+#ifdef IPV6_V6ONLY
+ /* Only communicate in IPv6 over AF_INET6 sockets. */
+ if (ai->ai_family == AF_INET6) {
+ if (setsockopt(listen_sock, IPPROTO_IPV6, IPV6_V6ONLY,
+ &on, sizeof(on)) == -1)
+ error("setsockopt IPV6_V6ONLY: %s",
+ strerror(errno));
+ }
+#endif
+