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 LEDE 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

$ ubus call mdns scan
# wait a second or two
$ ubus call mdns 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."
      },
   ....

• 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.

'mdns' 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 (*AT THE TIME OF WRITING* 2017-07-11) 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 "hoho" "tcp" "99" "daemon=special" "colour=fuschia"

will result in advertising _hoho._tcp.local with two text records.

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.