Configuring avahi-daemon on Raspberry Pi

I’ve been having intermittent issues connecting to my Raspberry Pi using the local domain name interwebs.local. After the most recent issue, I decided to actually figure out the root cause.

The problem #

I went to check the grafana dashboard on my pi today at http://interwebs.local:3000 and got the dreaded This site cannot be reached message. That’s not good, especially since this website is running on the pi. A quick check showed that the site was still up and running (whew!) and I was able to reach the pi by ip address.

Since the pi was still up and responding, I figured the most likely culprit was mDNS. I did a quick scan of the system logs in /var/log/syslog and was able to find the problem:

avahi-daemon[324]: Host name conflict, retrying with interwebs-2

So it looks like the avahi-daemon thinks the interwebs name already exists so it just adds a -2 to the name when registering the address. Checking this in the browser with http://interwebs-2.local:3000 does indeed bring up the grafana dashboard.

The solution #

It turns out this is a known bug in avahi: Spurious name conflicts · Issue #117 · lathiat/avahi · GitHub While it’s not fixed yet, it does look like there are two possible workarounds that we can try: limit the interfaces avahi interacts with and avoid IPv6 if you can.

Let’s start with specifying the interfaces. We can take a look at what interfaces are available with this command:

ip link | awk 'NR % 2 == 1 {print substr($2, 1, length($2)-1)}'

And the results look something like this:

lo
eth0
wlan0
docker0
br-97ab6ddb7319
veth236375f@if6
vetha16ca98@if8
veth1400258@if10
veth77d9a14@if12
veth7087693@if14
vethdb6a27c@if16
tun0

That’s a lot of interfaces, with the majority of them belonging to docker. In fact, any time a docker container starts or stops it creates/tears down an interface which causes avahi to register or withdraw a record. That means more chances for the bug to happen. The only interface we really care about is eth0. We can tell avahi to only listen on that interface in the config file /etc/avahi/avahi-daemon.conf. Add this line:

allow-interfaces=eth0

On my local network, I don’t really need IPv6 either so let’s disable that as well. By default, IPv6 is enabled with this line in /etc/avahi/avahi-daemon.conf:

use-ipv6=yes

We have to change that setting to no, and we also need to update the publish settings:

use-ipv6=no
publish-a-on-ipv6=no
publish-aaaa-on-ipv4=no

Restart the service with

sudo systemctl restart avahi-daemon

Restarting the daemon has fixed the issue, and interwebs.local is accessible again. Checking the logs, it appears to only be listening on eth0, and only on IPv4. When starting and stopping a docker container, I see the network interfaces getting created and destroyed in the logs, but now avahi does not try to advertise on those interfaces.

So far I have not seen any issues with this setup, but time will tell if this does actually fix the issue.

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