Multicast DNS/DNS-SD daemon
is a Multicast Domain Name System
(MDNS) daemon which acts as the host mdns querier and responder.
supports both raw MDNS as well as
DNS-SD (Service Discovery) as described in the MDNS and DNS-SD drafts.
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
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
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 22.214.171.124 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
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
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
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).
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,
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.
supports multiple interfaces, the
interfaces used must be specified as the arguments.
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
The options are as follows:
- Do not daemonize. If this option is specified, debugging will be enabled,
mdnsd will run in the foreground and
log to stderr.
- Do not publish a workstation service on startup.
- Print version and exit.
- UNIX-domain socket used for communication with
is released under the ISC license.x
program version 0.1 was released in
13 February 2011.
Christiano Farina Haesbaert <firstname.lastname@example.org>
No proper error return in mdnsl.c.