Port 913: APEX edge — The relay that stopped relaying

Official assignment: apex-edge (APEX endpoint-relay service)
Protocol: TCP/UDP
Status: Officially assigned but rarely used
Defined in: RFC 3340 (July 2002)

What Port 913 Was Meant For

Port 913 was assigned to APEX (Application Exchange Core), a protocol with ambitious goals: create an asynchronous message relaying service for loosely-coupled Internet applications. Think of it as trying to build infrastructure for applications to exchange messages without being tightly bound to each other.¹

APEX operated in two modes:

• Endpoint-relay mode (port 913, "apex-edge"): Applications connect to relay servers
• Relay-relay mode (port 912, "apex-mesh"): Relay servers connect to each other²

The idea was to create a mesh of relays across the Internet that could deliver datagrams between endpoints, providing services for access control, presence, and instant messaging.³

What Actually Happened

APEX was built on top of BEEP (Blocks Extensible Exchange Protocol), another protocol designed to be a general framework for building application protocols. Both were documented in RFCs in the early 2000s.⁴

Then they faded. The APEX RFCs were eventually marked as obsolete. The relay mesh never materialized at scale. The protocol that was supposed to revolutionize how applications exchange messages became a footnote.

Port 913 remains officially assigned to apex-edge, but if you scan the Internet looking for APEX relays listening on this port, you'll find almost nothing. The address is registered, but nobody's home.

Why This Port Still Matters

Unassigned or rarely-used well-known ports tell a story about the Internet's evolution. Port 913 represents an idea that didn't survive—a protocol that tried to solve problems that were eventually addressed in different ways by different technologies.

The well-known port range (0-1023) is reserved for services assigned by IANA. These assignments are meant to be permanent and universal—every APEX relay everywhere would listen on port 913. But permanence doesn't guarantee relevance.

What Might Be Using Port 913

Since APEX effectively disappeared, you might encounter:

Repurposed services: Some organizations use officially assigned but rarely-used ports for internal services, knowing there's little risk of conflict.

Confused software: Occasionally software that doesn't follow the APEX protocol shows up on ports 912-913. VMware Authentication Daemon, for example, has been observed on port 912 (apex-mesh) even though it has nothing to do with APEX.⁵

Nothing at all: Most likely, the port is simply closed on most systems.

How to Check What's Listening

To see if anything is listening on port 913 on your system:

# On Linux/macOS
sudo lsof -i :913
sudo netstat -tulpn | grep :913

# On Windows
netstat -ano | findstr :913

To scan a remote host (with permission):

nmap -p 913 target-host

If you find something listening and you don't know why, investigate. It could be legitimate software, or it could be something that shouldn't be there.

The Lesson of Empty Ports

Port 913 is a reminder that official assignment doesn't equal adoption. The IETF can publish an RFC, IANA can register the port, and the protocol can still fade into obscurity.

The Internet's port system has thousands of officially assigned ports. Some carry the weight of billions of connections daily. Others, like port 913, carry mostly history.