Add debug logging to DNS SRV requests. (#9305)

Author: Bubu

changelog.d/9305.misc

@@ -0,0 +1 @@
+Add debug logging for SRV lookups. Contributed by @Bubu.

synapse/http/federation/matrix_federation_agent.py

@@ -323,12 +323,19 @@ async def _resolve_server(self) -> List[Server]:
         if port or _is_ip_literal(host):
             return [Server(host, port or 8448)]
 
+        logger.debug("Looking up SRV record for %s", host.decode(errors="replace"))
         server_list = await self._srv_resolver.resolve_service(b"_matrix._tcp." + host)
 
         if server_list:
+            logger.debug(
+                "Got %s from SRV lookup for %s",
+                ", ".join(map(str, server_list)),
+                host.decode(errors="replace"),
+            )
             return server_list
 
         # No SRV records, so we fallback to host and 8448
+        logger.debug("No SRV records for %s", host.decode(errors="replace"))
         return [Server(host, 8448)]