Port 10028 — Unassigned, But Not Unused

What This Port Is

Port 10028 is unassigned. It has no official service name in the IANA (Internet Assigned Numbers Authority) registry. It lives in the user ports range (1024-49151)—the space where applications are allowed to register services, but not required to.

The IANA port registry is the authoritative list of which port numbers are assigned to which protocols. Port 10028 doesn't appear there. But that doesn't mean nothing uses it.

What Actually Uses It

Port 10028 is used by DKIMproxy, an open-source SMTP proxy for adding DKIM (DomainKeys Identified Mail) signatures to outgoing email.¹ The port works within Postfix, a popular mail server.

Here's the setup:

1. A mail server (Postfix) receives outgoing messages on port 587
2. It forwards them to DKIMproxy listening on port 10027
3. DKIMproxy signs the message with the sender's private cryptographic key
4. DKIMproxy sends the signed message back to Postfix listening on port 10028
5. The signed message is delivered

Every email that goes through this setup—one that can prove it actually came from your domain—touches port 10028. It's internal traffic, invisible to end users, but essential for fighting email spoofing and phishing.

Why It Doesn't Have an Assignment

IANA port assignments are optional. Services can request official registration, but administrators often don't bother. Port 10028 works fine without it. DKIMproxy chose this port number years ago, and it stuck. The protocol doesn't change, the use case is clear, and registration would require more paperwork than value.

This is normal. Thousands of applications use unassigned ports. They just aren't famous enough to warrant an IANA entry.

How to Check What's On Port 10028

If you're on a mail server running DKIMproxy, you can check:

Linux/Unix:

# See what's listening on port 10028
sudo netstat -tlnp | grep 10028
# Or with ss (newer)
sudo ss -tlnp | grep 10028
# Or with lsof
sudo lsof -i :10028

macOS:

# List listening ports
sudo lsof -i :10028

Windows:

# Find what's using port 10028
netstat -ano | findstr :10028

If nothing shows up, that's fine. Most servers don't run DKIMproxy.

Why Unassigned Ports Matter

The Internet has about 65,000 ports. Only a few thousand have official assignments. The unassigned ones are the frontier—where new protocols emerge, where local infrastructure runs, where experiments happen.

Unassigned doesn't mean unused. It means unsanctioned. It means an administrator chose a port and made it work. Some of these become famous (SSH on 22, HTTP on 80). Others stay quiet, carrying specific traffic between specific systems, never needing a name in a registry.

Port 10028 is one of the quiet ones. It does its job. That's enough.