Add logging for hostname canonicalization feature

Author: ronf

asyncssh/connection.py

@@ -281,18 +281,27 @@ async def _canonicalize_host(loop: asyncio.AbstractEventLoop,
 
     host = options.host
 
-    if not options.canonicalize_hostname or not options.canonical_domains or \
-            host.count('.') > options.canonicalize_max_dots:
+    if not options.canonicalize_hostname or not options.canonical_domains:
+        logger.info('Host canonicalization disabled')
+        return None
+
+    if host.count('.') > options.canonicalize_max_dots:
+        logger.info('Host canonicalization skipped due to max dots')
         return None
 
     try:
         ipaddress.ip_address(host)
     except ValueError:
         pass
     else:
+        logger.info('Hostname canonicalization skipped on IP address')
         return None
 
+    logger.debug1('Beginning hostname canonicalization')
+
     for domain in options.canonical_domains:
+        logger.debug1('  Checking domain %s', domain)
+
         canon_host = f'{host}.{domain}'
 
         try:
@@ -303,18 +312,28 @@ async def _canonicalize_host(loop: asyncio.AbstractEventLoop,
 
         cname = addrinfo[0][3]
 
-        if cname:
+        if cname and cname != canon_host:
+            logger.debug1('  Checking CNAME rules for hostname %s '
+                          'with CNAME %s', canon_host, cname)
+
             for patterns in options.canonicalize_permitted_cnames:
                 host_pat, cname_pat = map(WildcardPatternList, patterns)
 
                 if host_pat.matches(canon_host) and cname_pat.matches(cname):
+                    logger.info('Hostname canonicalization to CNAME '
+                                'applied: %s -> %s', options.host, cname)
                     return cname
 
+        logger.info('Hostname canonicalization applied: %s -> %s',
+                    options.host, canon_host)
+
         return canon_host
 
     if not options.canonicalize_fallback_local:
+        logger.info('Hostname canonicalization failed (fallback disabled)')
         raise OSError(f'Unable to canonicalize hostname "{host}"')
 
+    logger.info('Hostname canonicalization failed, using local resolver')
     return None
 
 

tests/test_connection.py

@@ -2693,6 +2693,16 @@ async def test_canonicalize(self):
                                 canonical_domains=['test']) as conn:
             self.assertEqual(conn.get_extra_info('host'), 'testhost.test')
 
+    @asynctest
+    async def test_canonicalize_max_dots(self):
+        """Test hostname canonicalization exceeding max_dots"""
+
+        async with self.connect('testhost.test', known_hosts=None,
+                                canonicalize_hostname=True,
+                                canonicalize_max_dots=0,
+                                canonical_domains=['test']) as conn:
+            self.assertEqual(conn.get_extra_info('host'), 'testhost.test')
+
     @asynctest
     async def test_canonicalize_ip_address(self):
         """Test hostname canonicalization with IP address"""