DNSServiceQueryRecord

DNSServiceQueryRecord is a function in Apple's DNS Service Discovery (DNS-SD) API used to query DNS resource records for a given fully qualified domain name. It can retrieve arbitrary records on the local multicast DNS network or on a conventional DNS server, depending on the system’s resolver configuration. The function is designed for asynchronous operation and is commonly used in Bonjour-enabled applications to obtain DNS data beyond the scope of service discovery.

The function takes several parameters: a pointer to a DNSServiceRef to identify the query, flags that modify behavior, an interfaceIndex to limit the query to a specific network interface (0 means all), the fullname of the DNS name to query, rrtype indicating the resource record type (for example, 1 for A, 28 for AAAA, 16 for TXT), rrclass (typically 1 for IN), a callback function of type DNSServiceQueryRecordReply, and a user-provided context pointer passed through to the callback. The callback is invoked for each matching resource record and may also signal completion or errors via an errorCode parameter. Each delivered record includes the record type, class, data length, data, and TTL.

Behavior and usage: DNSServiceQueryRecord returns immediately with a service reference; results arrive asynchronously through the callback, which may be called multiple times for multiple records. Applications must drive the run loop to receive events and eventually deallocate the DNSServiceRef when finished. This API supports both local (mDNS) and standard DNS resolution paths, depending on the network environment and resolver settings.

Common use cases include retrieving A/AAAA/TXT records for a host, obtaining additional data associated with a name, or validating DNS configurations published via DNS-SD. Typical errors include NoSuchRecord, BadParam, or NoMemory, which are reported to the callback.