Address https://github.com/cesanta/mongoose/discussions/2305 - bind to v6 only / v6 and v4

mongoose.c

@@ -6062,7 +6062,8 @@ bool mg_open_listener(struct mg_connection *c, const char *url) {
       // won't work! (setsockopt will return EINVAL)
       MG_ERROR(("setsockopt(SO_REUSEADDR): %d", MG_SOCK_ERR(rc)));
 #endif
-#if defined(IPV6_V6ONLY)
+#if MG_IPV6_V6ONLY
+      // Bind only to the V6 address, not V4 address on this port
     } else if (c->loc.is_ip6 &&
                (rc = setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY, (char *) &on,
                                 sizeof(on))) != 0) {

mongoose.h

@@ -709,6 +709,10 @@ struct timeval {
 #define MG_ENABLE_IPV6 0
 #endif
 
+#ifndef MG_IPV6_V6ONLY
+#define MG_IPV6_V6ONLY 0  // IPv6 socket binds only to V6, not V4 address
+#endif
+
 #ifndef MG_ENABLE_MD5
 #define MG_ENABLE_MD5 1
 #endif

src/config.h

@@ -44,6 +44,10 @@
 #define MG_ENABLE_IPV6 0
 #endif
 
+#ifndef MG_IPV6_V6ONLY
+#define MG_IPV6_V6ONLY 0  // IPv6 socket binds only to V6, not V4 address
+#endif
+
 #ifndef MG_ENABLE_MD5
 #define MG_ENABLE_MD5 1
 #endif

src/sock.c

@@ -212,7 +212,8 @@ bool mg_open_listener(struct mg_connection *c, const char *url) {
       // won't work! (setsockopt will return EINVAL)
       MG_ERROR(("setsockopt(SO_REUSEADDR): %d", MG_SOCK_ERR(rc)));
 #endif
-#if defined(IPV6_V6ONLY)
+#if MG_IPV6_V6ONLY
+      // Bind only to the V6 address, not V4 address on this port
     } else if (c->loc.is_ip6 &&
                (rc = setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY, (char *) &on,
                                 sizeof(on))) != 0) {