How exactly does mDNS resolve addresses?

Question

Once a service is discovered through DNS-SD, how exactly does the address of that host get resolved, and does it take significantly more time/overhead?

Also, if I am using JmDNS or Bonjour there are call-backs for both serviceFound and serviceResolved. If I am just interested in the IP address of the device publishing a certain service, is there a faster/more efficient way of getting the address than going through both serviceFound and serviceResolved?

Thanks

Answer 1

DNS-SD uses Multicast DNS (MDNS) which works by sending DNS packets over UDP to a certain multicast address. All mdns-capable hosts in the network also listen to this address. It uses UDP so, it's quite low overhead. Also, the clients are designed in a way that the amount of chatter on the network is kept to a minimum, by using extensive caching.

Service discovery is a two step process. The first step is finding the names of all hosts providing a certain service (e.g. printing). This will not yet give you the ip address, instead it gives you the mdns name (ending with .local). This is because the ip could possibly change, whereas the name will not.

The second step in service discovery is to resolve the .local name of the host over mdns. You ask via multicast who foo.local is, foo.local will see that packet, and respond via broadcast with its ip address, port number and other information.