Bonjour basics

Bonjour exchanges information by means of individual multicast DNS packets (mDNS) according to RFC 6762 and DNS-based service discovery (DNS-SD) according to RFC 6763. The clients exchange Bonjour information via the multicast address 224.0.0.251 (IPv4) or ff02::fb (IPv6) on port 5353. Bonjour packets are not routed (multicast packet, TTL = 1), which limits their use to the current local area network.

Note: Please note that the Bonjour proxy only serves to aid the discovery of Bonjour services. The actual routing between the communicating parties requires a separate configuration or restriction by means of, for example, routing or firewall entries.

It is often impractical to provide all services on a single network. This is why larger networks are often divided into several subnets. However, Bonjour is unable to operate in this situation.

Example application with two networks

At a school, students use a dedicated IP network to access the WLAN. In parallel to this, the local printer is made available on a second internal IP network. In principal, the appropriate routing and restrictions would make it possible for students to use their smartphones to access the local internal printer. However, because mDNS is only defined as link-local, Bonjour is unable to help students to discover the printer with their smartphones. The LANCOM Bonjour proxy mediates between two networks, which enables students to discover printers in other networks.

Basically, there are two ways of realizing such a scenario:

Multicast routing: A router forwards the search queries and service advertisements between the two networks.
Note: This option causes unnecessary traffic, which makes it rather inefficient.
Caching of services: The router stores discovered mDNS service advertisements in its local cache. A router that receives an mDNS query then responds on behalf of the original service. Before processing the advertisement and before transmitting anything from the cache, the router checks its policies to see whether the service is approved or blocked. The policies are used to control which services are approved for discovery and between which networks.
Note: Please note that reading out the mDNS cache content with the SNMP protocol is not supported.

The Bonjour proxy supports an mDNS query client, which at set time intervals queries an interface about the services of interest. This query keeps the cache entries for approved services up to date. In order for the cache to be up-to-date at all times, it is useful to enable automatic searches for services that are permanently available (e.g. print services).

Note: If no automatic queries about frequently used services are configured, the Bonjour proxy may be unable to respond to the corresponding queries even though the services are approved.

Bonjour proxies only operate on logical LAN / WLAN interfaces or on logical networks with an IP address. WAN interfaces / remote sites or tunnels (except for WLC L3 tunnels) and VLANs without address binding are not supported.