Port 1999: tcp-id-port — The Cisco Ghost

What Port 1999 Is

Port 1999 sits in the registered port range (1024–49151). These ports are tracked by IANA — the Internet Assigned Numbers Authority — meaning some organization at some point claimed them for a specific use. Unlike the well-known ports below 1024 (HTTP, SSH, DNS), registered ports don't require elevated privileges to bind, and their assignments vary wildly in how well-documented and actively used they are.

Port 1999 has an IANA registration: service name tcp-id-port, associated with Cisco's identification infrastructure. But the registration is sparse. There's no RFC, no protocol specification, no detailed documentation attached to it. The name was claimed, and the reasons largely evaporated into Cisco's internal history.

The Cisco Connection

Several sources identify port 1999 as used by Cisco networking equipment for device identification — a mechanism by which Cisco devices announce or verify themselves on a network segment. Some Cisco community discussions mention seeing it during device discovery processes.

But "Cisco identification" covers a lot of ground. Cisco has many proprietary protocols (CDP, LLDP, various management plane functions), and pinning port 1999 to a specific one is difficult. Network administrators who see it open on a Cisco device sometimes turn to forums for answers. The answers are rarely definitive.¹

If you see port 1999 open on a Cisco device in your environment, it's likely legitimate — part of a management or discovery function you may not have configured explicitly. If you see it on a non-Cisco device, that's worth investigating.

Security History

Port 1999 appears on older lists of ports associated with remote access trojans and backdoor tools. These associations come from an era (late 1990s, early 2000s) when malware authors chose arbitrary high-numbered ports to avoid detection, and security scanners flagged anything unexpected.²

There's no specific, widely-documented trojan that called port 1999 home the way Back Orifice owned 31337. The security flags are more the residue of that general era than evidence of a specific threat. That said: an unrecognized open port is always worth investigating.

How to Check What's Using It

On Linux or macOS:

# Show what process is listening on port 1999
sudo ss -tlnp | grep 1999
# or
sudo lsof -i :1999

On Windows:

# Show all listening ports with process IDs
netstat -ano | findstr :1999

# Map the PID to a process name
tasklist | findstr <PID>

On a Cisco device:

show control-plane host open-ports

This shows what the device itself has open, including management-plane ports.

Why Unassigned and Poorly-Documented Ports Matter

The port number system is a registry, not a specification. IANA tracks assignments, but it can't force anyone to use ports as registered, can't compel documentation, and can't revoke assignments when the original reason becomes unclear.

Port 1999 is a minor example of accumulated ambiguity: officially registered, historically associated with a real vendor's real equipment, but practically opaque. Multiply this across thousands of registered ports and you get a system where the gap between "what the registry says" and "what's actually running" is often wide.

For network defenders, this matters: knowing a port is "registered" tells you less than knowing what's actually listening on it in your environment.