Dual mode IPv4/IPv6 stack support

src/udprelay.c

@@ -312,26 +312,59 @@ int create_remote_socket(int ipv6)
 int create_server_socket(const char *host, const char *port)
 {
     struct addrinfo hints;
-    struct addrinfo *result, *rp;
+    struct addrinfo *result, *rp, *ipv4v6bindall;
     int s, server_sock;
 
     memset(&hints, 0, sizeof(struct addrinfo));
     hints.ai_family = AF_UNSPEC;    /* Return IPv4 and IPv6 choices */
     hints.ai_socktype = SOCK_DGRAM; /* We want a UDP socket */
-
+    hints.ai_flags = AI_ALL | AI_V4MAPPED | AI_PASSIVE | AI_ADDRCONFIG; /* For wildcard IP address */
+    hints.ai_protocol = IPPROTO_UDP;
+	
     s = getaddrinfo(host, port, &hints, &result);
     if (s != 0) {
         LOGE("[udp] getaddrinfo: %s", gai_strerror(s));
         return -1;
     }
 
-    for (rp = result; rp != NULL; rp = rp->ai_next) {
+    rp = result;
+
+    /*
+       On Linux, with net.ipv6.bindv6only = 0 (the default), getaddrinfo(NULL) with
+       AI_PASSIVE returns 0.0.0.0 and :: (in this order). AI_PASSIVE was meant to
+       return a list of addresses to listen on, but it is impossible to listen on
+       0.0.0.0 and :: at the same time, if :: implies dualstack mode.
+     */
+    if (!host) {
+        ipv4v6bindall = result;
+
+        /* Loop over all address infos found until a IPV6 address is found. */
+        while (ipv4v6bindall) {
+            if (ipv4v6bindall->ai_family == AF_INET6) {
+                rp = ipv4v6bindall; /* Take first IPV6 address available */
+                break;
+            }
+
+            ipv4v6bindall = ipv4v6bindall->ai_next; /* Get next address info, if any */
+        }
+    }
+	
+    for (/*rp = result*/; rp != NULL; rp = rp->ai_next) {
         server_sock = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol);
         if (server_sock == -1) {
             continue;
         }
 
         int opt = 1;
+		
+        if (rp->ai_family == AF_INET6) {
+            int ipv6only = 0;
+            if (host) {
+                ipv6only = 1;
+            }
+            setsockopt(server_sock, IPPROTO_IPV6, IPV6_V6ONLY, &ipv6only, sizeof(ipv6only));
+        }
+		
         setsockopt(server_sock, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
 #ifdef SO_NOSIGPIPE
         set_nosigpipe(server_sock);