Discovering all advertised Bonjour service types

Q:  Is it possible to dynamically discover every Bonjour service type being advertised on the network?

A: Is it possible to dynamically discover every Bonjour service type being advertised on the network?

Yes. Devices running mDNSResponder-58.6 (Mac OS X 10.3.4) or later will respond to the "Service Type Enumeration" meta-query described in Section 10 of the DNS-SD specification. Issuing a Multicast DNS PTR record query for the name "_services._dns-sd._udp.local." will return a list of service types being advertised on the local network.

Queries for "_services._dns-sd._udp.local." behave somewhat differently than standard mDNS queries. Normal mDNS long-lived queries will return ADD/REMOVE events as records are registered and deregistered, however, a service type enumeration meta-query will not be told about REMOVE events when a service is deregistered. Otherwise, the meta-query behaves identically to a standard mDNS query. It will receive REMOVE events when a network interface is disabled or if the PTR record expires from the cache naturally.

You can issue this meta-query by using DNSServiceQueryRecord from the socket-based DNSServiceDiscovery API, located in /usr/include/dns_sd.h. For each DNS-SD service type being advertised, the callback will return a DNS PTR record containing a service type and domain. For example, if someone is advertising an HTTP service on the local network, the DNSServiceQueryRecord callback will return a PTR record which points to "_http._tcp.local.".

The DNSServiceMetaQuery sample code shows how to issue the service type enumeration meta-query.

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