DNSSD

DNSSD stands for DNS Service Discovery. It is a networking API and protocol framework that enables automatic discovery and advertisement of network services on local networks. Implemented as part of Apple's Bonjour suite, DNSSD helps applications find services without manual configuration.

DNSSD combines DNS-Based Service Discovery with multicast DNS. Services are advertised by type, such as _http._tcp.local., and discovered by clients on the local network. The system uses DNS records, including PTR, SRV, TXT, and A/AAAA, to locate a service instance and determine its host and port. The API is asynchronous and can be integrated with a run loop or dispatch queues.

Programmatic access is provided through the dns_sd.h header. Core operations include DNSServiceRegister (advertise a service), DNSServiceBrowse (discover services of a type), DNSServiceResolve (obtain host and port), and DNSServiceGetAddrInfo (fetch IP addresses). TXT records can carry additional metadata about a service.

Availability and portability: on macOS and iOS DNSSD is distributed as part of Bonjour. Windows and Linux environments provide ported or compatible implementations, enabling the same API style across platforms (for example, via libdns_sd or Avahi-compatible layers).

Standards and transport: DNS-SD is described in RFC 6763, with the underlying local-network transport provided by multicast DNS per RFC 6762. The combination allows zero-configuration service discovery on local networks using standard DNS records over multicast.

Use cases and limitations: DNSSD enables zero-configuration discovery of printers, media servers, file shares, and other services on local networks. It relies on local-network broadcasting and can be affected by firewalls or network segmentation; it is not intended for wide-area service discovery.