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167 lines
7.5 KiB
167 lines
7.5 KiB
What is mDNSResponder?
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----------------------
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The mDNSResponder project is a component of Bonjour,
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Apple's ease-of-use IP networking initiative:
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<http://developer.apple.com/bonjour/>
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Apple's Bonjour software derives from the ongoing standardization
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work of the IETF Zero Configuration Networking Working Group:
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<http://zeroconf.org/>
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The Zeroconf Working Group has identified three requirements for Zero
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Configuration Networking:
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1. An IP address (even when there is no DHCP server to assign one)
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2. Name-to-address translation (even when there is no DNS server)
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3. Discovery of Services on the network (again, without infrastucture)
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Requirement 1 is met by self-assigned link-local addresses, as
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described in "Dynamic Configuration of IPv4 Link-Local Addresses"
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<http://files.zeroconf.org/draft-ietf-zeroconf-ipv4-linklocal.txt>
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Requirement 2 is met by sending DNS-like queries via Multicast (mDNS).
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Requirement 3 is met by DNS Service Dicsovery (DNS-SD).
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Self-assigned link-local address capability has been available since
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1998, when it first appeared in Windows '98 and in Mac OS 8.5.
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Implementations for other platforms also exist.
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The mDNSResponder project allows us to meet requirements 2 and 3.
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It provides the ability for the user to identify hosts using names
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instead of dotted-decimal IP addresses, even if the user doesn't have a
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conventional DNS server set up. It also provides the ability for the
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user to discover what services are being advertised on the network,
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without having to know about them in advance, or configure the machines.
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The name "mDNS" was chosen because this protocol is designed to be,
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as much as possible, similar to conventional DNS. The main difference is
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that queries are sent via multicast to all local hosts, instead of via
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unicast to a specific known server. Every host on the local link runs an
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mDNSResponder which is constantly listening for those multicast queries,
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and if the mDNSResponder receives a query for which it knows the answer,
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then it responds. The mDNS protocol uses the same packet format as
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unicast DNS, and the same name structure, and the same DNS record types.
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The main difference is that queries are sent to a different UDP port
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(5353 instead of 53) and they are sent via multicast to address
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224.0.0.251. Another important difference is that all "mDNS" names
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end in ".local." When a user types "yourcomputer.local." into their Web
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browser, the presence of ".local." on the end of the name tells the host
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OS that the name should be looked up using local multicast instead of by
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sending that name to the worldwide DNS service for resolution. This
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helps reduce potential user confusion about whether a particular name
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is globally unique (e.g. "www.apple.com.") or whether that name has only
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local significance (e.g. "yourcomputer.local.").
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About the mDNSResponder Code
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----------------------------
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Because Apple benefits more from widespread adoption of Bonjour than
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it would benefit from keeping Bonjour proprietary, Apple is making
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this code open so that other developers can use it too.
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Because Apple recognises that networks are hetrogenous environments
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where devices run many different kinds of OS, this code has been made
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as portable as possible.
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A typical mDNS program contains three components:
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+------------------+
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| Application |
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+------------------+
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| mDNS Core |
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+------------------+
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| Platform Support |
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+------------------+
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The "mDNS Core" layer is absolutely identical for all applications and
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all Operating Systems.
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The "Platform Support" layer provides the necessary supporting routines
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that are specific to each platform -- what routine do you call to send
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a UDP packet, what routine do you call to join multicast group, etc.
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The "Application" layer does whatever that particular application wants
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to do. It calls routines provided by the "mDNS Core" layer to perform
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the functions it needs --
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* advertise services,
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* browse for named instances of a particular type of service
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* resolve a named instance to a specific IP address and port number,
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* etc.
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The "mDNS Core" layer in turn calls through to the "Platform Support"
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layer to send and receive the multicast UDP packets to do the actual work.
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Apple currently provides "Platform Support" layers for Mac OS 9, Mac OS X,
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Microsoft Windows, VxWorks, and for POSIX platforms like Linux, Solaris,
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FreeBSD, etc.
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Note: Developers writing applications for OS X do not need to incorporate
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this code into their applications, since OS X provides a system service to
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handle this for them. If every application developer were to link-in the
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mDNSResponder code into their application, then we would end up with a
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situation like the picture below:
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+------------------+ +------------------+ +------------------+
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| Application 1 | | Application 2 | | Application 3 |
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+------------------+ +------------------+ +------------------+
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| mDNS Core | | mDNS Core | | mDNS Core |
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+------------------+ +------------------+ +------------------+
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| Platform Support | | Platform Support | | Platform Support |
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+------------------+ +------------------+ +------------------+
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This would not be very efficient. Each separate application would be sending
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their own separate multicast UDP packets and maintaining their own list of
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answers. Because of this, OS X provides a common system service which client
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software should access through the "/usr/include/dns_sd.h" APIs.
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The situation on OS X looks more like the picture below:
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-------------------
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/ \
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+---------+ +------------------+ +---------+ \ +---------+
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| App 1 |<-->| daemon.c |<-->| App 2 | ->| App 3 |
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+---------+ +------------------+ +---------+ +---------+
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| mDNS Core |
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+------------------+
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| Platform Support |
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+------------------+
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Applications on OS X make calls to the single mDNSResponder daemon
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which implements the mDNS and DNS-SD protocols.
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Vendors of products such as printers, which are closed environments not
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expecting to be running third-party application software, can reasonably
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implement a single monolithic mDNSResponder to advertise all the
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services of that device. Vendors of open systems which run third-party
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application software should implement a system service such as the one
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provided by the OS X mDNSResponder daemon, and application software on
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that platform should, where possible, make use of that system service
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instead of embedding their own mDNSResponder.
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See ReadMe.txt in the mDNSPosix directory for specific details of
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building an mDNSResponder on a POSIX Operating System.
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Compiling on Older C Compilers
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------------------------------
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We go to some lengths to make the code portable, but //-style comments
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are one of the modern conveniences we can't live without.
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If your C compiler doesn't understand these comments, you can transform
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them into classical K&R /* style */ comments with a quick GREP
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search-and-replace pattern.
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In BBEdit on the Mac:
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1. Open the "Find" dialog window and make sure "Use Grep" is selected
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2. Search For : ([^:])//(.*)
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3. Replace With: \1/*\2 */
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4. Drag your mDNSResponder source code folder to the Multi-File search pane
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5. Click "Replace All"
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For the more command-line oriented, cd into your mDNSResponder source code
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directory and execute the following command (all one line):
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find mDNSResponder \( -name \*.c\* -or -name \*.h \) -exec sed -i .orig -e 's,^//\(.*\),/*\1 */,' -e '/\/\*/\!s,\([^:]\)//\(.*\),\1/*\2 */,' {} \;
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