mDNSResponder

mDNSResponder is the daemon that implements multicast DNS (mDNS) and DNS Service Discovery (DNS-SD), forming the local-network discovery and name resolution component of Apple's Bonjour technology. It enables devices on the same local area network to resolve hostnames ending in .local and to discover services advertised by other devices without a central DNS server.

Technically, mDNSResponder listens for mDNS queries over UDP on IPv4 multicast address 224.0.0.251 and IPv6 multicast address FF02::FB, using port 5353. It also advertises services via DNS-SD by publishing DNS records for service instances, types, and domains such as _http._tcp.local. This enables clients to browse and resolve services using human-friendly names on a local network.

Platforms and usage: It is bundled with macOS and iOS as part of the Bonjour suite and is widely used by printers, media servers, and other network devices. The mDNS/DNS-SD mechanism has been implemented on other platforms through ports and independent projects (for example, Avahi on Linux) to provide similar functionality.

Open-source status: Apple has released mDNSResponder as open-source software under a permissive license, allowing developers to study, modify, and port the code for non-Apple environments.