Manpages - systemd.dnssd.5

systemd.dnssd - DNS-SD configuration

network_service.dnssd

DNS-SD setup is performed by *systemd-resolved*(8).

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 network service file contains a [Service] section, which specifies a discoverable network service announced in a local network with Multicast DNS broadcasts.

Name=

An instance name of the network service as defined in the section 4.1.1 of RFC 6763[1], e.g. “webserver”.

The option supports simple specifier expansion. The following expansions are understood:

Table 1. Specifiers available

Specifier	Meaning	Details
“%a”	Architecture	A short string identifying the architecture of the local system. A string such as x86, x86-64 or arm64. See the architectures defined for ConditionArchitecture= in *systemd.unit*(5) for a full list.
“%A”	Operating system image version	The operating system image version identifier of the running system, as read from the IMAGE_VERSION= field of /etc/os-release. If not set, resolves to an empty string. See *os-release*(5) for more information.
“%b”	Boot ID	The boot ID of the running system, formatted as string. See *random*(4) for more information.
“%B”	Operating system build ID	The operating system build identifier of the running system, as read from the BUILD_ID= field of /etc/os-release. If not set, resolves to an empty string. See *os-release*(5) for more information.
“%H”	Host name	The hostname of the running system.
“%m”	Machine ID	The machine ID of the running system, formatted as string. See *machine-id*(5) for more information.
“%M”	Operating system image identifier	The operating system image identifier of the running system, as read from the IMAGE_ID= field of /etc/os-release. If not set, resolves to an empty string. See *os-release*(5) for more information.
“%o”	Operating system ID	The operating system identifier of the running system, as read from the ID= field of /etc/os-release. See *os-release*(5) for more information.
“%v”	Kernel release	Identical to uname -r output.
“%w”	Operating system version ID	The operating system version identifier of the running system, as read from the VERSION_ID= field of /etc/os-release. If not set, resolves to an empty string. See *os-release*(5) for more information.
“%W”	Operating system variant ID	The operating system variant identifier of the running system, as read from the VARIANT_ID= field of /etc/os-release. If not set, resolves to an empty string. See *os-release*(5) for more information.
“%%”	Single percent sign	Use “%%” in place of “%” to specify a single percent sign.

Type=

A type of the network service as defined in the section 4.1.2 of RFC 6763[1], e.g. “_http._tcp”.

Port=

An IP port number of the network service.

Priority=

A priority number set in SRV resource records corresponding to the network service.

Weight=

A weight number set in SRV resource records corresponding to the network service.

TxtText=

A whitespace-separated list of arbitrary key/value pairs conveying additional information about the named service in the corresponding TXT resource record, e.g. “path=/portal/index.html”. Keys and values can contain C-style escape sequences which get translated upon reading configuration files.

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.

TxtData=

A whitespace-separated list of arbitrary key/value pairs conveying additional information about the named service in the corresponding TXT resource record where values are base64-encoded string representing any binary data, e.g. “data=YW55IGJpbmFyeSBkYXRhCg==”. Keys can contain C-style escape sequences which get translated upon reading configuration files.

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.

Example 1. HTTP service

    # /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 hosts 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 = [80]
       txt = ["path=/stats/index.html" "t=temperature_sensor"]
    = enp3s0 IPv4 meteo                                         Web Site             local
       hostname = [meteo.local]
       address = [169.254.208.106]
       port = [80]
       txt = ["path=/stats/index.html" "t=temperature_sensor"]

*systemd*(1), *systemd-resolved.service*(8), *resolvectl*(1)

1.

RFC 6763

https://tools.ietf.org/html/rfc6763