Sendmail

Sendmail. It is a mail transfer agent (MTA) originally developed by Eric Allman at the University of California, Berkeley in the early 1980s. For decades, it was the dominant software for routing electronic mail across the Internet, defining many early standards and protocols. Its complex but powerful configuration system allowed it to handle a vast array of mail routing scenarios, making it a foundational component of the early ARPANET and subsequent global network infrastructure.

History

The development of Sendmail began in 1979 within the Computer Systems Research Group at Berkeley, as a successor to the simpler delivermail program. Its creation was driven by the need to manage email across the transitioning network environments of the ARPANET, UUCP, and early LANs. The release of BSD 4.1c in 1983, which included Sendmail, cemented its role in the burgeoning Internet. Key figures like Eric Allman continued its development, with significant contributions documented in the influential RFC 821 and RFC 822 standards. The program's evolution was closely tied to the growth of Sun Microsystems and other commercial Unix vendors who adopted it widely. In 1998, Sendmail, Inc. was founded to provide commercial support, and the software itself was relicensed under the more permissive Sendmail License.

Functionality

As a mail transfer agent, its primary function is to transfer messages between machines using the Simple Mail Transfer Protocol and other protocols like UUCP. It performs critical operations such as MX record lookups using the Domain Name System, message queuing, header rewriting, and alias expansion. The software can act as both an SMTP client and server, interfacing with local delivery agents (MDAs) like procmail for final inbox placement. Its rule-based mailer and rewriting system, defined in the sendmail.cf file, provides extensive control over mail routing, enabling complex configurations for gateways, mailing lists, and spam filtering preprocessors.

Configuration

Configuration is notoriously complex, centered on the monolithic sendmail.cf configuration file written in a specialized macro language. Administrators often used the m4 macro processor with template files from the cf directory to generate this file. Key directives control the definition of mailers, feature classes, relay rules, and domain masquerading. The aliases database, typically processed by newaliases, maps recipient addresses. Tools like the sendmail command-line interface, with flags for testing and queue management, and the mailq utility for inspecting the queue, are essential for administration. The rise of easier alternatives like Postfix and Exim was a direct response to this configuration complexity.

Security

Historically, its monolithic design and privileged operation as a setuid root process made it a frequent target for network attacks, including buffer overflows and shell injection exploits. High-profile incidents like the Morris worm of 1988 exploited vulnerabilities in its debugging code. Over time, concerted efforts by developers and the CERT Coordination Center led to major security rewrites, adopting a separated architecture where most components run without root access. Features like SMTP AUTH, STARTTLS encryption, and integration with SASL and SPF were added to combat spam and forgery. The OpenBSD project maintains a heavily fortified version, and regular audits are conducted in response to advisories from CISA.

Alternatives

Due to its complexity and historical security issues, several modern MTAs were developed as alternatives. Postfix, created by Wietse Venema at IBM Research, emphasizes security, speed, and simplicity in configuration. Exim, developed at the University of Cambridge, offers great flexibility with a different configuration syntax. qmail, written by Daniel J. Bernstein, pioneered security through component isolation and has influenced many subsequent designs. Other notable replacements include OpenSMTPD from the OpenBSD project and Microsoft Exchange Server in corporate Windows environments. The proliferation of these alternatives has significantly reduced its once-ubiquitous deployment on ISP and enterprise servers.

Category:Message transfer agents Category:Unix mail-related software Category:Software using the Sendmail license Category:Software written in C