Copyright (c) 2010, Christiano F. Haesbaert <[email protected]>

Multicast Domain Name System for OpenBSD

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Intro

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This is an attempt to bring native mdns/dns-sd to OpenBSD. Mainly

cause all the other options suck and proper network browsing is a nice

feature these days.

Why not Apple's mdnsd ?

1 - It sucks big time.

2 - No BSD License (Apache-2).

3 - Overcomplex API.

4 - Not OpenBSD-like.

Why not Avahi ?

1 - No BSD License (LGPL).

2 - Overcomplex API.

3 - Not OpenBSD-like

4 - DBUS and lots of dependencies.

My goal is to first provide a solid mdns framework without the

service-discovery capabilities which are much more complex. When that

is working properly, work on service-discovery will begin, the

architecture was designed in order to support it. So goals:

* Have a mdns responder/querier as an user process (mdnsd).

* Have integration with libc's resolver, read: make

gethostbyname and friends resolve names through mdns.

* Have an application interface so other software can

publish names/services as they see fit.

* Have a mdns control program, which is mainly a crude

interface to the API.

MDNSD Design

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The "Multicast Domain Name System Daemon" is an user process that runs

without privileges, it binds to udp port 5353 and acts as the

querier/responder for all mdns requests. By responder we understand

who is responsible for answering mdns queries, and by querier the

other way around, read: This is different from libc's unicast

resolver, we *NEED* to cache answers and maintain state.

As most OpenBSD daemons, it drops privilege upon startup and none task

requires super-user privilege. All work is done on a single process,

my first design had a process per interface, but that became overkill

and overcomplex.

The API is designed above Henning Brauer imsg framework, so we have a

unix domain socket that listen to requests from other processes, much

like ripctl and ospfctl do, but we need a fancier API. We use

libevent and all it's glorious features to have a proper unix daemon.

There is a routing socket in order to be notified when link goes up

and down, or when an interface address changes, this is *required* in

order to publish records, every time link comes up we need to re-probe

for our records and things like that, we use as our main name the

short name from /etc/myname in the .local domain.

The basic data flow would be:

__________ _______

| Programs | <--Control Socket (API)--> | MDNSD | <-- Mcast Packets -->

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LIBMDNS Design (API)

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Libmdns is a shared library in which programs can link against in

order to have mdns capabilities, like publishing services, browsing

the network and so on, by now the only thing it does is resolving mdns

names and browsing for services, no publishing capabilities yet. All

the library does is sending and receiving messages to mdnsd through

the imsg framework. The library should provide in the near future:

* Means for looking up names. (DONE)

* Means for publishing names.

* Means for browsing services (dns-service-discovery). (DONE)

* Means for publishing services.

By now no library is being used to make it easier for people to test

mdnsctl, everything is being kept on mdnsl.c in mdnsctl/.

On libc integration:

After a chat with nicm@ we have decided not to worry about libc right

now, maybe never. See the README file on mdns-libc branch if you're

curious.

MDNSCTL

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It's a simple interface for the library, code is inspired in

ripctl. By now you can use it to lookup hosts and browse

services.

How do I use this thing ?

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Get the source from http://github.com/haesbaert/mdnsd

Add user and group _mdnsd

Compile and install mdnsd and mdnsctl

Make sure you have proper multicast setup (man 7 netstart), or

just add a route to lo0:

route delete 224/4

route add 224/4 127.0.0.1

Run mdnsd on the desired interface: mdnsd -d iface

Play with mdnsctl :)