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Cannot Contact the DNS Server via TCP Port 53

[This topic is intended to address a specific issue called out by the Exchange Server Analyzer Tool. You should apply it only to systems that have had the Exchange Server Analyzer Tool run against them and are experiencing that specific issue. The Exchange Server Analyzer Tool, available as a free download, remotely collects configuration data from each server in the topology and automatically analyzes the data. The resulting report details important configuration issues, potential problems, and nondefault product settings. By following these recommendations, you can achieve better performance, scalability, reliability, and uptime. For more information about the tool or to download the latest versions, see "Microsoft Exchange Analyzers" at http://go.microsoft.com/fwlink/?linkid=34707.]  

Topic Last Modified: 2008-05-20

The Microsoft Exchange Analyzer Tool uses the following Exchange Management Shell cmdlet to query for the values of the ExternalDNSServers and InternalDNSServers properties of any target Exchange 2007 Transport server roles. The ExternalDNSServers property specifies the list of external Domain Name System (DNS) servers that the server queries when it resolves a remote domain. The InternalDNSServers property specifies the list of DNS servers that should be used to resolve a local domain name for the target server.

get-TransportServer -Identity %ExchangeServerName% -DomainController %DomainControllerName%

After retrieving the list of the DNS servers, the Exchange Server Analyzer opens a TCP socket connection to port 53 on each server. This connection is made by using a custom object processor that returns a specific string if the connection is successful. If the Exchange Server Analyzer does not receive 53 Available as part of the returned string from a server, a warning is displayed.

This warning indicates that the server did not respond to a connection attempt on TCP port 53. By default, DNS servers listen on TCP socket 53 for communications such as name resolution queries.

If the target DNS server is unavailable for a connection on TCP socket 53, mail delivery may be slow or fail.

DNS lookups are used by Exchange 2007 transport servers to resolve a host name to an IP address.

When a Send connector is configured to use the external DNS lookup settings on a transport server, it uses the external DNS settings that you configure to resolve host or MX records to IP addresses. The external DNS settings also apply to Receive connectors for the anti-spam agents, such as the Sender ID agent and Connection Filter agent.

The internal DNS lookups configuration settings are used to resolve DNS queries for all communications that do not route through a Send connector that is set to use the external DNS lookup settings on a transport server. For example, all messages that are sent between Hub Transport servers in the organization by using the implicit intra-organization Send connector use the internal DNS lookups configuration.

To address this warning:

  • Make sure that the DNS server has been started, and that there is connectivity to it from the Exchange network.

  • Verify that the IP address of the DNS server is correct.

  • Reconfigure the DNS server(s).

For more information about configuring DNS settings for Exchange 2007 Servers, see the following Exchange Server resources:

 
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