Port 3353: FATPIPE — Claimed, Not Documented

What This Port Is

Port 3353 sits in the registered port range — the 48,128-port band between 1024 and 49151 that IANA maintains for named services. Unlike the well-known ports below 1024 (where HTTP lives at 80, SSH at 22, DNS at 53), registered ports don't require root privileges to open and don't carry the same weight of open documentation.

IANA lists port 3353 as FATPIPE on both TCP and UDP.¹

What FATPIPE Is

FatPipe Inc. is a Utah-based networking company that builds WAN redundancy and SD-WAN appliances. Their core product bonds multiple Internet connections from different ISPs together — if one link goes down, traffic continues flowing over the others without interruption. They call this WARP (Wide Area Redundancy Protocol).²

Port 3353 appears to be used internally by FatPipe's appliances, likely for coordination between devices or for management traffic. The protocol is proprietary and not publicly documented. Unlike open standards (where the RFC tells you exactly what bytes flow over the wire), this is a vendor-defined format that FatPipe controls entirely.

Why the Port Is Mostly Silent Everywhere Else

If you're not running FatPipe hardware, you almost certainly have nothing listening on port 3353. The IANA registration reserves the number — it's a flag in the sand saying "this is ours" — but it doesn't mean the port is active on any given network. Most registered ports are like this: claimed for a specific product, quiet everywhere else.

How to Check What's Listening

If you see activity on port 3353 and aren't running FatPipe equipment, it's worth investigating.

On Linux/macOS:

# Show what process is listening on port 3353
ss -tlnp | grep 3353
# or
lsof -i :3353

On Windows:

netstat -ano | findstr :3353
# Then look up the PID in Task Manager

With nmap (from another machine):

nmap -sV -p 3353 <target-ip>

The Registered Port Reality

There are 48,128 registered ports. Most people can name fewer than a dozen. The registry is not a map of the Internet's active services — it's closer to a land records office. FatPipe filed a claim for 3353. The claim is on record. Whether the port is doing anything on your network is a separate question entirely.

This is how most of the registered port space works: officially assigned, rarely seen in the wild outside of the specific product it was built for.