resolved: properly handle LLMNR/TCP connection errors

The LLMNR spec suggests to do do reverse address lookups by doing direct LLMNR/TCP connections to the indicated
address, instead of doing any LLMNR multicast queries. When we do this and the peer doesn't actually implement LLMNR
this will result in a TCP connection error, which we need to handle. In contrast to most LLMNR lookups this will give
us a quick response on whether we can find a suitable name. Report this as new transaction state, since this should
mostly be treated like an NXDOMAIN rcode, except that it's not one.

src/basic/fd-util.h

@@ -74,5 +74,6 @@ void cmsg_close_all(struct msghdr *mh);
 
 bool fdname_is_valid(const char *s);
 
+/* Hint: ENETUNREACH happens if we try to connect to "non-existing" special IP addresses, such as ::5 */
 #define ERRNO_IS_DISCONNECT(r) \
-        IN_SET(r, ENOTCONN, ECONNRESET, ECONNREFUSED, ECONNABORTED, EPIPE)
+        IN_SET(r, ENOTCONN, ECONNRESET, ECONNREFUSED, ECONNABORTED, EPIPE, ENETUNREACH)

src/resolve/resolved-bus.c

@@ -62,6 +62,11 @@ static int reply_query_state(DnsQuery *q) {
         case DNS_TRANSACTION_NETWORK_DOWN:
                 return sd_bus_reply_method_errorf(q->request, BUS_ERROR_NETWORK_DOWN, "Network is down");
 
+        case DNS_TRANSACTION_NOT_FOUND:
+                /* We return this as NXDOMAIN. This is only generated when a host doesn't implement LLMNR/TCP, and we
+                 * thus quickly know that we cannot resolve an in-addr.arpa or ip6.arpa address. */
+                return sd_bus_reply_method_errorf(q->request, _BUS_ERROR_DNS "NXDOMAIN", "'%s' not found", dns_query_string(q));
+
         case DNS_TRANSACTION_RCODE_FAILURE: {
                 _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
 

src/resolve/resolved-dns-query.c

@@ -563,7 +563,8 @@ static int dns_query_synthesize_reply(DnsQuery *q, DnsTransactionState *state) {
                     DNS_TRANSACTION_NO_SERVERS,
                     DNS_TRANSACTION_TIMEOUT,
                     DNS_TRANSACTION_ATTEMPTS_MAX_REACHED,
-                    DNS_TRANSACTION_NETWORK_DOWN))
+                    DNS_TRANSACTION_NETWORK_DOWN,
+                    DNS_TRANSACTION_NOT_FOUND))
                 return 0;
 
         r = dns_synthesize_answer(

src/resolve/resolved-dns-transaction.c

@@ -432,6 +432,13 @@ static int on_stream_complete(DnsStream *s, int error) {
         if (ERRNO_IS_DISCONNECT(error)) {
                 usec_t usec;
 
+                if (t->scope->protocol == DNS_PROTOCOL_LLMNR) {
+                        /* If the LLMNR/TCP connection failed, the host doesn't support LLMNR, and we cannot answer the
+                         * question on this scope. */
+                        dns_transaction_complete(t, DNS_TRANSACTION_NOT_FOUND);
+                        return 0;
+                }
+
                 log_debug_errno(error, "Connection failure for DNS TCP stream: %m");
                 assert_se(sd_event_now(t->scope->manager->event, clock_boottime_or_monotonic(), &usec) >= 0);
                 dns_server_packet_lost(t->server, IPPROTO_TCP, t->current_feature_level, usec - t->start_usec);
@@ -1463,6 +1470,12 @@ int dns_transaction_go(DnsTransaction *t) {
                 dns_transaction_complete(t, DNS_TRANSACTION_RR_TYPE_UNSUPPORTED);
                 return 0;
         }
+        if (t->scope->protocol == DNS_PROTOCOL_LLMNR && ERRNO_IS_DISCONNECT(-r)) {
+                /* On LLMNR, if we cannot connect to a host via TCP when doing revers lookups. This means we cannot
+                 * answer this request with this protocol. */
+                dns_transaction_complete(t, DNS_TRANSACTION_NOT_FOUND);
+                return 0;
+        }
         if (r < 0) {
                 if (t->scope->protocol != DNS_PROTOCOL_DNS) {
                         dns_transaction_complete(t, DNS_TRANSACTION_RESOURCES);
@@ -2989,6 +3002,7 @@ static const char* const dns_transaction_state_table[_DNS_TRANSACTION_STATE_MAX]
         [DNS_TRANSACTION_NO_TRUST_ANCHOR] = "no-trust-anchor",
         [DNS_TRANSACTION_RR_TYPE_UNSUPPORTED] = "rr-type-unsupported",
         [DNS_TRANSACTION_NETWORK_DOWN] = "network-down",
+        [DNS_TRANSACTION_NOT_FOUND] = "not-found",
 };
 DEFINE_STRING_TABLE_LOOKUP(dns_transaction_state, DnsTransactionState);
 

src/resolve/resolved-dns-transaction.h

@@ -41,6 +41,7 @@ enum DnsTransactionState {
         DNS_TRANSACTION_NO_TRUST_ANCHOR,
         DNS_TRANSACTION_RR_TYPE_UNSUPPORTED,
         DNS_TRANSACTION_NETWORK_DOWN,
+        DNS_TRANSACTION_NOT_FOUND, /* like NXDOMAIN, but when LLMNR/TCP connections fail */
         _DNS_TRANSACTION_STATE_MAX,
         _DNS_TRANSACTION_STATE_INVALID = -1
 };