Port 1704: bcs-broker — A Registered Ghost

What This Port Is

Port 1704 sits in the registered port range (1024-49151). These are ports that organizations or individuals have formally claimed through IANA's registration process, distinguishing them from the well-known ports (0-1023) that require IETF review.

According to the IANA registry, port 1704 is assigned to a service called bcs-broker, on both TCP and UDP, registered to one Andy Warner at a Dutch domain.¹

That's where the documentation ends.

The Ghost Registration

"bcs-broker" has no RFC. No public specification. No software project that announces it runs on port 1704. The name suggests something involving a broker pattern, perhaps message brokering or some kind of middleware, but that's inference from three letters and a word. The actual meaning is unknown.

This is not rare. The registered port range contains thousands of entries like this: claimed by companies or individuals for internal software, proprietary protocols, or projects that never shipped. Once registered, a port number is held indefinitely. There's no expiration, no requirement to publish a spec, no process for reclaiming abandoned entries.

Port 1704 is registered. What it was registered for has been lost to time, or was never public to begin with.

What's Actually on Port 1704

If you see traffic on port 1704 in the wild, it's almost certainly not "bcs-broker." It's more likely:

• Application-specific software that chose an uncontested port number
• Development or testing traffic from an internal tool
• Misconfigured services that ended up on a non-default port

The IANA name gives you no useful signal here.

How to Check What's Listening

To see if anything is running on port 1704 on your machine:

# macOS / Linux
lsof -i :1704

# Linux alternative
ss -tlnp | grep 1704

# Windows
netstat -ano | findstr :1704

To check what's running on a remote host (if you have access):

nmap -p 1704 <host>

If something shows up, the process name or PID will point you to the actual software far more reliably than the IANA registry ever could.

Why Unassigned Ports Matter

The port system works because of shared expectations. When your browser connects to port 443, both sides know what's coming. That shared understanding collapses with unassigned or ghost-registered ports.

This is why security teams monitor for unexpected traffic on unusual ports. A service quietly running on port 1704 means something decided not to use a standard port, which is worth understanding. It might be legitimate proprietary software. It might not be.

The registered range is large enough (48,128 ports) that most of it is either lightly used or occupied by registrations like this one, where the paper trail exists but the software doesn't.