Port 1387 carries a very specific kind of conversation: engineering software checking if it's allowed to run.
What Runs on This Port
Port 1387 is officially registered for CADSI-LM, a license management system originally from Computer Aided Design Software, Inc. (CADSI), later acquired by LMS International.1 LMS was subsequently acquired by Siemens in 2013 for approximately $870 million.2
The protocol handles license checkout and management for CAD and engineering simulation software. When an engineer launches a CAD application, the software contacts a license server on port 1387 to ask: "Can I run?" The server checks how many licenses are available, who's currently using them, and whether this user is allowed access.
Both TCP and UDP protocols are supported on port 1387, though TCP is more commonly used for the reliable request-response pattern that license management requires.3
How License Management Works
The exchange on port 1387 follows a pattern familiar to anyone who's ever seen "no licenses available" in a CAD program:
- License request — Application contacts the license server: "I need a license for SolidWorks."
- Availability check — Server verifies: How many licenses exist? How many are currently checked out? Is this user authorized?
- License grant or denial — Server either grants a license (and tracks it as "in use") or denies the request with an error.
- Periodic heartbeat — While the application runs, it periodically contacts the server to prove it's still active.
- License return — When the application closes (or crashes), the license gets returned to the pool.
This is how companies manage expensive software: they buy 50 licenses for 500 engineers, knowing not everyone needs the software simultaneously.
The Corporate License Manager Reality
Port 1387 represents something particular to enterprise software: concurrent licensing. Instead of buying a copy for every employee, companies buy a pool of licenses that employees share.
When more people try to use the software than licenses exist, someone gets locked out. Port 1387 is where that rejection happens. It's the moment when an engineer at 3pm discovers all the licenses are taken and has to wait for someone to close their copy.
Security Considerations
License servers are internal infrastructure, not Internet-facing services. Port 1387 should only be accessible within your corporate network.
If you see port 1387 open to the Internet, something is misconfigured. License managers don't need external access—they exist to serve internal users.
Alternative Uses
Port 1387 has also been observed carrying traffic for Cisco VIRL PE (Virtual Internet Routing Lab Personal Edition), a network simulation and modeling tool.4 VIRL uses this port for API communication with its server.
This dual use is common in the registered port range (1024-49151): IANA assigns ports, but multiple applications sometimes use the same number for different purposes.
Checking What's Listening
To see if something is listening on port 1387 on your system:
On Linux/macOS:
On Windows:
If you see something listening and you're not running engineering software or network simulation tools, investigate what's bound to this port.
The Registered Port Range
Port 1387 belongs to the registered port range (1024-49151). These ports are assigned by IANA to specific services, but the assignments aren't enforced by the operating system. Any application can technically listen on port 1387—the registration is more like a reservation than a rule.
This differs from well-known ports (0-1023), which typically require elevated privileges to use.
Why This Port Matters
Port 1387 represents the infrastructure behind expensive software. CAD applications can cost thousands of dollars per license. License managers exist because companies can't afford—or don't want—to buy a copy for every potential user.
The port carries the question: "Am I allowed to work right now?" And sometimes the answer is no, not because you lack permission, but because a colleague in another office is using the last available license.
It's a small technical detail that shapes how engineering teams actually work.
Frequently Asked Questions About Port 1387
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