Port 372: ListProcessor — The Mailing List Manager That Ran Universities

Port 372 is officially assigned to ulistproc (Unix ListProcessor), an automated mailing list management system that was widely deployed across universities and research institutions throughout the 1990s.¹

What ListProcessor Was

Before Slack, before Discord, before web forums—there was email. And before email lists became one-click subscriptions, there was ListProcessor.²

ListProcessor was software that automated the management of email discussion lists. It handled subscriptions, unsubscriptions, message distribution, and archive management. When you sent an email to listproc@university.edu with commands like subscribe courselist Your Name, ListProcessor running on port 372 processed that request.

The protocol operated on both TCP and UDP, though TCP was far more common for the reliable delivery requirements of email list management.

The History

ListProcessor was created by Anastasios Kotsikonas (known as "Tasos") at Boston University in the 1980s.³ It was originally called "Unix Listserv"—and that name caused problems.

Eric Thomas, the creator of the actual LISTSERV software, was not pleased with the naming confusion. When he founded L-Soft to commercialize LISTSERV, the potential for customer confusion became a business concern. Kotsikonas renamed his software to ListProcessor, or "Listproc" for short.⁴

By 1995, ListProcessor version 7.1 was in widespread use, managed by the Corporation for Research and Educational Networking (CREN).⁵ Universities deployed it to run academic discussion lists, course mailing lists, and research group communications. Before the web provided easy interfaces for group discussions, ListProcessor was how communities talked.

How It Worked

ListProcessor listened on port 372 for commands and list traffic. Users would interact with it through specially formatted email messages:

• subscribe listname Your Name — Join a list
• unsubscribe listname — Leave a list
• index listname — Get a list of available files
• get listname filename — Retrieve a file from the archives

The software parsed these commands, updated subscription databases, and distributed messages to list members. It was automated community management before we had the terminology for it.

The Decline

ListProcessor competed with LISTSERV (commercial, powerful) and Majordomo (free, simpler). But Listproc hasn't had a major code revision since the 1990s.⁶ As security standards evolved and web-based interfaces became expected, universities began migrating away.

Port 372 traffic became rare. Modern mailing list software like Mailman and Sympa offered web interfaces, better security, and active development. The last generation of ListProcessor servers was quietly shut down in the 2000s and 2010s.

Why It Matters

Port 372 represents a specific moment in Internet history: when mailing lists were the primary form of online community. Before web forums, before social media, there were lists. And ListProcessor was one of the three major systems that made those lists possible.

Thousands of academic discussions, research collaborations, and community announcements flowed through port 372. Every subscription confirmation, every digest email, every archived message—all carried by a protocol that most users never knew existed.

The fact that the software was born from a naming dispute makes it even more human. Even in the 1980s and 1990s, the people building the Internet were navigating trademark concerns, competitive pressures, and the need to differentiate their work. Port 372 carries that story.

Current Status

Port 372 is essentially unused today. ListProcessor has been replaced by modern mailing list software, and the protocol has no active implementations.

You can check if anything is listening on port 372 on your system:

# On Linux/Mac
sudo lsof -i :372
netstat -an | grep 372

# On Windows
netstat -an | findstr :372

You will almost certainly find nothing. Port 372 is a historical marker now—a reminder of how communities communicated before the web made everything visual.

Related Ports

• Port 25 (SMTP): The email protocol that ListProcessor relied on for message delivery
• Port 110 (POP3) and Port 143 (IMAP): Email retrieval protocols users needed to access their list messages

Security Considerations

If you somehow encounter a ListProcessor server still running on port 372, treat it as a significant security risk. The software hasn't been maintained in decades and predates modern security practices. It should not be exposed to the public Internet.