Port 540: UUCP — The protocol that connected Unix before TCP/IP

Port 540 is assigned to uucpd (UUCP daemon), the service that ran the Unix-to-Unix Copy Protocol. This is a well-known port in the reserved range (0-1023), officially assigned by IANA.¹

You won't find much traffic on port 540 anymore. UUCP belongs to a different era—before TCP/IP was ubiquitous, before always-on connections, before the Internet looked anything like it does now.

What UUCP Was

UUCP (Unix-to-Unix Copy Protocol) was a suite of programs that let Unix machines transfer files, execute remote commands, and forward email to each other. But here's what made it different: it worked over phone lines, using modems, with no permanent connection required.²

A machine would dial another machine, typically in the middle of the night when phone rates were cheap. They'd exchange whatever files and mail had queued up. Then they'd hang up. Your email didn't travel directly to its destination—it hopped from machine to machine, store-and-forward, until it eventually arrived. Hours or days later.³

The uucpd daemon listening on port 540 was the service that answered those calls when they came in over TCP/IP instead of phone lines.⁴

The Network That Came Before

In 1978, UUCP connected 82 Unix machines inside Bell Labs. By 1983, it connected 550 machines across the United States. By the mid-1980s, thousands of sites were connected—universities, research labs, early tech companies—forming networks like UUCPNET and enabling services like Usenet news.²

This was the Internet before the Internet. Email addresses looked like this: site1!site2!site3!username—the actual path your message would take, hop by hop. If you knew the network topology, you could route your own mail more efficiently.³

UUCP predates widespread TCP/IP adoption. When TCP/IP became standard and persistent connections became normal, UUCP faded. Port 540 remains in the IANA registry, a marker of what came before.

Why You Might Still See It

You probably won't. UUCP is obsolete outside of a few legacy systems that never migrated away. If you see port 540 open on a modern system, it's either:

• A very old Unix system still running original software
• Someone running UUCP deliberately for historical purposes
• A misconfiguration or leftover service that should be disabled

Security Considerations

UUCP was designed for a different threat model—trusted Unix administrators calling each other over phone lines. It includes authentication (the uucpd daemon expects a login and password), but by modern standards, it's not secure.

If port 540 is open on your network and you're not intentionally running UUCP:

• Close it
• Check what's listening: netstat -an | grep 540 or lsof -i :540
• Remove or disable the uucpd service

What This Port Teaches Us

Port 540 is a reminder that the Internet wasn't inevitable. Before always-on connections, before TCP/IP was standard, people built networks with what they had: Unix machines, modems, and phone lines.

UUCP worked because it matched the constraints of its time. Email could wait hours. Files could transfer overnight. The network didn't need to be real-time—it just needed to be patient.

That patience built communities. The UUCP network carried Usenet discussions, open-source software, and email between researchers who would later build the modern Internet. Port 540 is one of the foundations we built on top of.

Checking What's on Port 540

On Linux or macOS:

# See if anything is listening
sudo lsof -i :540

# Check with netstat
netstat -an | grep 540

# Test if the port is open
nc -zv localhost 540

On Windows:

# Check listening ports
netstat -an | findstr :540

# See which process is using it
Get-Process -Id (Get-NetTCPConnection -LocalPort 540).OwningProcess

You'll almost certainly find nothing. Port 540 is a ghost town now.

Related Ports

• Port 548 — Apple Filing Protocol (AFP), which some sources incorrectly associate with port 540
• Port 22 — SSH, which replaced protocols like UUCP for remote file transfer and command execution

Last Updated: 2026-02-13