systemd-resolved as mDNS responder

Question

I'm creating a custom distribution where I need a DNS responder. I'm already using systemd so I would like to use systemd-resolved to manage mDNS (the device should announce itself as capable of a couple of services); I'm not sure whether or not this is possible, but the systemd-resolved documentation pages reports

systemd-resolved is a system service that provides network name resolution to local applications. It implements [...] MulticastDNS resolver and responder.

I already added set MulticastDNS=yes in the configuration file, as well as under the [Network] section of the interfaces where I want mDNS to be enabled (I can verify that with systemd-resolve --status eth0).

However, I'm not able to understand how to configure the available services to be announces, as it was done with avahi by adding them in /etc/avahi/services.

Are there any other configuration file for systemd-resolved? Is this not possible at all?

Answer 1

The capability of using mDNS should be enabled in the /etc/systemd/resolved.conf file, in the [Resolve] section, by setting MulticastDNS=yes. Moreover, it should be enabled in the [Network] section of each interface configuration file (the one for systemd-network) by setting MulticastDNS=yes.

The status of the MulticastDNS setting can be verified with:

~# systemd-resolve --status
Global
           Protocols: +LLMNR +mDNS -DNSOverTLS DNSSEC=no/unsupported
    resolv.conf mode: uplink
Fallback DNS Servers: 1.1.1.1#cloudflare-dns.com 8.8.8.8#dns.google 1.0.0.1#cloudflare-dns.com 8.8.4.4#dns.google 2606:4700:4700::1111#cloudflare-dns.com 2001:4860:4860::8888#dns.google 2606:4700:4700::1001#cloudflare-dns.com
                      2001:4860:4860::8844#dns.google

Link 2 (eth0)
Current Scopes: LLMNR/IPv4 LLMNR/IPv6 mDNS/IPv4 mDNS/IPv6
     Protocols: -DefaultRoute +LLMNR +mDNS -DNSOverTLS DNSSEC=no/unsupported

Link 3 (enp1s0)
Current Scopes: none
     Protocols: -DefaultRoute +LLMNR +mDNS -DNSOverTLS DNSSEC=no/unsupported

For each interface, +mDNS means that MulticastDNS is enabled in that interface. Global refers to the systemd-resolved global configuration.

Services can be configured by creating /service/.dnssd files with the following format:

[Service]
Name=%H
Type=_http._tcp
Port=80
TxtText=path=/stats/index.html t=temperature_sensor

see https://www.freedesktop.org/software/systemd/man/systemd.dnssd.html for more informations.

Configuration files can be saved in:

• /etc/systemd/dnssd
• /run/systemd/dnssd
• /usr/lib/systemd/dnssd