Port 550: New-rwho — The Abandoned Who's Online Service

Port 550 sits in the well-known port range (0-1023), but if you check IANA's current registry, you'll find it empty. No official assignment. No standardized service. Just a number that once meant something.

What Used to Live Here

Port 550 was historically assigned to new-rwho (new remote who) and new-who—services that let UNIX administrators see who was logged into remote systems across their network.¹²

The protocol was simple. Machines would periodically broadcast user login status over UDP port 550. Other machines listened, collected these updates, and maintained a network-wide list of who was logged in where. No encryption. No authentication. Just announcements into the void, trusting that everyone listening had legitimate reasons.³

New-rwho was part of the broader family of Berkeley r-commands—remote utilities developed in 1982 at UC Berkeley that formed the foundation of early networked UNIX systems.⁴ The suite included rlogin (remote login), rsh (remote shell), and rwho (remote who). The "new" versions were iterations attempting to improve on the originals.

Why It Was Abandoned

The problem wasn't technical. The protocol worked fine for what it did. The problem was what it did.

Broadcasting user login information in plaintext across a network creates obvious privacy and security issues. Anyone on the network could monitor who was using which systems and when. In the trusted, small networks of early UNIX computing, this seemed acceptable. As networks grew and connected to the wider Internet, it became indefensible.

Modern systems use authentication, encryption, and access controls. The idea of broadcasting "here's who's logged in" without any security layer feels quaint—or dangerous, depending on your perspective.

By the time IANA updated its registry, new-rwho had faded from common use. The port assignment was dropped. Port 550 became officially unassigned.⁵

What This Port Represents

Port 550 exists in the well-known range, which means it was important enough to warrant a reserved spot in the first 1024 ports. These numbers were precious in the early days of networking—assigned only to services that mattered.

That it now sits empty tells you something about how the Internet evolved. Services that once seemed essential became obsolete. Security concerns that once seemed paranoid became mandatory. The networks we built outgrew the trust model they were designed for.

Unassigned ports in the well-known range are rare. Most have either kept their original assignments for decades or been reassigned to newer protocols. Port 550's abandonment is a small monument to a service that couldn't adapt.

Current Status

Today, port 550 is officially unassigned. Organizations could theoretically use it for custom applications, but doing so is uncommon. Legacy UNIX systems in controlled environments might still run new-rwho, though this would be unusual and generally inadvisable.⁶

If you find something listening on port 550, it's either:

• A very old system running legacy software
• A custom application that chose this port arbitrarily
• Something that shouldn't be there

You can check what's listening with:

# Linux/macOS
sudo lsof -i :550
sudo netstat -tulpn | grep :550

# Windows
netstat -ano | findstr :550

Why Unassigned Ports Matter

The existence of unassigned ports in the well-known range serves a purpose. They're reminders that the Internet's infrastructure isn't static. Protocols rise and fall. Services that once seemed permanent fade away. Security standards evolve.

Port 550 represents a different era—when networks were small, users were trusted, and broadcasting "who's online" seemed like a reasonable administrative tool. That it now sits empty is evidence that we learned better.