Always set IPV6_ONLY option, either to on or off

src/civetweb.c

@@ -13809,6 +13809,7 @@ set_ports_option(struct mg_context *phys_ctx)
 #endif
 
 		if (ip_version > 4) {
+/* Could be 6 for IPv6 onlyor 10 (4+6) for IPv4+IPv6 */
 #if defined(USE_IPV6)
 			if (ip_version > 6) {
 				if (so.lsa.sa.sa_family == AF_INET6
@@ -13821,7 +13822,21 @@ set_ports_option(struct mg_context *phys_ctx)
 					/* Set IPv6 only option, but don't abort on errors. */
 					mg_cry_internal(
 					    fc(phys_ctx),
-					    "cannot set socket option IPV6_V6ONLY (entry %i)",
+					    "cannot set socket option IPV6_V6ONLY=off (entry %i)",
+					    portsTotal);
+				}
+			} else {
+				if (so.lsa.sa.sa_family == AF_INET6
+				    && setsockopt(so.sock,
+				                  IPPROTO_IPV6,
+				                  IPV6_V6ONLY,
+				                  (void *)&on,
+				                  sizeof(on)) != 0) {
+
+					/* Set IPv6 only option, but don't abort on errors. */
+					mg_cry_internal(
+					    fc(phys_ctx),
+					    "cannot set socket option IPV6_V6ONLY=on (entry %i)",
 					    portsTotal);
 				}
 			}