ReadMe About mDNSPosix ---------------------- mDNSPosix is a port of Apple's core mDNS code to the Posix platform. The sample shows how you might implement an mDNS responder inside an embedded device, such as a printer or a web camera. mDNS is short for "multicast DNS", which is a technology that allows you to register IP services and browse the network for those services. For more information about mDNS, see the mDNS web site. mDNS is part of a family of technologies developed by the IETF zeroconf working group. For information about other zeroconf technologies, see the zeroconf web site. Apple uses the brand name "Rendezvous" to describe our implementation of zeroconf technologies. This sample is designed to show how easy it is to make a device "Rendezvous compatible". The code in this sample was compiled and tested on Mac OS X (10.1.x, 10.2), Solaris (SunOS 5.6), Linux (Redhat 2.4.9-21), and OpenBSD (2.9). YMMV. IMPORTANT This sample is not a full port of Apple's Rendezvous APIs to Posix. Specifically, the sample includes a responder daemon that registers entities based on its command line arguments (or a text file). This is perfect for a embedded device, but is not suitable for a general purpose computer. A real implementation of the Rendezvous APIs would require a mDNS daemon, client libraries that applications link with, and some form of RPC between them. Client libraries and client-to-daemon RPC are beyond the scope of this sample, however, this would be a good place to start if you were interested in implementing these facilities on your platform. Packing List ------------ The sample includes the following files and directories: o ReadMe.txt -- This file. o mDNSCore -- A directory containing the core mDNS code. This code is written in pure ANSI C and has proved to be very portable. o mDNSPosix.h -- The interface to the platform support code. o mDNSPosix.c -- The platform support code for the Posix platform. This code glues the mDNS core to Posix. o mDNSUNP.h -- Interface to the code in "mDNSUNP.c". o mDNSUNP.c -- A few routines from the "Unix Network Programming" book that I borrowed to make the port easier. The routines are slightly modified from the originals to meet my specific needs. You can get the originals at the URL below. o Client.c -- The main program for the sample mDNS client. o Responder.c -- The main program for the sample mDNS responder. o Services.txt -- A sample configuration file for the mDNS responder. You can test with this file using the option "-f Services.txt". o ProxyResponder.c -- Another sample mDNS responder, this one intended for creating proxy registrations for other network devices that don't have their own mDNS responders. o ExampleClientApp.h o ExampleClientApp.c -- shared code prioviding the "ExampleClientEventLoop" used by Client.c and ProxyResponder.c. o Makefile -- A makefile for building on Mac OS X and other platforms. Building the Sample ------------------- The sample does not use autoconf technology, primarily because I didn't want to delay shipping while I learnt how to use it. Thus the code builds using a very simple make file. To build the sample you should type "make os=myos", e.g. make os=osx For Linux you would change that to: make os=linux There are definitions for each of the platforms I ported to. If you're porting to any other platform you'll have to add appropriate definitions for it. Using the Sample ---------------- Once you've built the sample you can test it by first running the client, as shown below. quinn% build/mDNSClientPosix Hit ^C when you're bored waiting for responses. By default the client starts a search for AppleShare servers and then sits and waits, printing a message when services appear and disappear. To continue with the test you should start the responder in another shell window. quinn% build/mDNSResponderPosix -n Foo This will start the responder and tell it to advertise a AppleShare service "Foo". In the client window you will see the client print out the following as the service shows up on the network. quinn% build/mDNSClientPosix Hit ^C when you're bored waiting for responses. *** Found name = 'Foo', type = '_afpovertcp._tcp.', domain = 'local.' Back in the responder window you can quit the responder cleanly using SIGINT (typically ^C). quinn% build/mDNSResponderPosix -n Foo ^C quinn% As the responder quits it will multicast that the "Foo" service is disappearing and the client will see that notification and print a message to that effect (shown below). Finally, when you're done with the client you can use SIGINT to quit it. quinn% build/mDNSClientPosix Hit ^C when you're bored waiting for responses. *** Found name = 'Foo', type = '_afpovertcp._tcp.', domain = 'local.' *** Lost name = 'Foo', type = '_afpovertcp._tcp.', domain = 'local.' ^C quinn% If things don't work, try starting each program in verbose mode (using the "-v 1" option, or very verbose mode with "-v 2") to see if there's an obvious cause. That's it for the core functionality. Each program supports a variety of other options. For example, you can advertise and browse for a different service type using the "-t type" option. Use the "-?" option on each program for more user-level information. How It Works ------------ A typical mDNS program is divided into three sections. +----------------+ | Application | +----------------+ | mDNS Core | +----------------+ | Posix Platform | +----------------+ The mDNS core code comprises the files in the "mDNSCore" directory. It's standard ANSI C that's very portable. It relies on the underlying platform code for all external functionality. In this example the external platform code glues the mDNS core to a POSIX-ish platform. This code is contained in the files: o mDNSPosix.h o mDNSPosix.c o mDNSUNP.h o mDNSUNP.c The guts of the code is in "mDNSPosix.c". I should be clear that true POSIX isn't powerful enough to accomplish the job, so this code doesn't compile with _POSIX_SOURCE defined and there's a bunch of conditional code that does different things on different Unixen. I've isolated the hairiest parts of this code in the "mDNSUNP". Above the mDNS core code is the code that actually does application-specific tasks. In this example I've supplied two application programs: the responder (Responder.c) acts as a simple mDNS responder, listening for mDNS service lookup requests and answering them, and the client (Client.c), which is a simple mDNS browser, making simple mDNS search queries. Both programs use the same mDNS core and Posix platform code. A discussion of the mDNS protocol itself is beyond the scope of this sample. Quite frankly, my goal here was to demonstrate how it easy it is to use Apple's mDNS core without actually understanding mDNS, and because I achieved that goal I never had to learn a lot about how the mDNS core code works. It's just a black box that I call. If you want to learn more about mDNS, see the references at the top of this file. The mDNS Posix platform code is actually pretty simple. mDNS core requires six key features in its platform support. o the core calls the platformm at startup (mDNSPlatformInit) and shutdown (mDNSPlatformClose) o the core calls the platform to send a UDP packet (mDNSPlatformSendUDP) o the core calls the platform to set a timer (mDNSPlatformScheduleTask) o the platform calls the core (mDNSCoreTask) when the timer expires o the platform calls the core (mDNSCoreReceive) when a UDP datagram arrives o the platform calls the core when network interfaces are added (mDNS_RegisterInterface) or removed (mDNS_DeregisterInterface) All of these features are implemented in "mDNSPosix.c". The runtime behaviour of the code is as follows. 1. The application calls mDNS_Init, which in turns calls the platform (mDNSPlatformInit). 2. mDNSPlatformInit gets a list of interfaces (get_ifi_info) and registers each one with the core (mDNS_RegisterInterface). For each interface it also creates a multicast socket (SetupSocket). 3. The application then calls select() repeatedly to handle file descriptor events. Before calling select() each time, the application calls mDNSPosixGetFDSet() to give mDNSPosix.c a chance to add its own file descriptors to the set, and then after select() returns, it calls mDNSPosixProcessFDSet() to give mDNSPosix.c a chance to receive and process any packets that may have arrived. 4. When the core needs to send a UDP packet it calls mDNSPlatformSendUDP. That routines finds the interface that corresponds to the source address requested by the core, and sends the datagram using the UDP socket created for the interface. If the socket is flow send-side controlled it just drops the packet. 5. When SocketDataReady runs it uses a complex routine, "recvfrom_flags", to actually receive the packet. This is required because the core needs information about the packet that is only available via the "recvmsg" call, and that call is complex to implement in a portable way. I got my implementation of "recvfrom_flags" from Stevens' "UNIX Network Programming", but I had to modify it further to work with Linux. One thing to note is that the Posix platform code is very deliberately not multi-threaded. I do everything from a main loop that calls "select()". This is good because it avoids all the problems that often accompany multi-threaded code. If you decide to use threads in your platform, you will have to implement the mDNSPlatformLock() and mDNSPlatformUnlock() calls which are no-ops in mDNSPosix.c. Caveats ------- Currently the program uses a simple make file. There are various problems with loopback-only self discovery. The code will attempt service discovery on the loopback interface only if no other interfaces are available. However, this exposes a number of problems with the underlying network stack (at least on Mac OS X). o On Mac OS X 10.1.x the code fails to start on the loopback interface because the IP_ADD_MEMBERSHIP option returns ENOBUFS. o On Mac OS X 10.2 the loopback-only case fails because mDNSPlatformSendUDP's call to "sendto" fails with error EHOSTUNREACH [Radar ID 3016042]. I haven't been able to test the loopback-only case on other platforms because I don't have access to the physical machine. Licencing --------- This code is distributed under the Apple Public Source License. Information about the licence is included at the top of each source file. Credits and Version History --------------------------- If you find any problems with this sample, mail and I will try to fix them up. 1.0a1 (Jul 2002) was a prerelease version that was distributed internally at Apple. 1.0a2 (Jul 2002) was a prerelease version that was distributed internally at Apple. 1.0a3 (Aug 2002) was the first shipping version. The core mDNS code is the code from Mac OS 10.2 (Jaguar) GM. Share and Enjoy Apple Developer Technical Support Networking, Communications, Hardware 6 Aug 2002 To Do List ---------- ¥ port to a System V that's not Solaris ¥ use sig_atomic_t for signal to main thread flags ¥ test and debug the daemon function, including properly logging