Merge pull request #23 from matrix-org/paul/ipv6-srv-failure

Mention AAAA as well as A records; remark on default port 8448 for no…

specification/30_server_server_api.rst

@@ -59,10 +59,13 @@ and an optional TLS port.
 
 .. **
 
-If the port is present then the server is discovered by looking up an A record
-for the DNS name and connecting to the specified TLS port. If the port is
-absent then the server is discovered by looking up a ``_matrix._tcp``
-SRV record for the DNS name.
+If the port is present then the server is discovered by looking up an AAAA or
+A record for the DNS name and connecting to the specified TLS port.
+
+If the port is absent then the server is discovered by looking up a
+``_matrix._tcp`` SRV record for the DNS name. If this record does not exist
+then the server is discovered by looking up an AAAA or A record on the DNS
+name and taking the default fallback port number of 8448.
 
 Home servers may use SRV records to load balance requests between multiple TLS
 endpoints or to failover to another endpoint if an endpoint fails.