Port 125: Locus PC-Interface — A Bridge Between Two Worlds

What Port 125 Does

Port 125 is registered with IANA as locus-map, the service name for the Locus PC-Interface Net Map Server¹. It operates on both TCP and UDP and falls within the well-known port range (0-1023), meaning it was assigned through formal IETF review processes².

The Net Map Server maintained a directory of available shared disk areas and machines on a network. Think of it as a browser for shared resources: when a PC client wanted to know what UNIX file systems were available, it asked the map server on port 125. Its companion, the Locus PC-Interface Conn Server on port 127 (locus-con), handled the actual connections to exported partitions³.

In practice, almost no one runs this service today. The software it belonged to has been defunct for decades.

The Story Behind Port 125

Locus Computing Corporation

In 1982, three researchers from UCLA, Gerald J. Popek, Charles S. Kline, and Gregory I. Thiel, founded Locus Computing Corporation to commercialize technologies from the LOCUS distributed operating system project⁴. LOCUS was ambitious: a transparently distributed UNIX system where processes and files could move between machines without users noticing. It was ahead of its time.

The company turned that research into real products. They built a version of AIX for IBM's PS/2 and System/370 lines. They created Merge, which let DOS applications run under native UNIX. Intel commissioned them to build a multiprocessor operating system for the Paragon, a massively parallel supercomputer⁴.

PC-Interface

But their bread-and-butter product was PC-Interface (PCI), a cross-platform file sharing toolkit⁴. In the mid-1980s, offices were filling up with IBM PCs and clones, but the serious computing still happened on UNIX servers. Getting those two worlds to talk to each other was a genuine engineering problem.

Sun Microsystems had NFS for UNIX-to-UNIX file sharing. Locus built the equivalent for UNIX-to-PC. PC-Interface let DOS, Windows, and Macintosh clients mount UNIX file systems over a LAN, using the map server on port 125 to discover resources and the connection server on port 127 to access them³.

IBM thought enough of it to bundle the server-side components with AIX. Users still had to license the PC clients separately, but the server came free³.

Acquisition and Disappearance

On April 12, 1995, Platinum Technology announced it would acquire Locus Computing Corporation for approximately $33 million⁴. PC-Interface was renamed PC-Enterprise when Windows 95 support was added. Eventually, the product line faded into irrelevance as native networking became standard in every operating system.

The port assignments, registered under the name Eric Peterson, remain in the IANA registry. They are among the quietest monuments in the history of networking: proof that someone once solved a problem so thoroughly that the world forgot the problem ever existed.

How the Protocol Worked

The Locus PC-Interface system used a client-server architecture with two dedicated ports:

When a PC client started up, it contacted the map server on port 125 to get a list of available UNIX file systems and shared directories. Once the user selected a resource, the client established a connection through the conn server on port 127 to mount the remote file system locally.

This two-port architecture separated discovery from connection, a clean design that kept the map server lightweight and responsive even on busy networks.

Security Considerations

Port 125 has been flagged in some security databases as historically associated with Trojan horse activity⁵. This does not mean the Locus protocol itself was malicious. It means that malware authors have, at various points, used port 125 for command-and-control communication, likely because they expected the port to be unmonitored on most networks.

Since the Locus PC-Interface software is effectively extinct, there is no legitimate reason for port 125 to be open on a modern system. If you find it listening:

# Check what's using port 125 on Linux/macOS
sudo lsof -i :125

# Check on Windows
netstat -ano | findstr :125

An open port 125 on a modern machine warrants investigation. It is not a port that any current software should be using.

Port 125 in Context

Port 125 sits in the well-known range (0-1023), ports reserved for system-level services assigned by IANA². Its neighbors tell the story of 1980s networking:

• Port 123: NTP (Network Time Protocol), still in heavy use
• Port 124: ANSA REX Trader, another defunct service
• Port 125: Locus PC-Interface Net Map Server
• Port 126: NXEdit (Unisys), yet another relic
• Port 127: Locus PC-Interface Conn Server

Many well-known ports in this range are relics. They were assigned during an era when port numbers were handed out to specific products and protocols, many of which did not survive. The ports remain assigned because IANA rarely reclaims them. Each one is a fossil record of a problem someone needed to solve.