multicast ip address - blocked in call to recvfrom

Question

i am writing a simple multicast application. i intend to run it on localhost.

i have done the following:

char *maddr;
.
.
.
sendfd = socket(...);

struct sockaddr_in sasend;
sasend.sin_family = AF_INET;
sasend.sin_port = htonl(portno);
inet_ntop(maddr, &(sasend.sin_addr.s_addr));


struct sockaddr_in sarecv;
memcpy(&sarecv, &sasend);

recvfd = socket(...);

const int on = 1;
setsockopt(recvfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));  // can you explain why
                                                                // this  is needed

bind(recvfd, &sarecv);

struct ip_mreq mreq;
memcpy(&mreq.imr_multiaddr, &(sasend.sin_addr));
mreq.imr_interface = htonl(INADDR_ANY);

setsockopt(recvfd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));

char flag = 1;
setsockopt(sendfd, IPPROTO_IP, IP_MULTICAST_LOOP, &flag, 1);

if (fork() == 0) {
    while (recvfrom(recvfd)) {
    }
}
else {
  while (sendto(sendfd)) {
    sleep(3);
  }
}

in the actual code i am checking the return values of all system calls. the problem is that recvfrom does not return. the process stays blocked in the call to recvfrom.

i have tried running two instances of the program on different shells. it does not help.

also i tried setting loopback flag to 0, it does not help.

i want to run both the programs from localhost.

the multicast address i am using is 239.255.1.2 which i have seen from the book. i think we can use any class D address as we are making the required setsockopt call.

connecting on port 1025

running linux kernel 2.6.25

also how do i check if multicasting support has been compiled into the kernel.

Update:

i did route add -net 224.0.0.0 netmask 240.0.0.0 dev eth0 on the shell. still the problem exists.

Answer 1

ok i disabled the firewall and i could get the program running. got help from here

now more problems:

• how to add rules to the firewall to specifically my code to run
• if i do not disable loopback, then i do not get the messages from another process sending on the same multicast group, and if i enable it then my process receives the messages it transmits. this is because i am running both the programs on localhost. is there a way around.