Port 591 carries HTTP traffic—but only for FileMaker databases. It's registered in the well-known ports range (0-1023) as "http-alt" and assigned specifically to FileMaker, Inc.1
The Problem This Port Solved
In the late 1990s and early 2000s, FileMaker wanted their database software to publish data directly to the web. The obvious choice was port 80, the standard HTTP port. But port 80 was almost always already occupied—by Apache, IIS, or whatever web server was already running on the machine.
You can't have two services listening on the same port. FileMaker needed their own door.
FileMaker's Web Publishing Journey
FileMaker 4.0 (1997) introduced Web Companion, a plugin that turned the database into a web server.2 FileMaker 5.5 (2001) enhanced it with a multithreaded version. FileMaker 7 (2004) brought Instant Web Publishing (IWP), which rendered database layouts directly in web browsers without requiring custom HTML.3
All of these features needed an HTTP port. Port 591 became FileMaker's designated alternate.4
How It Worked
When you enabled web publishing in FileMaker Server, you could configure it to listen on port 591 instead of port 80.5 Users would access your database at URLs like:
The :591 explicitly told the browser "connect to this port, not the default port 80." FileMaker's Web Publishing Engine would handle the request, generate the HTML, and send it back—all over port 591.
What Happened to Instant Web Publishing
FileMaker deprecated Instant Web Publishing in FileMaker 13 (2013) and removed it entirely in FileMaker 16 (2017), replacing it with FileMaker WebDirect, which uses standard ports 80 and 443.6 Port 591 became less relevant.
Modern FileMaker Server deployments typically use the standard web ports, with FileMaker WebDirect running on port 443 (HTTPS). Port 591 still appears in the IANA registry as FileMaker's assigned alternate, but it's a legacy of an era when database software needed to avoid port conflicts.
Why This Port Matters
Port 591 represents a practical solution to a common architectural problem: what do you do when the standard port is occupied? The answer is: get your own port, register it officially, and document it so firewalls and network administrators know what it's for.
The Internet is full of these alternate ports—http-alt services scattered across the well-known range, each one a story about software that needed HTTP but couldn't have port 80.
Checking What's Listening
To see if anything is listening on port 591 on your system:
Linux/macOS:
Windows:
If you find something listening on port 591 and you're not running FileMaker Server, you might want to investigate what it is.
Security Considerations
Port 591 carries HTTP traffic, which is unencrypted. Any data sent over this port—including database queries and responses—travels in plaintext across the network. Anyone with access to the network path can read it.
If you're still running FileMaker on port 591, consider migrating to HTTPS on port 443. FileMaker WebDirect (the modern replacement for Instant Web Publishing) uses HTTPS by default.
Related Ports
- Port 80 — HTTP (the standard this port was avoiding)
- Port 443 — HTTPS (what FileMaker WebDirect uses now)
- Port 8080 — Another common HTTP alternate
- Port 5003 — FileMaker Server Admin Console (another FileMaker-specific port)
Frequently Asked Questions About Port 591
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