mdns discovery of sessions

[Antoine Martin]

Make mdns more useful.

It should be easier to see which sessions are available on the LAN.
Tools like ​avahi-discover aren't very user friendly and don't know about our service type.. (#731)

[Antoine Martin]

• r14008 cleanup, move mdns bits to a sub-package
• r14009 + r14010: expose the server uuid in the mdns data
• r14011 + r14013: "list-mdns" subcommand
• r14012 add support for building --without-mdns

Notes:

• there is no support for OSX or win32 yet (#1333)
• a GUI would be nice: #1334

---

Example:

$ xpra start --bind-tcp=0.0.0.0: --mdns=yes --session-name=hello

$ xpra list-mdns
Looking for xpra services via mdns
* user 'antoine' on 'desktop.local'
 session 'hello', uuid=bce14c3aeed34c7892aea576ea8d78a8
 + ssh endpoint on host 192.168.1.200, port 22, interface eth1
   "ssh/antoine@192.168.1.200:22/2"
 + tcp endpoint on host 192.168.1.200, port 14500, interface eth1
   "tcp/antoine@192.168.1.200:14500/2"
1 service found

$ xpra info | grep uuid
server.uuid=bce14c3aeed34c7892aea576ea8d78a8

(works equally well for ssl sockets and the quoted uri should be usable as-is)

@afarr: mostly a FYI.

[Antoine Martin]

Pull request for avahi: ​add Xpra service type

[J. Max Mena]

Neat. Closing.

Note: if a server is on the non-standard port, this will not find it. Makes sense(because one would be deviating from the standard, and it'd be unreasonable to try to check every single port), but just wanted to note it for future reference.