Port 743: FlexLM — The license checker

Port 743 is officially assigned to FlexLM (Flexible License Manager), a software licensing system that controls access to commercial applications.¹

What FlexLM Does

FlexLM—now part of Flexera's FlexNet licensing system—is the infrastructure behind software licensing for expensive commercial applications. When you launch CAD software, engineering tools, or professional design applications, they often connect to a license server to verify you're authorized to use them.

That conversation happens on port 743.

The protocol handles:

• License checkout — Applications request permission to run
• License monitoring — Servers track who's using what
• Floating licenses — Shared licenses that move between users
• License return — Applications release licenses when closed

How It Works

A typical FlexLM setup has three components:

1. License server (lmgrd) — Runs on port 743, manages the license pool
2. Vendor daemon — Handles vendor-specific licensing rules
3. Client application — The software requesting a license

When you launch licensed software:

1. Application contacts the license server on port 743
2. Server checks if licenses are available
3. If yes, grants a license and records the checkout
4. Application runs until closed or license expires
5. License returns to the pool

This is why you sometimes see "no licenses available"—someone else is using all the seats.

The Commercial Reality

FlexLM exists because software vendors need to control how their products are used. A company might buy 10 licenses for software that costs $5,000 per seat. FlexLM ensures that only 10 people can use it simultaneously.

Port 743 is the enforcement mechanism. The software won't run without permission from this port.

Why It's a Well-Known Port

Port 743 sits in the well-known range (0-1023), which requires IANA assignment. This was granted because FlexLM became ubiquitous in enterprise software licensing—enough installations that a standard port made sense.

Most license servers actually run on configurable ports (often 27000-27009), but port 743 remains the official assignment.²

Security Considerations

License servers are valuable targets:

• They control access to expensive software — Compromising them could grant unauthorized access
• They track usage data — Information about who uses what software when
• They're often poorly secured — Old license servers run outdated software

Best practices:

• Firewall license servers—only authorized clients should reach port 743
• Keep FlexLM/FlexNet updated—old versions have known vulnerabilities
• Monitor license server logs for suspicious checkouts
• Don't expose license servers to the public Internet

Checking Port 743

To see if a FlexLM server is listening:

# Check if port 743 is open
lsof -i :743

# On the license server, check the license manager
lmstat -a

# Test connection to a license server
telnet license-server.company.com 743

The Bigger Picture

Port 743 represents a specific model of software distribution—the idea that you don't own software, you license it. The port is the technical expression of that commercial relationship.

Every connection to port 743 is software asking permission to run. The answer determines whether you can do your work today.