Resolve HOST_* to *_ADDRESS only if *_ADDRESS is not already set

core/admin/mailu/internal/nginx.py

@@ -3,6 +3,9 @@ from flask import current_app as app
 
 import re
 import urllib
+import ipaddress
+import socket
+import tenacity
 
 
 SUPPORTED_AUTH_METHODS = ["none", "plain"]
@@ -88,4 +91,18 @@ def get_server(protocol, authenticated=False):
             hostname, port = extract_host_port(app.config['AUTHSMTP_ADDRESS'], 10025)
         else:
             hostname, port = extract_host_port(app.config['SMTP_ADDRESS'], 25)
+    try:
+        # test if hostname is already resolved to an ip adddress
+        ipaddress.ip_address(hostname)
+    except:
+        # hostname is not an ip address - so we need to resolve it
+        hostname = resolve_hostname(hostname)
     return hostname, port
+
+@tenacity.retry(stop=tenacity.stop_after_attempt(100),
+                wait=tenacity.wait_random(min=2, max=5))
+def resolve_hostname(hostname):
+    """ This function uses system DNS to resolve a hostname.
+    It is capable of retrying in case the host is not immediately available
+    """
+    return socket.gethostbyname(hostname)