In computer networking, the multicast Domain Name System (mDNS) resolves host names to IP addresses within small networks that do not include a.local name server. It is a zero-configuration service, using essentially the same programming interfaces, packet formats and operating semantics as the unicast Domain Name System (DNS). Although Stuart Cheshire designed mDNS to be stand-alone capable, it can work in concert with unicast DNS servers.
Contents
The mDNS protocol is published as RFC 6762, uses IP multicast User Datagram Protocol (UDP) packets, and is implemented by the Apple Bonjour and open source Avahi (software) software packages.
mDNS can work in conjunction with DNS Service Discovery (DNS-SD), a companion zero-configuration technique specified separately in RFC 6763.
Protocol overview
When an mDNS client needs to resolve a host name, it sends an IP multicast query message that asks the host having that name to identify itself. That target machine then multicasts a message that includes its IP address. All machines in that subnet can then use that information to update their mDNS caches.
Any host can relinquish its claim to a domain name by sending a response packet with a time to live (TTL) equal to zero.
By default, mDNS only and exclusively resolves host names ending with the .local top-level domain (TLD). This can cause problems if that domain includes hosts which do not implement mDNS but which can be found via a conventional unicast DNS server. Resolving such conflicts requires network-configuration changes that violate the zero-configuration goal.
Packet structure
An mDNS Ethernet frame is a multicast UDP packet to:
01:00:5E:00:00:FB (for IPv4) or 33:33:00:00:00:FB (for IPv6)224.0.0.251 or IPv6 address FF02::FB5353The payload structure is based on the unicast DNS packet format, consisting of two parts—the header and the data. The header is identical to that found in unicast DNS, as are the sub-sections in the data part: queries, answers, authoritative-nameservers, and additional records. The number of records in each sub-section matches the value of the corresponding *COUNT field in the header.
Queries
The wire format for records in the query section is slightly modified from that in unicast DNS, adding one single-bit field.
As in unicast DNS, the QNAME field consists of a series of length/value sub-fields called "labels". Each label represents one of the dot-separated substrings in a fully qualified domain name (FQDN). The list is terminated by a single null-byte, representing the "root" of the DNS.
The UNICAST-RESPONSE field is used to minimize unnecessary broadcasts on the network: if the bit is set, responders SHOULD send a directed-unicast response directly to the inquiring node rather than broadcasting the response to the entire network.
The QCLASS field is identical to that found in unicast DNS.
Resource Records
All records in the answers, authoritative-nameservers, and additional records sections have the same format and are collectively known as "Resource Records" (RR).
Resource Records in mDNS also have a slightly modified general format from unicast DNS:
The CACHE-FLUSH bit is used to instruct neighbor-nodes that the record should overwrite, rather than be appended onto, any existing cached entries for this RRNAME and RRTYPE.
The formats of the RDATA fields are the same as those found in unicast DNS. However, DNS Service Discovery (DNS-SD), the most common use-case for mDNS, specifies slight modifications to some of their formats (notably TXT records).
Example
Trying to ping the appletv.local host would cause an mDNS client computer to multicast the following UDP packet:
The appletv.local host would respond by multicasting its mDNS response packet. For example:
00 00 84 00 00 00 00 01 00 00 00 02 07 61 70 706c 65 74 76 05 6c 6f 63 61 6c 00 00 01 80 01 0000 78 00 00 04 99 6d 07 5a c0 0c 00 1c 80 01 0000 78 00 00 10 fe 80 00 00 00 00 00 00 02 23 32ff fe b1 21 52 c0 0c 00 2f 80 01 00 00 78 00 0008 c0 0c 00 04 40 00 00 08In its header, the non-zero fields are the Flags word (84 00), the ANCOUNT word (00 01), and the ARCOUNT word (00 02). The data again begin with the FQDN (hex 07 61 70 70 6c 65 74 76 05 6c 6f 63 61 6c 00 for appletv.local), followed by that host's DNS information: