IO::Socket::Multicast(3) Send and receive multicast messages


use IO::Socket::Multicast;
# create a new UDP socket ready to read datagrams on port 1100
my $s = IO::Socket::Multicast->new(LocalPort=>1100);
# Add a multicast group
# Add a multicast group to eth0 device
# now receive some multicast data
# Drop a multicast group
# Set outgoing interface to eth0
# Set time to live on outgoing multicast packets
# Turn off loopbacking
# Multicast a message to group
$s->mcast_send('hello world!','');
$s->mcast_send('hello again!');


The IO::Socket::Multicast module subclasses IO::Socket::INET to enable you to manipulate multicast groups. With this module (and an operating system that supports multicasting), you will be able to receive incoming multicast transmissions and generate your own outgoing multicast packets.

This module requires IO::Interface version 0.94 or higher.


Multicasting is designed for streaming multimedia applications and for conferencing systems in which one transmitting machines needs to distribute data to a large number of clients.

IP addresses in the range and are reserved for multicasting. These addresses do not correspond to individual machines, but to multicast groups. Messages sent to these addresses will be delivered to a potentially large number of machines that have registered their interest in receiving transmissions on these groups. They work like TV channels. A program tunes in to a multicast group to receive transmissions to it, and tunes out when it no longer wishes to receive the transmissions.

To receive transmissions from a multicast group, you will use IO::Socket::Multicast->new() to create a UDP socket and bind it to a local network port. You will then subscribe one or more multicast groups using the mcast_add() method. Subsequent calls to the standard recv() method will now receive messages incoming messages transmitted to the subscribed groups using the selected port number.

To send transmissions to a multicast group, you can use the standard send() method to send messages to the multicast group and port of your choice. The mcast_set() and mcast_send() methods are provided as convenience functions. Mcast_set() will set a default multicast destination for messages which you then send with mcast_send().

To set the number of hops (routers) that outgoing multicast messages will cross, call mcast_ttl(). To activate or deactivate the looping back of multicast messages (in which a copy of the transmitted messages is received by the local machine), call mcast_loopback().


$socket = IO::Socket::Multicast->new([LocalPort=>$port,...])
The new() method is the constructor for the IO::Socket::Multicast class. It takes the same arguments as IO::Socket::INET, except that the Proto argument, rather than defaulting to ``tcp'', will default to ``udp'', which is more appropriate for multicasting.

To create a UDP socket suitable for sending outgoing multicast messages, call new() without arguments (or with "Proto=>'udp'"). To create a UDP socket that can also receive incoming multicast transmissions on a specific port, call new() with the LocalPort argument.

If you plan to run the client and server on the same machine, you may wish to set the IO::Socket ReuseAddr argument to a true value. This allows multiple multicast sockets to bind to the same address.


$success = $socket->mcast_add($multicast_address [,$interface])
The mcast_add() method will add the provided multicast address to the list of subscribed multicast groups. The address may be provided either as a dotted-quad decimal, or as a packed IP address (such as produced by the inet_aton() function). On success, the method will return a true value.

The optional $interface argument can be used to specify on which network interface to listen for incoming multicast messages. If the IO::Interface module is installed, you may use the device name for the interface (e.g. ``tu0''). Otherwise, you must use the IP address of the desired network interface. Either dotted quad form or packed IP address is acceptable. If no interface is specified, then the multicast group is joined on INADDR_ANY, meaning that multicast transmissions received on any of the host's network interfaces will be forwarded to the socket.

Note that mcast_add() operates on the underlying interface(s) and not on the socket. If you have multiple sockets listening on a port, and you mcast_add() a group to one of those sockets, subsequently all the sockets will receive mcast messages on this group. To filter messages that can be received by a socket so that only those sent to a particular multicast address are received, pass the LocalAddr option to the socket at the time you create it:

  my $socket = IO::Socket::Multicast->new(LocalPort=>2000,

By combining this technique with IO::Select, you can write applications that listen to multiple multicast groups and distinguish which group a message was addressed to by identifying which socket it was received on.

$success = $socket->mcast_drop($multicast_address)
This reverses the action of mcast_add(), removing the indicated multicast address from the list of subscribed groups.
$loopback = $socket->mcast_loopback
$previous = $socket->mcast_loopback($new)
The mcast_loopback() method controls whether the socket will receive its own multicast transmissions (default yes). Called without arguments, the method returns the current state of the loopback flag. Called with a boolean argument, the method will set the loopback flag, and return its previous value.
$ttl = $socket->mcast_ttl
$previous = $socket->mcast_ttl($new)
The mcast_ttl() method examines or sets the time to live (TTL) for outgoing multicast messages. The TTL controls the numbers of routers the packet can cross before being expired. The default TTL is 1, meaning that the message is confined to the local area network. Values between 0 and 255 are valid.

Called without arguments, this method returns the socket's current TTL. Called with a value, this method sets the TTL and returns its previous value.

$interface = $socket->mcast_if
$previous = $socket->mcast_if($new)
By default, the OS will pick the network interface to use for outgoing multicasts automatically. You can control this process by using the mcast_if() method to set the outgoing network interface explicitly. Called without arguments, returns the current interface. Called with the name of an interface, sets the outgoing interface and returns its previous value.

You can use the device name for the interface (e.g. ``tu0'') if the IO::Interface module is present. Otherwise, you must use the interface's dotted IP address.

NOTE: To set the interface used for incoming multicasts, use the mcast_add() method.

$dest = $socket->mcast_dest
$previous = $socket->mcast_dest($new)
The mcast_dest() method is a convenience function that allows you to set the default destination group for outgoing multicasts. Called without arguments, returns the current destination as a packed binary sockaddr_in data structure. Called with a new destination address, the method sets the default destination and returns the previous one, if any.

Destination addresses may be provided as packed sockaddr_in structures, or in the form ``XX.XX.XX.XX:YY'' where the first part is the IP address, and the second the port number.

$bytes = $socket->mcast_send($data [,$dest])
Mcast_send() is a convenience function that simplifies the sending of multicast messages. $data is the message contents, and $dest is an optional destination group. You can use either the dotted IP form of the destination address and its port number, or a packed sockaddr_in structure. If the destination is not supplied, it will default to the most recent value set in mcast_dest() or a previous call to mcast_send().

The method returns the number of bytes successfully queued for delivery.

As a side-effect, the method will call mcast_dest() to remember the destination address.


  $socket->mcast_send('Hi there group members!','') || die;
  $socket->mcast_send("How's the weather?") || die;

Note that you may still call IO::Socket::Multicast->new() with a PeerAddr, and IO::Socket::INET will perform a connect(), creating a default destination for calls to send().


The following is an example of a multicast server. Every 10 seconds it transmits the current time and the list of logged-in users to the local network using multicast group, port 2000 (these are chosen arbitrarily).

 # server
 use strict;
 use IO::Socket::Multicast;
 use constant DESTINATION => ''; 
 my $sock = IO::Socket::Multicast->new(Proto=>'udp',PeerAddr=>DESTINATION);
 while (1) {
   my $message = localtime;
   $message .= "\n" . `who`;
   $sock->send($message) || die "Couldn't send: $!";
 } continue {
   sleep 10;

This is the corresponding client. It listens for transmissions on group, port 2000, and echoes the messages to standard output.

 # client
 use strict;
 use IO::Socket::Multicast;
 use constant GROUP => '';
 use constant PORT  => '2000';
 my $sock = IO::Socket::Multicast->new(Proto=>'udp',LocalPort=>PORT);
 $sock->mcast_add(GROUP) || die "Couldn't set group: $!\n";
 while (1) {
   my $data;
   next unless $sock->recv($data,1024);
   print $data;


None by default. However, if you wish to call mcast_add(), mcast_drop(), mcast_if(), mcast_loopback(), mcast_ttl, mcast_dest() and mcast_send() as functions you may import them explicitly on the use line or by importing the tag ``:functions''.


The mcast_if(), mcast_ttl() and mcast_loopback() methods will cause a crash on versions of Linux earlier than 2.2.0 because of a kernel bug in the implementation of the multicast socket options.


Lincoln Stein, [email protected].

This module is distributed under the same terms as Perl itself.