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ETRN: An SMTP Extension for Linux Distros

Iqsom successfully delivered an SMTP extension of Serialmail which was built to be modular, and was re-used by our client with the addition of some tools to handle ETRN, RADIUS and dynamic IPs.

Various components of the projects which were developed by Iqsom:

ETRN Daemon

The ETRN Daemon was written in C. The daemon listens on port 25 (or another if required by the load balancer) and implemented a subset of the SMTP protocol sufficient to handle ETRN connections (RFC1985). Any unsupported commands are declined with an appropriate SMTP response code. The current system of our client supports the triggering of a delivery by the connection itself (ignoring the content of the SMTP/ETRN session) and this is now supported as an option. To support RADIUS triggers, the ETRN daemon also accepts customized proxy ETRN requests from listed RADIUS servers.

Delivery Agent Wrapper

The wrapper is a compiled C program, which accepts various mail delivery parameters such as "Sender Address", "Recipient Address", and then after parsing these parameters, this agent executed a maildrop command and then the message is piped from standard input to the maildrop process.

IP Lookup Tool

The lookup tool was developed to map between usernames and IP addresses, either statically defined or dynamically retrieved via SNMP from the Portmasters. This tool requires a configuration file to map from IP ranges to Portmaster IPs, and to store static IP mappings.

UCSPI Connector

This was a compiled C tool to replace the standard ‘tcpclient’ application used by serialmail. The primary addition in the Client's version was the handling of dynamic IP addresses.

RADIUS Connector

The RADIUS tool was also a compiled C program, called by the RADIUS daemon for each new dialup login.

We also developed some other components to support logging and debugging of this software.