Port 10027 — Unassigned But in Use

The Port's Status

Port 10027 lives in the registered port range (1024-49151), which means it could be assigned by IANA on request from a software developer or organization. But it never has been. It remains unassigned in the official registry. ¹

What Actually Runs Here

Despite its orphan status, port 10027 has a practical home in mail infrastructure:

DKIMproxy listens on port 10027 to perform DKIM signing and verification. In typical configurations, outgoing mail passes through port 10027 (dkimproxy.out) where it gets signed, then continues to port 10028. ² This creates a signing pipeline: mail arrives on the submission port (587), gets filtered, signed at 10027, and forwarded onward.

Amavisd-new, a content filtering system used in mail servers, also commonly listens on port 10027 (typically on localhost) for local mail filtering. ³ If you see port 10027 in use on a mail server, this is probably what's listening.

Checking What's Listening

To see if something is using port 10027 on your system:

Linux:

netstat -tlnp | grep 10027
lsof -i :10027

macOS:

lsof -i :10027

Windows:

netstat -ano | findstr :10027
Get-NetTCPConnection -LocalPort 10027

If nothing returns, the port is quiet on your machine. If DKIMproxy or amavisd-new appear, you're running a mail filtering setup.

Why Unassigned Ports Matter

The fact that port 10027 gets used without official assignment reveals how port allocation actually works:

• IANA's registry is advisory, not enforceable. No global authority can stop you from listening on 10027.
• Stability matters more than approval. Applications pick high-numbered registered ports because they're unlikely to conflict with system services (0-1023) or dynamic ports (49152+). They don't need IANA's permission to be stable.
• The system scales through restraint. DKIMproxy picked 10027 and 10028 deliberately. amavisd-new also uses this range. They work together because they were designed thoughtfully, not because IANA forced coordination.

Port 10027 is useful precisely because it's unassigned. No standards committee will change what it does. Applications can rely on it.

The Bigger Picture

The port system divides into three ranges:

• 0-1023: Well-known, system services (HTTP, SSH, DNS)
• 1024-49151: Registered ports like 10027, assigned by IANA or claimed by applications
• 49152-65535: Dynamic/ephemeral ports, temporary by design

Port 10027 chose to live in the stable middle ground. It's not official, but it works.