Port 10031 — The Unassigned Port with a Job

What Is Port 10031?

Port 10031 belongs to the registered ports range (1024–49151). This range exists in between the well-known system ports (0–1023) and the ephemeral/dynamic ports (49152–65535). Registered ports are meant for specific services that have been officially assigned by IANA, the Internet Assigned Numbers Authority.

But port 10031 has no official assignment in the IANA registry. ¹ It's unassigned. And yet it has a job.

The Common Use: Policyd

Port 10031 is most frequently found running Policyd (also called Cluebringer), a policy daemon used by mail server administrators.² Policyd acts as a gatekeeper between mail clients and mail servers. When a message arrives, the mail server (usually Postfix) queries Policyd before accepting or rejecting the message. Policyd checks policy rules: rate limits, spam thresholds, domain whitelists, authentication requirements.

It's middleware for email. A decision engine. Thousands of messages funneled through a single daemon on a single port, each one asking the same question: "Should I accept this?"

Why Port 10031?

There's no RFC that says "use 10031 for Policyd." The port became conventional through adoption. Mail server administrators configured Policyd to listen on 10031. Others copied the setup. The port became a standard by use, not by decree.

Policyd is now largely deprecated. ³ Newer mail systems use iRedAPD or other policy frameworks. But legacy installations still run it. If you see port 10031 open on a mail server, you're looking at infrastructure from the 2000s–early 2010s era, still working, still filtering mail.

How to Check What's Listening

On Linux/macOS:

# See if anything is listening on port 10031
sudo lsof -i :10031

# Or use netstat
sudo netstat -tuln | grep 10031

# Or check with ss
sudo ss -tuln | grep 10031

On Windows:

# Check listening ports
Get-NetTCPConnection -LocalPort 10031

If nothing appears, port 10031 is not in use. If you see a process listening, check the process name. It will likely be policyd or postfix if this is a mail server.

Why Unassigned Ports Matter

The IANA registry is the closest thing the Internet has to an authoritative phone book. But it's incomplete. Thousands of ports run applications that were never officially registered. They coexist in the registered range, unclaimed but useful.

This creates a risk: two applications could claim the same port by accident. It's rare but it happens. That's why IANA exists—to prevent chaos by creating a shared namespace.

Port 10031 escaped that system. It found a use anyway. It works. Nobody collides with it because nobody knows about it. That's not a bug. That's how the edge of the system actually functions.

Related Ports

• Port 25 (SMTP) — The primary mail submission port
• Port 587 (Submission) — Modern authenticated mail submission
• Port 10024, 10025 — Other unofficial policy daemon ports in the same range