Multicast DNS Daemon

This is early stage documentation, but at least attempts to cover some basic usage, and bring umdns usage out of the dark.

mDNS, also known as Bonjour or zero-configuration networking, enables automatic discovery of computers, devices, and services on IP networks. It is an internet standard documented in RFC6762.

The umdns package provides a compact implementation of this standard, well integrated with the OpenWrt system environment. In particular, almost all interaction with the daemon is via ubus.

• mdnsd - provided by Apple's mDNSResponder package
• avahi - A fairly full, but quite large implementation

/etc/config/umdns

config umdns
        option jail 1
        list network lan
        list network dmz # Provides visibility into both networks, but does not act as a repeater

Note that it may be unsafe to enable umdns on wan interface.

$ ubus call umdns update
# wait a second or two
$ ubus call umdns browse
# big json dump example...
        ....
	"_printer._tcp": {
		"HP\\032Color\\032LaserJet\\032CP2025dn\\032(28A6CC)": {
			"port": 515,
			"txt": "txtvers=1",
			"txt": "qtotal=1",
			"txt": "rp=RAW",
			"txt": "ty=HP Color LaserJet CP2025dn",
			"txt": "product=(HP Color LaserJet CP2025dn)",
			"txt": "priority=50",
			"txt": "adminurl=http:\/\/NPI28A6CC.local.",
			"txt": "Transparent=T",
			"txt": "Binary=T",
			"txt": "TBCP=T"
		},
		"HP\\032LaserJet\\032P3010\\032Series\\032[46A14F]": {
			"port": 515,
			"txt": "txtvers=1",
			"txt": "qtotal=4",
			"txt": "rp=RAW",
			"txt": "pdl=application\/postscript,application\/vnd.hp-PCL,application\/vnd.hp-PCLXL",
			"txt": "ty=HP LaserJet P3010 Series",
			"txt": "product=(HP LaserJet P3010 Series)",
			"txt": "usb_MFG=Hewlett-Packard",
			"txt": "usb_MDL=HP LaserJet P3010 Series",
			"txt": "priority=52",
			"txt": "adminurl=http:\/\/NPI46A14F.local."
      },
   ....
$ ubus call umdns hosts 
#Show hosts discovered via mDns
        "SteakPrinter.local": {
                "ipv4": "192.168.1.159"
        },
        "Upstairs.local": {
                "ipv4": "192.168.1.151"
        },

• IP addresses are missing.
• TXT records aren't valid json in the dump, so jsonfilter can't be used.
• How long is data cached? What causes it to update? No idea.
• You may not see locally advertised services with ubus call umdns browse. See the discussion

The umdns scans all the services listed in ubus (ubus call service list) and looks for mdns objects in their data object. You can view this more selectively for example with:

# ubus call service list | jsonfilter -e '@[*].instances[*].data.mdns'
{ "ssh_22": { "service": "_ssh._tcp.local", "port": 22, "txt": [ "daemon=dropbear" ] } }

Here we can see that ssh is being advertised locally.

If you want to advertise your own service, your service needs to be a procd managed service. You can use the procd_add_mdns call to provide a basic definition.

procd_open_instance
....
procd_add_mdns <service> <proto> <port> [<textkey=textvalue> ... ]
...
procd_close_instance

As an example, the following call

procd_add_mdns "webdav" "tcp" "80" "path=/nextcloud/remote.php/dav/files/YOUR_USER/" "u=YOUR_USER"

will result in advertising _webdav._tcp.local with two text records. In the example we published a WebDAV folder from Nextcloud and now it can be seen in Network folder of a file manager in GNOME and KDE and can be discovered from a Kodi media player. The service names may be taken from the IANA register and the txt-records may be taken from the official DNS-SD keys (see ServiceTypes under “Defined TXT keys”.

If you wish to create a more complicated mdns information block, see procd_add_mdns_service in /lib/functions/procd.sh but be warned that umdns probably can't automatically support everything you can represent in json.

Alternatively you can create a service description in JSON file and put it into /etc/umdns/ folder:

/etc/umdns/nextcloud_webdav.json

{
  "nextcloud_webdav": {
    "service": "_webdav._tcp.local",
    "port": 80,
    "txt": [
      "path=/nextcloud/remote.php/dav/files/YOUR_USER/",
      "u=YOUR_USER"
    ]
  }
}

See examples in umdns sources