DNS discovery leverages DNS-SD (DNS Service Discovery) queries to find services on a network. The method varies depending on whether the requested domain is local or not.
Understanding the Basics of DNS-SD
DNS-SD is a way for devices to advertise their services on a network and for clients to discover these services. It works by using standard DNS records to hold service information.
The Discovery Process
The discovery process is handled differently for local and non-local domains:
- Local Domain (.local):
- When an application queries for a name under the
.localtop-level domain (TLD), the query is sent using multicast DNS (mDNS). - This means the query is broadcast to all enabled network interfaces on the local network.
- Devices on the local network that offer the requested service respond directly.
- When an application queries for a name under the
- Non-Local Domains:
- If the query is not for a name under
.local, then a regular unicast DNS query is used. - This means the query is sent to a specific DNS server configured for the network, usually specified via DHCP.
- The DNS server, which might be a local resolver or a remote server on the internet, searches its database for relevant records and responds to the client.
- If the query is not for a name under
Key Differences Summarized
| Feature | Local Domain (.local) | Non-Local Domains |
|---|---|---|
| Query Type | Multicast DNS (mDNS) | Unicast DNS |
| Scope | Local Network | Wider network (potentially internet) |
| Target | Broadcast to all devices on the local network | Specific DNS server |
Practical Example
Imagine a printer on your local network which supports mDNS discovery for printer services.
- When you try to add a printer, your computer sends out an mDNS query on the network for "_ipp._tcp.local".
- The printer, which offers IPP printing services, listens for this query.
- The printer responds to your computer directly with its service details, including its IP address and port.
- Your computer can then connect to the printer without needing manual configuration.
In Summary
DNS discovery relies on DNS-SD queries and differs based on the domain type. Local domain queries are multicast, allowing devices to discover services in their immediate network; while non-local domain queries are unicast, relying on the standard DNS resolution process.
