MDNS

Multicast DNS is a protocol that is normally used for the discovery of printers. Avahi is a popular implementation by but more setup is needed for the regular name resolution to see the results.

Setup avahi

Install, enable and start avahi with:

$ doas apk add avahi $ doas rc-update add avahi-daemon $ doas rc-service avahi-daemon start

It should now be possible to browse results. To look for a printer, use:

$ doas apk add avahi-tools $ avahi-browse --resolve --terminate _ipp._tcp

Make note of the hostname, as we will use it afterwards.

Setup avahi2dns

Name resolution is implemented by musl, and it only supports DNS, so we have to map the avahi results to a regular DNS server. This is done by avahi2dns. As of now, avahi2dns package is available in the testing repository. It can be safely installed by following the guidelines.

$ doas apk add avahi2dns@testing

since we will want a full DNS server running at port 53, we need to configure avahi2dns to use another port. This is done by default with /etc/conf.d/avahi2dns containing:

 command_args="-p 5354"

Enable and start avahi2dns with the command:

$ doas rc-update add avahi2dns $ doas rc-service avahi2dns start

It should now be possible to use DNS to query the address of the printer:

$ drill -p 5354 @127.0.0.1 <printer_name>.local

Where printer_name is the hostname given by avahi-browse.

Setup Networkmanager

If you are already using Networkmanager, you can leverage dnsmasq as your DNS server which can easily forward mDNS requests to another server.

Install the networkmanager-dnsmasq package:

$ doas apk add networkmanager-dnsmasq

Configure networkmanager to use dnsmasq as it's dns server by editing /etc/NetworkManager/NetworkManager.conf as follows

Contents of /etc/NetworkManager/NetworkManager.conf

[main] dhcp=internal dns=dnsmasq

Then we need to tell dnsmasq to forward all mDNS queries to avahidns by editing the file /etc/NetworkManager/dnsmasq.d/mdns.conf as follows

Contents of /etc/NetworkManager/dnsmasq.d/mdns.conf

# Forward queries for the "local" domain to 127.0.0.1 port 5354 server=/local/127.0.0.1#5354

Restart networkmanager:

$ doas rc-service networkmanager restart

Without NetworkManager

If you are not using Networkmanager, you will need to setup a DNS resolver that will forward request of .local domain to avahi2dns and handle other requests normally. There is more than one way to do it.

Setup unbound as DNS resolver

We will use unbound as the server and resolvconf tool to inform unbound about the DHCP results. This option is probably most convenient for a laptop: using the DHCP provided by the server for the regular DNS requests.

Install the programs:

$ doas apk add openresolv unbound

Create /etc/resolvconf.conf as follows:

Contents of /etc/resolvconf.conf

name_servers=127.0.0.1 unbound_conf=/etc/unbound/unbound.conf.d/resolvconf.conf

This tells resolveconf to use a local nameserver and pass the DHCP provided DNS server to unbound. Create /etc/unbound/unbound.conf.d/avahi-local.conf as follows

Contents of /etc/unbound/unbound.conf.d/avahi-local.conf

forward-zone: name: "local" forward-addr: 127.0.0.1@5354 server: do-not-query-localhost: no domain-insecure: "local"

This reads the information provided by resolvconf, but forwards .local requests to avahi2dns. We also need to disable dnssec for .local and tell unbound that it is OK to query localhost.

Enable and start unbound.

$ doas rc-update add unbound $ doas rc-service unbound start

Using existing bind server as DNS resolver

It is assumed that a bind(named) server is already installed and running, if not, then you can use the instructions Small-Time_DNS_with_BIND9

Once the bind(named) server is up and running, then the solution is to set an access "zone" for postfix addresses.local

To do this, add the following lines to the bind configuration file /etc/bind/named.conf as follows

Contents of /etc/bind/named.conf

zone "local" { type forward; # type requests forward only; # rule to use only forwards forwarders { 127.0.0.1 port 5354; # Avahi2dns address and port }; };

Additionally, you may need to disable dnssec verification for the .local postfix. To do this, add the following lines inside the options configuration

Contents of /etc/bind/named.conf

... options { ... validate-except { "local"; }; ... };

After adding the configuration, double-check that you have a .local zone. To do this, use the command:

$ named-checkconf -l # OUTPUT: local IN _default forward

If there were no errors, you can reload the bind(named) service:

$ doas rc-service named reload

Setup DHCP client

How send the DHCP provided DNS to resolvconf depends on the DHCP client being used.

udhcpc

This is the DHCP client in busybox, and will work for both wired and wireless interfaces. Create /etc/udhcpc/udhcpc.conf as follows

Create the file /etc/udhcpc/post-bound/resolvconf as follows

Contents of /etc/udhcpc/post-bound/resolvconf

#!/bin/sh cat /etc/udhcpc-resolv.conf

and make it executable

$ doas chmod 755 /etc/udhcpc/post-bound/resolvconf

iwd

When using iwd an inconvenience of this setup is that udhcpc will not reconfigure the interface when connecting to other wifi networks. For that to happen one has to run

$ iwctl station wlan0 connect <network_name> $ doas kill -USR2 $(cat /run/udhcpc.wlan0.pid) $ doas kill -USR1 $(cat /run/udhcpc.wlan0.pid)

To avoid having to manually reconfigure the wifi interface, we can configure iwd to use DHCP internally and forward DNS server info to resolveconf. To do that create the file /etc/iwd/main.conf as follows

Contents of /etc/iwd/main.conf

[General] EnableNetworkConfiguration=True [Network] NameResolvingService=resolvconf

Troubleshooting

Test the setup

You should be able to query for both the printer address and regular addresses with:

$ drill @127.0.0.1 <printer_name>.local $ drill @127.0.0.1 alpinelinux.org

Your /etc/resolv.conf should also contain

Contents of /etc/resolv.conf

nameserver 127.0.0.1

Printer discovery should now be working.

Failure in parallel query of A and AAAA requsts

An issue with parallel query of A and AAAA requsts was reported to upstream related to ipv4/ipv6 dual-setups and since been resolved in Alpine Linux !82560 by adding a timeout 2.5s.