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POMulticast groups Linux - Joining and Leaving
 

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    <p>I am building a overlay network system to test a protocol (Uni research). I need to join and leave multicast groups in order to receive packets from different sources. </p> <p>I am unsure about the correct setup of the socket so I can close the socket and leave the multicast group and then rejoin the same multicast group later. When I try to join the same multicast group I get "<code>bind error: Address already in use</code>". </p> <pre><code>//for setting up individual groups int setUpForGroup(struct locgro_node* node, const char* port) { char mcastaddr[INET6_ADDRSTRLEN]; struct in6_addr* full_addr_gro = &amp;(node-&gt;group); if( NULL == inet_ntop(AF_INET6, full_addr_gro, mcastaddr, INET6_ADDRSTRLEN)) { printf("error inet_pton, retval: \n"); return -1; } if (buildAdd(mcastaddr, port, AF_INET6, SOCK_DGRAM, &amp;(node-&gt;addr_st)) &lt;0) { fprintf(stderr, "get_addr error:: could not find multicast, address=[%s] port=[%s]\n", mcastaddr, port); return -1; } node-&gt;sockfd = socket(AF_INET6, SOCK_DGRAM, 0); if (bind(node-&gt;sockfd, (struct sockaddr *)&amp;(node-&gt;addr_st), sizeof(node-&gt;addr_st)) &lt; 0) { perror("bind error:: "); close(node-&gt;sockfd); return -1; } if (joinGroup(node-&gt;sockfd, 0 , 8, &amp;(node-&gt;addr_st)) &lt;0) { close(node-&gt;sockfd); return -1; } return 0; } //internal function int joinGroup(int sockfd, int loopBack, int mcastTTL, struct sockaddr_in6 *addr_st) { int r1, r2, r3, retval; retval=-1; struct ipv6_mreq mreq6; memcpy(&amp;mreq6.ipv6mr_multiaddr, &amp;((addr_st)-&gt;sin6_addr), sizeof(struct in6_addr)); mreq6.ipv6mr_interface= 0; // allow any interface //set the loopback case r1 = setsockopt(sockfd, IPPROTO_IPV6, IPV6_MULTICAST_LOOP, &amp;loopBack, sizeof(loopBack)); if (r1&lt;0) perror("joinGroup:: IPV6_MULTICAST_LOOP:: "); /* setsockopt(sock_fd, IPPROTO_IPV6, SO_REUSEADDR, &amp;mreq6, sizeof(mreq6)); setsockopt(sock_fd, IPPROTO_IPV6, IPV6_DROP_MEMBERSHIP, &amp;multicast_req, sizeof(multicast_req)); */ //set the time to live for the packets (hops) r2 = setsockopt(sockfd, IPPROTO_IPV6, IPV6_MULTICAST_HOPS, &amp;mcastTTL, sizeof(mcastTTL)); if (r2&lt;0) perror("joinGroup:: IPV6_MULTICAST_HOPS:: "); //add this address to the group r3 = setsockopt(sockfd, IPPROTO_IPV6, IPV6_ADD_MEMBERSHIP, &amp;mreq6, sizeof(mreq6)); if (r3&lt;0) perror("joinGroup:: IPV6_ADD_MEMBERSHIP:: "); if ((r1&gt;=0) &amp;&amp; (r2&gt;=0) &amp;&amp; (r3&gt;=0)) retval=0; return retval; } //internal function int buildAdd(const char *hostname, const char *service, int family, int socktype, struct sockaddr_in6 *addr_st) { struct addrinfo hints, *res, *ressave; int n, sockfd, retval; retval = -1; memset(&amp;hints, 0, sizeof(struct addrinfo)); hints.ai_family = family; hints.ai_socktype = socktype; n = getaddrinfo(hostname, service, &amp;hints, &amp;res); if (n &lt;0) { fprintf(stderr, "getaddrinfo error: [%s]\n", gai_strerror(n)); return retval; } ressave = res; sockfd=-1; while(res) { sockfd = socket(res-&gt;ai_family, res-&gt;ai_socktype, res-&gt;ai_protocol); if (!(sockfd &lt; 0)) { int opval = 1; setsockopt(sockfd, IPPROTO_IPV6, SO_REUSEADDR, &amp;opval, res-&gt;ai_addrlen); if (bind(sockfd, res-&gt;ai_addr, res-&gt;ai_addrlen) == 0) { //to test that the address really is correct close(sockfd); memcpy(addr_st, res-&gt;ai_addr, sizeof(*addr_st)); retval=0; break; } perror("build addr : bind error"); close(sockfd); sockfd=-1; } res=res-&gt;ai_next; } freeaddrinfo(ressave); //free the struct return retval; } </code></pre> <p>This are the functions I used for joining a group. When leaving I simply do: (that happens inside another function)</p> <pre><code> struct ipv6_mreq mreq6; memcpy(&amp;mreq6.ipv6mr_multiaddr, &amp;(temp_lnode-&gt;group), sizeof(struct in6_addr)); mreq6.ipv6mr_interface= 0; // allow any interface setsockopt(temp_lnode-&gt;sockfd, IPPROTO_IPV6, IPV6_DROP_MEMBERSHIP, &amp;mreq6, sizeof(mreq6)); close(temp_lnode-&gt;sockfd); //close socket </code></pre> <p>I am getting a "Address already in use" error when I try to bind the address inside the build address function. I tried solving that by setting SO_REUSEADDR but it didn't help, also added the DROP_MEMBERSHIP but bind still fails. </p> <p>Do I need to bind in order to get it to work? What should I call or do to allow for joining and leaving of groups without this issues? I would need to do this during intervals of 30s. </p> <p>Thanks a lot M</p>
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    1. COProblem solved. Using SO_REUSEADDR when creating the groups for the first time and using DROP_MEMBERSHIP do the trick. I am not sure which one did the actual fix, as I realised I wasn't seeing DROP_MEMBERSHIP in one case, either way I read that it is good practice to set SO_REUSEADDR always when dealing with multicast. Cheers, M
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    2. CO@ unixsnob: good that you solved it youself. You can probably put that as your answer.. Offtopic: Please void comments in code- which are really trivial : for example //free the struct has not value.
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