1 What is mDNSResponder? 2 ---------------------- 3 4 The mDNSResponder project is a component of Bonjour, 5 Apple's ease-of-use IP networking initiative: 6 <http://developer.apple.com/bonjour/> 7 8 Apple's Bonjour software derives from the ongoing standardization 9 work of the IETF Zero Configuration Networking Working Group: 10 <http://zeroconf.org/> 11 12 The Zeroconf Working Group has identified three requirements for Zero 13 Configuration Networking: 14 1. An IP address (even when there is no DHCP server to assign one) 15 2. Name-to-address translation (even when there is no DNS server) 16 3. Discovery of Services on the network (again, without infrastucture) 17 18 Requirement 1 is met by self-assigned link-local addresses, as 19 described in "Dynamic Configuration of IPv4 Link-Local Addresses" 20 <http://files.zeroconf.org/draft-ietf-zeroconf-ipv4-linklocal.txt> 21 22 Requirement 2 is met by sending DNS-like queries via Multicast (mDNS). 23 24 Requirement 3 is met by DNS Service Dicsovery (DNS-SD). 25 26 Self-assigned link-local address capability has been available since 27 1998, when it first appeared in Windows '98 and in Mac OS 8.5. 28 Implementations for other platforms also exist. 29 30 The mDNSResponder project allows us to meet requirements 2 and 3. 31 It provides the ability for the user to identify hosts using names 32 instead of dotted-decimal IP addresses, even if the user doesn't have a 33 conventional DNS server set up. It also provides the ability for the 34 user to discover what services are being advertised on the network, 35 without having to know about them in advance, or configure the machines. 36 37 The name "mDNS" was chosen because this protocol is designed to be, 38 as much as possible, similar to conventional DNS. The main difference is 39 that queries are sent via multicast to all local hosts, instead of via 40 unicast to a specific known server. Every host on the local link runs an 41 mDNSResponder which is constantly listening for those multicast queries, 42 and if the mDNSResponder receives a query for which it knows the answer, 43 then it responds. The mDNS protocol uses the same packet format as 44 unicast DNS, and the same name structure, and the same DNS record types. 45 The 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 47 224.0.0.251. Another important difference is that all "mDNS" names 48 end in ".local." When a user types "yourcomputer.local." into their Web 49 browser, the presence of ".local." on the end of the name tells the host 50 OS that the name should be looked up using local multicast instead of by 51 sending that name to the worldwide DNS service for resolution. This 52 helps reduce potential user confusion about whether a particular name 53 is globally unique (e.g. "www.apple.com.") or whether that name has only 54 local significance (e.g. "yourcomputer.local."). 55 56 57 About the mDNSResponder Code 58 ---------------------------- 59 60 Because Apple benefits more from widespread adoption of Bonjour than 61 it would benefit from keeping Bonjour proprietary, Apple is making 62 this code open so that other developers can use it too. 63 64 Because Apple recognises that networks are hetrogenous environments 65 where devices run many different kinds of OS, this code has been made 66 as portable as possible. 67 68 A typical mDNS program contains three components: 69 70 +------------------+ 71 | Application | 72 +------------------+ 73 | mDNS Core | 74 +------------------+ 75 | Platform Support | 76 +------------------+ 77 78 The "mDNS Core" layer is absolutely identical for all applications and 79 all Operating Systems. 80 81 The "Platform Support" layer provides the necessary supporting routines 82 that are specific to each platform -- what routine do you call to send 83 a UDP packet, what routine do you call to join multicast group, etc. 84 85 The "Application" layer does whatever that particular application wants 86 to do. It calls routines provided by the "mDNS Core" layer to perform 87 the 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. 92 The "mDNS Core" layer in turn calls through to the "Platform Support" 93 layer to send and receive the multicast UDP packets to do the actual work. 94 95 Apple currently provides "Platform Support" layers for Mac OS 9, Mac OS X, 96 Microsoft Windows, VxWorks, and for POSIX platforms like Linux, Solaris, 97 FreeBSD, etc. 98 99 Note: Developers writing applications for OS X do not need to incorporate 100 this code into their applications, since OS X provides a system service to 101 handle this for them. If every application developer were to link-in the 102 mDNSResponder code into their application, then we would end up with a 103 situation 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 +------------------+ +------------------+ +------------------+ 112 113 This would not be very efficient. Each separate application would be sending 114 their own separate multicast UDP packets and maintaining their own list of 115 answers. Because of this, OS X provides a common system service which client 116 software should access through the "/usr/include/dns_sd.h" APIs. 117 118 The 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 +------------------+ 129 130 Applications on OS X make calls to the single mDNSResponder daemon 131 which implements the mDNS and DNS-SD protocols. 132 133 Vendors of products such as printers, which are closed environments not 134 expecting to be running third-party application software, can reasonably 135 implement a single monolithic mDNSResponder to advertise all the 136 services of that device. Vendors of open systems which run third-party 137 application software should implement a system service such as the one 138 provided by the OS X mDNSResponder daemon, and application software on 139 that platform should, where possible, make use of that system service 140 instead of embedding their own mDNSResponder. 141 142 See ReadMe.txt in the mDNSPosix directory for specific details of 143 building an mDNSResponder on a POSIX Operating System. 144 145 146 Compiling on Older C Compilers 147 ------------------------------ 148 149 We go to some lengths to make the code portable, but //-style comments 150 are one of the modern conveniences we can't live without. 151 152 If your C compiler doesn't understand these comments, you can transform 153 them into classical K&R /* style */ comments with a quick GREP 154 search-and-replace pattern. 155 156 In BBEdit on the Mac: 157 1. Open the "Find" dialog window and make sure "Use Grep" is selected 158 2. Search For : ([^:])//(.*) 159 3. Replace With: \1/*\2 */ 160 4. Drag your mDNSResponder source code folder to the Multi-File search pane 161 5. Click "Replace All" 162 163 For the more command-line oriented, cd into your mDNSResponder source code 164 directory and execute the following command (all one line): 165 166 find mDNSResponder \( -name \*.c\* -or -name \*.h \) -exec sed -i .orig -e 's,^//\(.*\),/*\1 */,' -e '/\/\*/\!s,\([^:]\)//\(.*\),\1/*\2 */,' {} \; 167
