PMDF System Manager's Guide

The extended SMTP command ETRN (RFC 1985) allows an SMTP client to request that a remote SMTP server start up processing of the remote side's message queues destined for sending to the original SMTP client; that is, it allows an SMTP client and SMTP server to negotiate "switching roles", where the side originally the sender becomes the receiver, and the side originally the receiver becomes the sender. Or in other words, ETRN provides a way to implement "polling" of remote SMTP systems for messages incoming to one's own system. This can be useful for systems that only have transient connections between each other, for instance, over dial-up lines. When the connection is brought up and one side sends to the other, via the ETRN command the SMTP client can also tell the remote side that it should now try to deliver any messages that need to travel in the reverse direction.

The SMTP client specifies on the SMTP ETRN command line the name of the system to which to send messages (generally the SMTP client system's own name). If the remote SMTP server supports the ETRN command, it will trigger execution of a separate process to connect back to the named system and send any messages awaiting delivery for that named system.

See also Section 2.3.4.33 and Section 2.3.4.34 for a general discussion of the SMTP ETRN command and PMDF channel keywords affecting PMDF's sending and behavior upon receipt of ETRN commands.

The ETRN command can be quite useful on an e-mail firewall system, particularly if communication partners have only dial-up or other intermittently scheduled connectivity. But for general external SMTP connections, you can want to limit the number of ETRN commands to which PMDF will respond in a single session, so that a single remote site cannot attempt to "monopolize" the PMDF system's message delivery processing. For this, the ALLOW_ETRNS_PER_SESSION channel option can be used in the external TCP/IP channel's option file; see Section 21.1.2.2.

Also, in the interest of limiting the amount of information about the firewall's configuration visible externally, you can want to block PMDF's normal echo of the name of the PMDF channel an ETRN command domain matches on the tcp_local channel handling general external SMTP connections. For this, specify the silentetrn channel keyword on the tcp_local channel.