XCON: Configuring X.400 Connectors to Communicate Through a Firewall (199326)

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The following diagram and list of Internet protocol (IP) addresses is an example of the relationship between Exchange Server computers and a firewall:

   Server A <--------> Firewall <--------> Server B

   Server A's IP address:          172.16.1.10
   Firewall's internal IP address: 172.16.1.11
   Firewall's external IP address: 192.168.1.1
   Server B's IP address:          192.168.1.2
				

For the Exchange Server computers to communicate through the firewall using X.400 Connectors, the firewall must allow traffic in both directions over Transmission Control Protocol (TCP) port 102. Firewalls accomplish this using one of two methods, which are described below.

Proxy

The firewall accepts a packet from an Exchange Server computer, encapsulates the packet into a new TCP/IP packet, and then sends this new packet to the destination Exchange Server computer. This new packet appears to have been sent by the firewall instead of the sending Exchange Server computer. The firewall behaves in this manner because of the rules that the system administrator has configured within the firewall.

In an environment where the firewall is configured to behave in this manner, the Exchange Server computers' X.400 Connectors must be configured to connect to the firewall instead of the destination Exchange Server computer. From the above example, the X.400 Connector installed on the first Exchange Server computer connects to the internal network interface of the firewall. This is accomplished by typing the IP address or fully qualified domain name (FQDN) of the firewall's internal network interface on the Stack tab in the properties for the X.400 Connector. Similarly, the X.400 Connector installed on the second Exchange Server computer connects to the external network interface of the firewall. In the above example, the IP address of 172.16.1.11 is typed on the Stack tab in the properties for the first X.400 Connector and the IP address of 192.168.1.1 is typed on the Stack tab in the properties for the second X.400 Connector.

Pass-Through

The firewall accepts a packet from an Exchange Server computer and forwards it to the destination Exchange Server computer without making any changes. In other words, the firewall routes the packet to the destination Exchange Server computer untouched. Therefore, the packet retains the source IP address of the sending Exchange Server computer.

In an environment where the firewall is configured to behave in this manner, the IP address or FQDN of the destination Exchange Server computer is typed on the Stack tab in the properties for the X.400 Connector. In the above example, the IP address of 192.168.1.2 or the FQDN of the second Exchange Server computer is typed on the Stack tab in the properties for the first X.400 Connector. Similarly, the IP address of 172.16.1.10 or the FQDN of the first Exchange Server computer is typed on the Stack tab in the properties for the second X.400 Connector.

The pass-through method is the preferred method of configuring the Exchange Server X.400 Connector to communicate through a firewall.