The mDNSResponder project is a component of Bonjour, Apple's ease-of-use IP networking initiative.
Apple's Bonjour software derives from the ongoing standardization work of the IETF Zero Configuration Networking Working Group.
The Zeroconf Working Group has identified three requirements for Zero Configuration Networking:
- An IP address (even when there is no DHCP server to assign one)
- Name-to-address translation (even when there is no DNS server)
- Discovery of Services on the network (again, without infrastructure)
Requirement 1 is met by self-assigned link-local addresses, as described in Dynamic Configuration of IPv4 Link-Local Addresses
Requirement 2 is met by sending DNS-like queries via Multicast.
Requirement 3 is met by DNS-based Service Discovery (DNS-SD).
Self-assigned link-local address capability has been available since 1998, when it first appeared in Windows '98 and in Mac OS 8.5. Implementations for other platforms also exist.
The mDNSResponder project allows us to meet requirements 2 and 3. It provides the ability for the user to identify hosts using names instead of dotted-decimal IP addresses, even if the user doesn't have a conventional DNS server set up. It also provides the ability for the user to discover what services are being advertised on the network, without having to know about them in advance, or configure the machines.
The name "mDNS" was chosen because this protocol is designed to be, as much as possible, similar to conventional DNS. The main difference is that queries are sent via multicast to all local hosts, instead of via unicast to a specific known server. Every host on the local link runs an mDNSResponder which is constantly listening for those multicast queries, and if the mDNSResponder receives a query for which it knows the answer, then it responds.
The mDNS protocol uses the same packet format as unicast DNS, and the same name structure, and the same DNS record types. The main difference is that queries are sent to a different UDP port (5353 instead of 53) and they are sent via multicast to address 224.0.0.251 or ff02::fb. Another important difference is that all "mDNS" names end in ".local."
When a user types "yourcomputer.local." into their Web browser, the presence of ".local." on the end of the name tells the host OS that the name should be looked up using local multicast instead of by sending that name to the worldwide DNS service for resolution. This helps reduce potential user confusion about whether a particular name is globally unique (e.g. "www.apple.com.") or whether that name has only local significance (e.g. "yourcomputer.local.").
Because Apple benefits more from widespread adoption of Bonjour than it would benefit from keeping Bonjour proprietary, Apple is making this code open so that other developers can use it too.
Because Apple recognises that networks are heterogeneous environments where devices run many different kinds of OS, this code has been made as portable as possible.
A typical mDNS program contains three components:
+------------------+
| Application |
+------------------+
| mDNS Core |
+------------------+
| Platform Support |
+------------------+
The "mDNS Core" layer is absolutely identical for all applications and all Operating Systems.
The "Platform Support" layer provides the necessary supporting routines that are specific to each platform -- what routine do you call to send a UDP packet, what routine do you call to join multicast group, etc.
The "Application" layer does whatever that particular application wants to do. It calls routines provided by the "mDNS Core" layer to perform the functions it needs:
- advertise services,
- browse for named instances of a particular type of service
- resolve a named instance to a specific IP address and port number
- etc.
The "mDNS Core" layer in turn calls through to the "Platform Support" layer to send and receive the multicast UDP packets to do the actual work.
Apple currently provides "Platform Support" layers for macOS, Microsoft Windows, and for POSIX platforms like Linux, Solaris, FreeBSD, etc.
Note: Developers writing applications for OS X do not need to incorporate this code into their applications, since OS X provides a system service to handle this for them. If every application developer were to link-in the mDNSResponder code into their application, then we would end up with a situation like the picture below:
+------------------+ +------------------+ +------------------+
| Application 1 | | Application 2 | | Application 3 |
+------------------+ +------------------+ +------------------+
| mDNS Core | | mDNS Core | | mDNS Core |
+------------------+ +------------------+ +------------------+
| Platform Support | | Platform Support | | Platform Support |
+------------------+ +------------------+ +------------------+
This would not be very efficient. Each separate application would be sending their own separate multicast UDP packets and maintaining their own list of answers. Because of this, macOS and POSIX versions of mDNSResponder provide a common system service which client software should access through the "/usr/include/dns_sd.h" APIs.
The situation on Apple and POSIX platforms looks more like the picture below:
-------------------
/ \
+---------+ +------------------+ +---------+ \ +---------+
| App 1 |<-->| daemon.c |<-->| App 2 | ->| App 3 |
+---------+ +------------------+ +---------+ +---------+
| mDNS Core |
+------------------+
| Platform Support |
+------------------+
In this configuration, applications make interprocess-communication (IPC) calls to the single mDNSResponder daemon which implements the mDNS and DNS-SD protocols.
Vendors of products such as printers, which are closed environments not expecting to be running third-party application software, can reasonably implement a single monolithic mDNSResponder to advertise all the services of that device. Vendors of open systems which run third-party application software should implement a system service such as the one provided by the OS X mDNSResponder daemon, and application software on that platform should, where possible, make use of that system service instead of embedding their own mDNSResponder.
See ReadMe.txt in the mDNSPosix directory for specific details of building an mDNSResponder on a POSIX Operating System.