The main network service file must have the extension .dnssd; other extensions are ignored.
The .dnssd files are read from the files located in the system network directories /usr/lib/systemd/dnssd and /usr/local/lib/systemd/dnssd, the volatile runtime network directory /run/systemd/dnssd and the local administration network directory /etc/systemd/dnssd. All configuration files are collectively sorted and processed in lexical order, regardless of the directories in which they live. However, files with identical filenames replace each other. Files in /etc have the highest priority, files in /run take precedence over files with the same name in /usr/lib. This can be used to override a system-supplied configuration file with a local file if needed.
Along with the network service file foo.dnssd, a "drop-in" directory foo.dnssd.d/ may exist. All files with the suffix ".conf" from this directory will be parsed after the file itself is parsed. This is useful to alter or add configuration settings, without having to modify the main configuration file. Each drop-in file must have appropriate section headers.
In addition to /etc/systemd/dnssd, drop-in ".d" directories can be placed in /usr/lib/systemd/dnssd or /run/systemd/dnssd directories. Drop-in files in /etc take precedence over those in /run which in turn take precedence over those in /usr/lib or /usr/local/lib. Drop-in files under any of these directories take precedence over the main network service file wherever located.
The option supports simple specifier expansion. The following expansions are understood:
Table 1. Specifiers available
|"%m"||Machine ID||The machine ID of the running system, formatted as string. See machine-id(5) for more information.|
|"%b"||Boot ID||The boot ID of the running system, formatted as string. See random(4) for more information.|
|"%H"||Host name||The hostname of the running system.|
|"%v"||Kernel release||Identical to uname -r output.|
This option together with TxtData= may be specified more than once, in which case multiple TXT resource records will be created for the service. If the empty string is assigned to this option, the list is reset and all prior assignments will have no effect.
This option together with TxtText= may be specified more than once, in which case multiple TXT resource records will be created for the service. If the empty string is assigned to this option, the list is reset and all prior assignments will have no effect.
# /etc/systemd/dnssd/http.dnssd [Service] Name=%H Type=_http._tcp Port=80 TxtText=path=/stats/index.html t=temperature_sensor
This makes the http server running on the host discoverable in the local network given MulticastDNS is enabled on the network interface.
Now the utility "resolvectl" should be able to resolve the service to the host's name:
$ resolvectl service meteo._http._tcp.local meteo._http._tcp.local: meteo.local:80 [priority=0, weight=0] 169.254.208.106%senp0s21f0u2u4 fe80::213:3bff:fe49:8aa%senp0s21f0u2u4 path=/stats/index.html t=temperature_sensor (meteo/_http._tcp/local) -- Information acquired via protocol mDNS/IPv6 in 4.0ms. -- Data is authenticated: yes
"Avahi" running on a different host in the same local network should see the service as well:
$ avahi-browse -a -r + enp3s0 IPv6 meteo Web Site local + enp3s0 IPv4 meteo Web Site local = enp3s0 IPv6 meteo Web Site local hostname = [meteo.local] address = [fe80::213:3bff:fe49:8aa] port =  txt = ["path=/stats/index.html" "t=temperature_sensor"] = enp3s0 IPv4 meteo Web Site local hostname = [meteo.local] address = [169.254.208.106] port =  txt = ["path=/stats/index.html" "t=temperature_sensor"]
- RFC 6763