Port 1750: SSLP — The Socket Broker Nobody Knows

What Port 1750 Does

Port 1750 is assigned to SSLP — the Simple Socket Library's PortMaster — for both TCP and UDP. ¹

The PortMaster is a connection broker. When distributed C programs built with the Simple Socket Library (SSL) need to find each other across a network, they don't connect directly at first. They ask the PortMaster. It knows where everything is, negotiates the introduction, and hands the programs a socket to talk through.

Think of it as a matchmaker for network processes: you tell it who you're looking for, it tells you where they are.

The Simple Socket Library

The Simple Socket Library is a C library created by Dr. Charles E. Campbell Jr. It wraps Berkeley streaming sockets — the fundamental UNIX networking API — to make it easier to build systems of cooperating programs over TCP/IP.

The library has three parts:

• The library itself — the socket API, simplified
• The PortMaster — the connection broker, running on port 1750
• Utilities — tools for managing the setup

The PortMaster also includes a firewall facility: a file listing the IP addresses of approved machines. Connections from unapproved addresses are refused. Basic, but functional.

Who Made This

Dr. Charles E. Campbell Jr. is a physicist and programmer who may be better known in a different corner of the Internet entirely — as the author of several widely used Vim plugins, including the LargeFile plugin that lets Vim open files too big for it to handle normally. ²

He registered port 1750 for his own tool. That's how the port registry works in the registered range: you build something, you apply, and if the number is free, it's yours.

The Registered Port Range

Port 1750 sits in the registered ports range: 1024 through 49151. ³

• Well-known ports (0-1023): Reserved for major protocols — HTTP, HTTPS, SSH, DNS. Require root/administrator privileges to bind on most systems.
• Registered ports (1024-49151): Assigned by IANA to specific services on request. No privilege required to bind, but the assignments are tracked.
• Dynamic/ephemeral ports (49152-65535): Untracked. Used temporarily by clients for outgoing connections.

Being in the registered range means IANA has a record of what port 1750 is supposed to be for. In practice, that record is only as meaningful as the software is widely deployed.

How to Check What's on Port 1750

If you see traffic on port 1750 and you're wondering whether it's actually SSLP — or something else entirely — check what's listening:

macOS / Linux:

sudo lsof -i :1750
sudo ss -tlnp sport = :1750

Windows:

netstat -aon | findstr :1750

The process name in the output will tell you what's actually there.

Why This Port Exists

The port registry is not just a list of famous protocols. It's also an archive of tools that individuals and small teams built, registered, and shipped — most of them long forgotten, some still quietly running somewhere.

Port 1750 is one of those. Not HTTP. Not SSH. Not SMTP. Just a socket matchmaker written by a physicist who also wrote Vim plugins, registered in the official list, and still technically yours to use if you're running his library.

The Internet runs on protocols everyone knows. It's held together, in part, by protocols almost nobody does.