implement ZeroConf/Bonjour support

[wkornewald]

We need:

• an API that can be used by applications (including a host/service browser)
• the possibility to extend the known hosts and services list via an instant messenger, for example
• detection of duplicate host names
• automatic home network configuration

[wkornewald]

Since I was the one who added this and Alex asked on the mailing list, here is the clarification of the second point:

It would be cool if other apps could extend the ZeroConf services and hosts list. For example, an instant messenger could automagically display your friends' computers in the known hosts list although they're not on the local network. Also, the services running on those (non-local) hosts could be enumerated, too. The result would be that you could, for example, easily connect to your friend to use a collaborative text editing tool or directly access your friend's file server, no matter whether he's on a local or remote network. Well, firewalls might get in the way, but that's a different problem.

[pulkomandy]

Apple's implementation is open source under the Apache license:

​http://opensource.apple.com//source/mDNSResponder/ ​https://opensource.apple.com/tarballs/mDNSResponder/

The first step is getting mDNS working, so name of machines can be resolved. This can be integrated as a network kit add-on, complementing our existing DNS resolver: ​https://tools.ietf.org/html/rfc6762 The second step is DSN-SD (service discovery), which allows to discover the services from other machines: ​http://www.ietf.org/rfc/rfc6763.txt

The firewall problem is then solved with uPNP or PCP (​http://www.ietf.org/rfc/rfc6887.txt) - if the router supports that, or in less friendly environments, with pwnat (​http://samy.pl/pwnat)