Proof of concept: Add really basically support for srv records.

breezy/bzr/smart/medium.py

@@ -51,7 +51,7 @@
     )
 from breezy.i18n import gettext
 from breezy.bzr.smart import client, protocol, request, signals, vfs
-from breezy.transport import ssh
+from breezy.transport import ssh, lookup_srv_single
 """)
 from ... import (
     errors,
@@ -1029,9 +1029,9 @@ def _ensure_connection(self):
             vendor = ssh._get_ssh_vendor()
         else:
             vendor = self._vendor
+        (host, port) = lookup_srv_single('_bzr+ssh._tcp', self._ssh_params.host, self._ssh_params.port)
         self._ssh_connection = vendor.connect_ssh(self._ssh_params.username,
-                                                  self._ssh_params.password, self._ssh_params.host,
-                                                  self._ssh_params.port,
+                                                  self._ssh_params.password, host, port,
                                                   command=[self._ssh_params.bzr_remote_path, 'serve', '--inet',
                                                            '--directory=/', '--allow-writes'])
         io_kind, io_object = self._ssh_connection.get_sock_or_pipes()

breezy/transport/__init__.py

@@ -1629,6 +1629,24 @@ def do_catching_redirections(action, transport, redirected):
         raise errors.TooManyRedirections
 
 
+def lookup_srv_single(service, host, port=None):
+    """Lookup a single host/port combination to connect to."""
+    try:
+        import dns.resolver
+    except ModuleNotFoundError:
+        mutter('dns.resolver not found, ignoring SRV record')
+        return (host, port)
+    srv_name = service + '.' + host
+    try:
+        srv_records = dns.resolver.query(srv_name, 'SRV')
+    except dns.resolver.NXDOMAIN:
+        mutter('srv record %s not found', srv_name)
+        return (host, port)
+    for srv in srv_records:
+        return (str(srv.target).rstrip('.'), port or srv.port)
+    return (host, port)
+
+
 class Server(object):
     """A Transport Server.
 

breezy/transport/sftp.py

@@ -52,6 +52,7 @@
     FileFileStream,
     _file_streams,
     ssh,
+    lookup_srv_single,
     ConnectedTransport,
     )
 
@@ -407,8 +408,10 @@ def _create_connection(self, credentials=None):
             auth = config.AuthenticationConfig()
             user = auth.get_user('ssh', self._parsed_url.host,
                                  self._parsed_url.port)
-        connection = vendor.connect_sftp(self._parsed_url.user, password,
-                                         self._parsed_url.host, self._parsed_url.port)
+        (host, port) = lookup_srv_single(
+            '_sftp._tcp', self._parsed_url.host, self._parsed_url.port)
+        connection = vendor.connect_sftp(
+            self._parsed_url.user, password, host, port)
         return connection, (user, password)
 
     def disconnect(self):