added code to attempt to resolve DNS issues on connection failure and implemented DNSName method in WebDav backend handler

Duplicati/Library/Backend/WEBDAV/WEBDAV.cs

@@ -33,6 +33,7 @@ namespace Duplicati.Library.Backend
         private string m_reverseProtocolUrl;
         private string m_rawurl;
         private string m_rawurlPort;
+        private string m_dnsName;
         private bool m_useIntegratedAuthentication = false;
         private bool m_forceDigestAuthentication = false;
         private bool m_useSSL = false;
@@ -61,6 +62,7 @@ namespace Duplicati.Library.Backend
         {
             var u = new Utility.Uri(url);
             u.RequireHost();
+            m_dnsName = u.Host;
 
             if (!string.IsNullOrEmpty(u.Username))
             {
@@ -311,7 +313,12 @@ namespace Duplicati.Library.Backend
             get { return Strings.WEBDAV.Description; }
         }
 
-        public string DNSName => throw new NotImplementedException();
+        public string DNSName 
+        {
+            get { return m_dnsName; }
+        }
+
+        //=> throw new NotImplementedException();
 
         public void Test()
         {

Duplicati/Library/Main/BackendManager.cs

@@ -519,6 +519,19 @@ namespace Duplicati.Library.Main
                                 throw;
                             }
 
+                            // Refresh DNS name if we fail to connect in order to prevent issues with incorrect DNS entries
+                            if (ex is System.Net.WebException)
+                            {
+                                try
+                                {
+                                    System.Net.Dns.GetHostEntry(m_backend.DNSName);
+                                }
+                                catch
+                                {
+                                    Logging.Log.WriteMessage("Failed to resolve IP for " + m_backend.DNSName, Logging.LogMessageType.Warning);
+                                }
+                            }
+
                             m_statwriter.SendEvent(item.BackendActionType, retries < m_numberofretries ? BackendEventType.Retrying : BackendEventType.Failed, item.RemoteFilename, item.Size);
 
                             bool recovered = false;