl start avahi-daemon.service
 sudo systemctl enable avahi-daemon.service

In order for normal commands (like ping, ssh, etc) to resolve Avahi host names, you need to have the following configuration line in your /etc/nsswitch.conf file:

hosts:      files mdns4_minimal [NOTFOUND=return] dns myhostname

This line is typically added automatically when you add the nss-mdns package.

What Happens If My Router Resolves .local Domains?

I have run into AT&T WIFI access points that resolve .local domains under the normal DNS protocol. This breaks normal Avahi lookups and was driving me crazy for several weeks. I finally came across an answer to this issue at Red Hat Bugzilla – Bug 1690078. Basically on an NST system, you need to:

• Change the /etc/nsswitch.conf file to use the mdns4 library instead of the mdns4_minimal library:

# Work around to dang AT&T Access Point resolving .local domains
hosts:      files mdns4 [NOTFOUND=return] dns myhostname

• Create a /etc/mdns.allow file with the following entries:

# Work around to dang AT&T Access Point resolving .local domains
.local.
.local

How Do I Scan My Network For Avahi Service?

The easy was is to use the NST Web Interface. From the menu bar, select Security -> Active Scanners -> NST Avahi (mDNS) Discovery. This will generate a table of available services and hosts with active links that allow further inspection.

Alternatively, you can run the following from the command line:

sudo avahi-browse --all