standalone: add an auth_hint

certbot/certbot/_internal/plugins/standalone.py

@@ -5,6 +5,7 @@ import logging
 import socket
 from typing import DefaultDict
 from typing import Dict
+from typing import List
 from typing import Set
 from typing import Tuple
 from typing import TYPE_CHECKING
@@ -184,6 +185,14 @@ class Authenticator(common.Plugin):
             if not self.served[servers]:
                 self.servers.stop(port)
 
+    def auth_hint(self, failed_achalls: List[achallenges.AnnotatedChallenge]) -> str:
+        port, addr = self.config.http01_port, self.config.http01_address
+        neat_addr = f"{addr}:{port}" if addr else f"port {port}"
+        return ("The Certificate Authority failed to download the challenge files from "
+                f"the temporary standalone webserver started by Certbot on {neat_addr}. "
+                "Ensure that the listed domains point to this machine and that it can "
+                "accept inbound connections from the internet.")
+
 
 def _handle_perform_error(error):
     if error.socket_error.errno == errno.EACCES:

certbot/tests/plugins/standalone_test.py

@@ -177,6 +177,13 @@ class AuthenticatorTest(unittest.TestCase):
             "server1": set(), "server2": set()})
         self.auth.servers.stop.assert_called_with(2)
 
+    def test_auth_hint(self):
+        self.config.http01_port = "80"
+        self.config.http01_address = None
+        self.assertIn("on port 80", self.auth.auth_hint([]))
+        self.config.http01_address = "127.0.0.1"
+        self.assertIn("on 127.0.0.1:80", self.auth.auth_hint([]))
+
 
 if __name__ == "__main__":
     unittest.main()  # pragma: no cover