NAME

SYNOPSIS

DESCRIPTION

MDNS is a way to perform DNS-like operations via multicast in the local link, there is no hierarchy or multiple domains as in conventional unicast DNS. MDNS provides a way for hosts to co-operate and maintain a cache name database which can then be used to resolve local host names without the need of a central DNS server.

DNS-SD is a convention on some names used in MDNS to provide hosts with Service Discovery capabilities.

A host can publish a service of any type, be it a HTTP server, NTP server, a Printer and so on, this services can then be browsed and resolved by the other hosts in the local network.

There are basically two roles in a MDNS environment, the Querier, and the Responder.

The Querier is the entity responsible for sending questions and MDNS requests in the local link, it can't be done as it is in libc, where each process does its own lookup, there must be something centralizing all the requests as there are various complications implied: cache, timers and so on.

The Responder is the entity responsible for answering those queries, there should be only one responder per host. Both roles are performed by MDNS.

MDNS operates on the All-Link-Local Multicast address 224.0.0.251 under UDP port 5353. There are no multiple domains in MDNS as in unicast DNS, the .local domain name is the single MDNS domain name and it's where all the queries and answers take place.

There are three basic types of MDNS question, in which mdnsd uses two of them.

The One-Shot Query, which resembles unicast DNS, where a single question is sent and an answer is expected, if no answer is received it means no one can answer that question. This question is used for simple lookups.

Continuous Multicast Query is a more complex way of querying, the querier will send the same question multiple times, doubling the interval between each time,
multiple answers may be received, it's used as a way for monitoring the Resource Records of the network. This question is mainly used by network browsing in DNS-SD, where a question for a type of service may enumerate one or more instances, for example, if browsing for the HTTP servers, there may be one or more servers (instances).

To diminish the volume of redundant answers, a feature called Known Answer Suppression is present, in which the querier when performing a Continuous Multicast Query places all the previous known answers in the additional section of the MDNS packet, thus, any answer that would be given which is already in the additional section, is suppressed.

There are two type of Resource Records, Unique and Shared.

Unique records are the ones which there may be only one answer for it in the local name, the A, PTR and HINFO under the hostname.local name are examples of Unique records, it would be strange if two hosts would answer an address for the same foobar.local. All Unique records must be Probed to verify its uniqueness, if a conflict is found, another name must be chosen (Unimplemented).

A Shared record is used for PTR records in DNS-SD, a host may have as many answers as necessary for a shared record, it's used only in network browsing, where there may be multiple instances of the same service. To access the MDNS services, a libmdns library will be provided in the near future, programs will then, be able to link with libmdns and publish its own services though MDNS. By now, only mdnsctl(8) is provided which is a command line interface to the daemon in the same fashion as ripctl(8) and ospfctl(8).

When mdnsd starts up, it probes for its hostname (fetched from /etc/myname), if there isn't a conflict, it publishes an A and a PTR record for itself, both records will be under the .local domain, which is the MDNS single domain name. All the other domain names in /etc/myname will be stripped, therefore foo.bar.midearth becomes foo.local, which can be resolved through MDNS. If a conflict is found, then, there is another foo.local in the network and conflict resolution takes place.

If -w was not specified, mdnsd will also publish a Workstation service, this service has no data itself, it's used to state that the host is up, it can be used for example, to browse every powered host on the local network.

mdnsd supports multiple interfaces, the interfaces used must be specified as the arguments. mdnsd must be started as root and upon start up it will drop privileges, change it's euid/egid to _mdnsd and chroot. Therefore make sure you have user and group _mdnsd created.

MDNS operations can be done with the mdnsctl(8) utility.

The options are as follows:

-d

Do not daemonize. If this option is specified, debugging will be enabled, mdnsd will run in the foreground and log to stderr.

-w

Do not publish a workstation service on startup.

-v

Print version and exit.

FILES

/var/run/mdnsd.sock

UNIX-domain socket used for communication with mdnsctl(8).

SEE ALSO

MDNS Draft, http://files.multicastdns.org/draft-cheshire-dnsext-multicastdns.txt, July 2010.

DNS-SD Draft, http://files.dns-sd.org/draft-cheshire-dnsext-dns-sd.txt, January 2011.

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