What Port 1903 Is
Port 1903 belongs to the registered port range (1024–49151). These ports are managed by IANA — the Internet Assigned Numbers Authority — which tracks which organizations have claimed which port numbers for which purposes.1
IANA lists port 1903 as assigned to a service called linkname, described as "Local Link Name Resolution," registered to Dan Harrington. Both TCP and UDP are claimed.2
But here's the thing: linkname never shipped.
The Protocol That Wasn't
In 1997, the IPv6 working group faced a real problem. IPv6 introduced link-local addresses — addresses that work between directly connected devices without going through a router. Useful in theory. Awkward in practice, because how do you find out what's on your local network when there's no central directory?
The linkname proposal was a solution: when an IPv6 host came online, it would multicast its name and link-local address to a well-known multicast group (FF02::1:1). Neighbors would listen, learn, and update their local tables automatically. No manual /etc/hosts editing required.3
It was a reasonable idea for 1997. It never made it past experimental draft status. The IETF working group eventually abandoned it, the draft expired, and no RFC was ever published.
The Internet solved the same problem differently. Multicast DNS (mDNS) on port 5353 and Link-Local Multicast Name Resolution (LLMNR) on port 5355 arrived later and actually made it into production.4
Port 1903 kept the reservation. Nothing came to claim it.
What's Actually on Port 1903
Almost certainly nothing, on almost any machine you check.
Occasionally port 1903 appears in network scans as open, typically because:
- A piece of software chose it arbitrarily for its own purposes (applications can bind to any available port)
- Malware has been documented using uncommon registered ports to blend into background noise
- A development tool or custom service picked a "random" port that happens to be 1903
None of these are linkname. That protocol was never implemented.
How to Check What's Listening
On Linux or macOS:
On Windows:
If something shows up, the process ID will help you identify it. On Linux/macOS, lsof will show the process name directly.
Why Unassigned (and Ghost-Assigned) Ports Matter
The registered port range exists to prevent collisions — two applications trying to use the same port on the same machine. IANA assignments are the directory that makes this work.
But the directory has gaps and ghosts. Some ports were reserved for protocols that were later abandoned, like linkname. Some were registered decades ago by companies that no longer exist. Some are legitimately unassigned, waiting.
When a port shows up open in a scan and you don't recognize it, the IANA registry is your first stop — but it won't always have the answer. A ghost assignment like port 1903 is proof that the registry records intent, not reality.
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