CODEWITHSUNDEEP
clinuxbindmulticast
Karoly Horvath
Karoly Horvath

Reputation: 96286

multicast bind - Address already in use

I try to do bind a multicast port in my app. Previously the code always worked, but on this server it (often, but not always) fails...

The error message is Address already in use, which I don't quite understand, as it's possible to bind the same address from multiple applications (and even from the same application)...

What could cause this? I know someone would ask for it, so here is the code:

int fd = socket(PF_INET, SOCK_DGRAM, 0);
/* yes, that's a valid socket, verified.... */

u_int val = 1;
if(setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &val, sizeof(val)) < 0) {
    perror("Reusing ADDR failed");
    exit(1);
}

struct sockaddr_in saddr;
saddr.sin_family = PF_INET;
saddr.sin_addr.s_addr = inet_addr(multicast_group_ip);
saddr.sin_port = htons(port);
/* yes, valid multicast ip address and port, verified */

if(bind(fd, (struct sockaddr *) &saddr, sizeof(struct sockaddr_in)) == -1)
    //FAILS....

Upvotes: 1

Views: 7888

Answers (2)

geeves
geeves

Reputation: 141

You can actually have multiple datagram sockets bound to the same multicast group and port. However, all of these sockets should set the SO_REUSEADDR option. Based on your code you seem to be doing this, but it sounds like there is another process on your server that has bound a socket to the same multicast group and port without setting that option. The solution would be to ensure that all the processes on this particular server which are binding sockets to that multicast group and port set the SO_REUSEADDR option.

EDIT:
To respond to your recent comments above, one way you can actually reproduce this is to create two simple multicast listening programs that both bind a datagram socket to the same port and group. Have one of the programs set the SO_REUSEADDR option but not the other. Run the program that does not have SO_REUSEADDR set and you should see multicast data coming through. While leaving this program up, run the second program that does have SO_REUSEADDR set and you should see that it will not receive any multicast data while the first program is still receiving it (this should replicate the problem you originally described).

Finally, shut down both programs, then modify the first program to set SO_REUSEADDR, and repeat the steps above. You should now see both programs receiving the multicast traffic.

Upvotes: 3

Victor Sorokin
Victor Sorokin

Reputation: 12006

It's possible to have multiple open sockets on the same host:port from the one process, it's impossible to have more than process to be listening (i.e., bound) to the same host:port.

Perhaps, you have previous instance of your server process unkilled.

Another option mentioned by Wug is that you trying to bind to port lower than 1024 while not being root. Range of low ports 1-1024 is reserved for applications with effective UID == 0, e.g., started by root. seems to be wrong assumption since you'd get different error in such case, not already in use.

Upvotes: 0

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