dns: do not treat unreachable DNS server as permanent error

getaddrinfo() does not distinguish between "DNS server not reachable"
and "DNS server told me host does not exist". For SMARTHOSTS it is
better to defer than to bounce in order to survive network outages.

Bug: fixes #30

Author: sig11b

dns.c

@@ -68,17 +68,33 @@ add_host(int pref, const char *host, int port, struct mx_hostentry **he, size_t
 	char servname[10];
 	struct mx_hostentry *p;
 	const int count_inc = 10;
-	int err;
 
 	memset(&hints, 0, sizeof(hints));
 	hints.ai_family = PF_UNSPEC;
 	hints.ai_socktype = SOCK_STREAM;
 	hints.ai_protocol = IPPROTO_TCP;
 
 	snprintf(servname, sizeof(servname), "%d", port);
-	err = getaddrinfo(host, servname, &hints, &res0);
-	if (err)
-		return (err == EAI_AGAIN ? 1 : -1);
+	switch (getaddrinfo(host, servname, &hints, &res0)) {
+	case 0:
+		break;
+	case EAI_AGAIN:
+	case EAI_NONAME:
+		/*
+		 * EAI_NONAME gets returned for:
+		 * SMARTHOST set but DNS server not reachable -> defer
+		 * SMARTHOST set but DNS server returns "host does not exist"
+		 *           -> buggy configuration
+		 *           -> either defer or bounce would be ok -> defer
+		 * MX entry was returned by DNS server but name doesn't resolve
+		 *           -> hopefully transient situation -> defer
+		 * all other DNS problems should have been caught earlier
+		 * in dns_get_mx_list().
+		 */
+		goto out;
+	default:
+		return(-1);
+	}
 
 	for (res = res0; res != NULL; res = res->ai_next) {
 		if (*ps + 1 >= roundup(*ps, count_inc)) {