Port 3553: Red Box Recorder — The Call Recording Discovery Port

What Port 3553 Is

Port 3553 is a registered port — assigned by IANA in July 2002 to rbr-discovery, the auto-discovery protocol for Red Box Recorder ADP (Auto Discovery Protocol). It operates on both TCP and UDP.

Red Box (now part of Motorola Solutions) makes call recording systems used by financial institutions, contact centers, and emergency services. Their software deploys across multiple servers, and components need to find each other on the network. Port 3553 is how they do it: a dedicated discovery channel where Red Box services announce their presence and locate peers.

The Registered Port Range

Port 3553 sits in the registered ports range (1024–49151). These ports are:

• Not reserved for system use (that's 0–1023, the well-known ports)
• Registered with IANA by organizations for specific applications
• Not protected by the OS — any process can bind to them without root privileges
• Documented in the IANA Service Name and Transport Protocol Port Number Registry

Registration means IANA has recorded an organization's claim to a port for a specific purpose. It doesn't mean the port is locked or enforced — just documented. If Red Box isn't on your network, port 3553 is available for anything else to use.

Who Uses This Port

Organizations running Red Box call recording software. This includes:

• Financial services firms (where call recording is often legally required)
• Emergency communication centers
• Large contact centers

If you see traffic on port 3553 and you're not running Red Box, it's worth investigating — though the more likely explanation is a misconfigured or forgotten Red Box installation rather than something malicious.

How to Check What's Listening

On Linux or macOS:

# See what process is bound to port 3553
sudo ss -tlnp | grep 3553
# or
sudo lsof -i :3553

On Windows:

netstat -ano | findstr :3553

From another machine:

nmap -p 3553 <target-ip>

Why Unassigned-Looking Ports Matter

Most registered ports are running on your network right now without you knowing what they are. Every major enterprise software suite claims a handful — sometimes more. The IANA registry documents tens of thousands of them.

This isn't a problem. It's the system working. When software registers a port, it reduces collisions — two applications that both want port 3553 would conflict. The registry is how the Internet coordinates without central enforcement.

Port 3553 is one of thousands of small solutions to small problems, registered once and mostly forgotten. The call it's recording might be a financial advisor, a 911 dispatcher, or a customer service agent. The discovery protocol humming on port 3553 is what makes the recording infrastructure hold together.