README.txt
1What is mDNSResponder?
2----------------------
3
4The mDNSResponder project is a component of Bonjour,
5Apple's ease-of-use IP networking initiative:
6<http://developer.apple.com/bonjour/>
7
8Apple's Bonjour software derives from the ongoing standardization
9work of the IETF Zero Configuration Networking Working Group:
10<http://zeroconf.org/>
11
12The Zeroconf Working Group has identified three requirements for Zero
13Configuration Networking:
141. An IP address (even when there is no DHCP server to assign one)
152. Name-to-address translation (even when there is no DNS server)
163. Discovery of Services on the network (again, without infrastucture)
17
18Requirement 1 is met by self-assigned link-local addresses, as
19described in "Dynamic Configuration of IPv4 Link-Local Addresses"
20<http://files.zeroconf.org/draft-ietf-zeroconf-ipv4-linklocal.txt>
21
22Requirement 2 is met by sending DNS-like queries via Multicast (mDNS).
23
24Requirement 3 is met by DNS Service Dicsovery (DNS-SD).
25
26Self-assigned link-local address capability has been available since
271998, when it first appeared in Windows '98 and in Mac OS 8.5.
28Implementations for other platforms also exist.
29
30The mDNSResponder project allows us to meet requirements 2 and 3.
31It provides the ability for the user to identify hosts using names
32instead of dotted-decimal IP addresses, even if the user doesn't have a
33conventional DNS server set up. It also provides the ability for the
34user to discover what services are being advertised on the network,
35without having to know about them in advance, or configure the machines.
36
37The name "mDNS" was chosen because this protocol is designed to be,
38as much as possible, similar to conventional DNS. The main difference is
39that queries are sent via multicast to all local hosts, instead of via
40unicast to a specific known server. Every host on the local link runs an
41mDNSResponder which is constantly listening for those multicast queries,
42and if the mDNSResponder receives a query for which it knows the answer,
43then it responds. The mDNS protocol uses the same packet format as
44unicast DNS, and the same name structure, and the same DNS record types.
45The main difference is that queries are sent to a different UDP port
46(5353 instead of 53) and they are sent via multicast to address
47224.0.0.251. Another important difference is that all "mDNS" names
48end in ".local." When a user types "yourcomputer.local." into their Web
49browser, the presence of ".local." on the end of the name tells the host
50OS that the name should be looked up using local multicast instead of by
51sending that name to the worldwide DNS service for resolution. This
52helps reduce potential user confusion about whether a particular name
53is globally unique (e.g. "www.apple.com.") or whether that name has only
54local significance (e.g. "yourcomputer.local.").
55
56
57About the mDNSResponder Code
58----------------------------
59
60Because Apple benefits more from widespread adoption of Bonjour than
61it would benefit from keeping Bonjour proprietary, Apple is making
62this code open so that other developers can use it too.
63
64Because Apple recognises that networks are hetrogenous environments
65where devices run many different kinds of OS, this code has been made
66as portable as possible.
67
68A typical mDNS program contains three components:
69
70 +------------------+
71 | Application |
72 +------------------+
73 | mDNS Core |
74 +------------------+
75 | Platform Support |
76 +------------------+
77
78The "mDNS Core" layer is absolutely identical for all applications and
79all Operating Systems.
80
81The "Platform Support" layer provides the necessary supporting routines
82that are specific to each platform -- what routine do you call to send
83a UDP packet, what routine do you call to join multicast group, etc.
84
85The "Application" layer does whatever that particular application wants
86to do. It calls routines provided by the "mDNS Core" layer to perform
87the functions it needs --
88 * advertise services,
89 * browse for named instances of a particular type of service
90 * resolve a named instance to a specific IP address and port number,
91 * etc.
92The "mDNS Core" layer in turn calls through to the "Platform Support"
93layer to send and receive the multicast UDP packets to do the actual work.
94
95Apple currently provides "Platform Support" layers for Mac OS 9, Mac OS X,
96Microsoft Windows, VxWorks, and for POSIX platforms like Linux, Solaris,
97FreeBSD, etc.
98
99Note: Developers writing applications for OS X do not need to incorporate
100this code into their applications, since OS X provides a system service to
101handle this for them. If every application developer were to link-in the
102mDNSResponder code into their application, then we would end up with a
103situation like the picture below:
104
105 +------------------+ +------------------+ +------------------+
106 | Application 1 | | Application 2 | | Application 3 |
107 +------------------+ +------------------+ +------------------+
108 | mDNS Core | | mDNS Core | | mDNS Core |
109 +------------------+ +------------------+ +------------------+
110 | Platform Support | | Platform Support | | Platform Support |
111 +------------------+ +------------------+ +------------------+
112
113This would not be very efficient. Each separate application would be sending
114their own separate multicast UDP packets and maintaining their own list of
115answers. Because of this, OS X provides a common system service which client
116software should access through the "/usr/include/dns_sd.h" APIs.
117
118The situation on OS X looks more like the picture below:
119
120 -------------------
121 / \
122 +---------+ +------------------+ +---------+ \ +---------+
123 | App 1 |<-->| daemon.c |<-->| App 2 | ->| App 3 |
124 +---------+ +------------------+ +---------+ +---------+
125 | mDNS Core |
126 +------------------+
127 | Platform Support |
128 +------------------+
129
130Applications on OS X make calls to the single mDNSResponder daemon
131which implements the mDNS and DNS-SD protocols.
132
133Vendors of products such as printers, which are closed environments not
134expecting to be running third-party application software, can reasonably
135implement a single monolithic mDNSResponder to advertise all the
136services of that device. Vendors of open systems which run third-party
137application software should implement a system service such as the one
138provided by the OS X mDNSResponder daemon, and application software on
139that platform should, where possible, make use of that system service
140instead of embedding their own mDNSResponder.
141
142See ReadMe.txt in the mDNSPosix directory for specific details of
143building an mDNSResponder on a POSIX Operating System.
144
145
146Compiling on Older C Compilers
147------------------------------
148
149We go to some lengths to make the code portable, but //-style comments
150are one of the modern conveniences we can't live without.
151
152If your C compiler doesn't understand these comments, you can transform
153them into classical K&R /* style */ comments with a quick GREP
154search-and-replace pattern.
155
156In BBEdit on the Mac:
1571. Open the "Find" dialog window and make sure "Use Grep" is selected
1582. Search For : ([^:])//(.*)
1593. Replace With: \1/*\2 */
1604. Drag your mDNSResponder source code folder to the Multi-File search pane
1615. Click "Replace All"
162
163For the more command-line oriented, cd into your mDNSResponder source code
164directory and execute the following command (all one line):
165
166find mDNSResponder \( -name \*.c\* -or -name \*.h \) -exec sed -i .orig -e 's,^//\(.*\),/*\1 */,' -e '/\/\*/\!s,\([^:]\)//\(.*\),\1/*\2 */,' {} \;
167
README.version