Port 508: XVTTP — The Protocol That Never Was

Port 508 sits in the well-known port range (0-1023), officially assigned by IANA to something called XVTTP—eXtensible Virtual Terminal Transfer Protocol.¹ The contact listed is Keith J. Alphonso. The assignment exists in the official registry.

But the protocol doesn't exist anywhere else.

What Port 508 Was Supposed to Be

According to the IANA registry, port 508 is assigned to "xvttp" for both TCP and UDP.¹ Some sources describe it as being for "remote X Window System services,"² but there's no RFC defining XVTTP. No specification. No implementation you can download. No documentation explaining how it works.

Search for XVTTP and you'll find the port assignment repeated across databases. Nothing more.

The Well-Known Range

Ports 0-1023 are called "well-known ports" or "system ports."³ They're assigned by IANA and traditionally required root privileges to bind on Unix systems. These are the foundation ports—the ones that define how the Internet works.

Port 508 has a well-known port number but not a well-known protocol.

What Actually Uses Port 508

In practice: probably nothing legitimate. Security databases have flagged port 508 as being abused by malware in the past,⁴ which is what happens to abandoned ports. When a registered port has no active service defending it, malicious software sometimes squats there.

If you find something listening on port 508, it's likely either:

• Malware using an obscure port to avoid detection
• A proprietary application that chose an "available" port
• Nothing (the port is closed)

Checking Port 508

On Linux or macOS:

# See what's listening on port 508
sudo lsof -i :508

# Or using netstat
sudo netstat -tlnp | grep :508

On Windows:

netstat -ano | findstr :508

If something is listening and you don't recognize it, investigate. Port 508 has no standard service that should be there.

Why Unassigned Ports Matter

Port 508 is technically assigned, but functionally unassigned. It's a reminder that port numbers are finite resources. IANA can give someone a port number, but they can't make the protocol real.

The well-known range has 1,024 slots. Some carry the entire Internet (80, 443, 53, 22). Some carry niche but essential services (123 for NTP, 25 for email). And some, like port 508, carry nothing but a name in a registry.

The ports that matter aren't the ones with assignments. They're the ones with protocols that people actually built, documented, and used.