Linux UDP Multicast

Setting up a UDP multicast server/client in Linux should be pretty straightforward. An example is provided below with some comments.

Get the files hosted at https://www.cubby.com/ using the link below:
UDP sender/listener example

UDP Multicast Sender in Linux (C/C++)
(udp_sender.cxx - see the link above)

1. Create a socket

This line creates a UDP connectionless socket
fd = socket( AF_INET, SOCK_DGRAM, 0 ) ;
 
2. Fill in information about the interface you will use

These lines fill a data structure that provide out going interface address and port number.
if_addr.sin_family = AF_INET ; if_addr.sin_addr.s_addr = inet_addr( INTERFACE_ADDR ) ; if_addr.sin_port = htons( INTERFACE_PORT ) ;
 
3. Bind the socket to the interface

This line binds the interface address/port number to the socket that was just created.

bind( fd, (struct sockaddr*) &if_addr, sizeof( if_addr ) ) ;
 
4. Fill in information about the multicast group you will use

Fill in another data structure that provides information about the destination multicast address and port number.
mc_addr.sin_family = AF_INET ; mc_addr.sin_addr.s_addr = inet_addr( MULTICAST_ADDR ) ; mc_addr.sin_port = htons( MULTICAST_PORT ) ;
 
5. Send the data

The sendto() command will transmit the data out of the socket and onto the network with the multicast destination address.

result = sendto( fd,
msg,
sizeof( msg ),
0,
(struct sockaddr *) &mc_addr,
sizeof( mc_addr ) ) ;

UDP Multicast Listener in Linux (C/C++)
(udp_listener.cxx - see the link above)

1. Create a socket

Create a UDP connectionless socket...add non-blocking reads
fd = socket( AF_INET, SOCK_DGRAM | SOCK_NONBLOCK, 0 ) ;


2. Fill in information about the interface you will use

In this case the listener can listen on any interface as long as the multicast port is correct.
if_addr.sin_family = AF_INET ; if_addr.sin_addr.s_addr = htonl( INADDR_ANY ) ; if_addr.sin_port = htons( MULTICAST_PORT ) ;


3. Bind the socket to the interface

Bind the socket to the 'interface'
bind( fd, (struct sockaddr *) &if_addr, sizeof( if_addr ) ) ;


4. Request to join the multicast group

This is the important part...here you indicate the multicast address you want to listen to and the interface that will listen for the multicast.
mreq.imr_multiaddr.s_addr = inet_addr( MULTICAST_ADDR ) ; mreq.imr_interface.s_addr = inet_addr( INTERFACE_ADDR ) ; ret_val = setsockopt( fd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof( mreq ) ) ;


5. Receive data

The recvfrom() function will read multicast data off the socket.
recv_bytes = recvfrom( fd,
msgbuf,
MSGBUFSIZE,
0,
(struct sockaddr *) &if_addr,
(socklen_t *) &addrlen ) ;