What This Port Is
Port 10103 is officially registered to eZrelay, a C++11 library designed to build simple TCP relay services.1 It handles connection management and allows applications to relay network traffic, useful in situations where you need a proxy-like intermediary between clients and servers.
The port lives in the registered ports range (1024–49151), which IANA allocates to applications that go through official registration processes. It's TCP/UDP, so it can operate on either protocol.
The Reality
If you scan networks, you're unlikely to find port 10103 listening. eZrelay exists as a legitimate tool for developers who need to build relay functionality, but it hasn't achieved the kind of widespread adoption that would make this port common.
The registration exists. The port number is allocated. The service is real. And yet, in practice, most systems never touch it. This is completely normal.
How to Check What's Listening
To see if something is using port 10103 on your system:
On Linux/macOS:
On Windows:
Cross-platform (if you have curl):
If nothing returns, port 10103 is free. It's not a security issue. It's just an empty slot in the number space.
Why Unassigned Ports Matter
The Internet has 65,535 port numbers total. Only about 500 are heavily used (well-known ports like 80, 443, 22). The registered range (1024–49151) contains thousands of official assignments—many perfectly legitimate, many never instantiated.
This sprawl is intentional. It solves a coordination problem: if every application had to negotiate a single global port number, we'd have conflicts and scarcity. Instead, IANA pre-allocates, applications register, and the vast majority never collide. Port 10103 is part of that buffer.
It's not that port 10103 is unknown. It's that it's reserved for when someone needs it.
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