Port 2604: OSPFd VTY — The OSPF daemon's back door

Port 2604 has no official IANA assignment. But on any Linux router running Zebra, Quagga, or FRRouting — the open-source routing suites that power a significant chunk of the Internet's infrastructure — port 2604 is where the OSPF daemon listens for configuration commands.

The Routing Suite That Claimed It

Zebra was a Unix routing daemon project from the late 1990s. Its successor, Quagga, forked in 2003. Its current successor, FRRouting (FRR), is maintained today by a broad coalition of networking companies and is widely deployed in production networks.

All three suites share the same port convention. Each routing protocol daemon gets its own VTY (Virtual TeletYpe) port — a Telnet-based command interface for live configuration:

To reach ospfd directly:¹

telnet 127.0.0.1 2604

You get a Cisco-like CLI where you can inspect OSPF neighbors, adjacency states, link-state databases, and routing tables — or make live configuration changes.

Why Telnet? Why Not SSH?

Because it only listens on 127.0.0.1 by default. The assumption is that you're already on the router, so you've already authenticated to the system. The VTY is a local configuration interface, not a remote access protocol. Boundary security rather than transport encryption.

That said: if a misconfigured router ever binds these ports to an external interface, you're handing anyone on the network a direct line to your routing daemon.

What Range This Port Belongs To

Port 2604 falls in the registered port range (1024–49151). These ports are registered with IANA for specific services, but IANA has not assigned port 2604 to anything. The routing suite's use of this port is entirely informal — a convention that became de facto standard through widespread adoption.

Checking What's Listening

To see if ospfd (or anything else) is using port 2604 on your system:

# Linux
ss -tlnp | grep 2604

# macOS
lsof -i :2604

# Cross-platform
netstat -an | grep 2604

If you see it listening on 127.0.0.1:2604, you're running FRRouting or Quagga. If it's listening on 0.0.0.0:2604, that router has a misconfiguration worth fixing.

Why Unassigned Ports Still Matter

The registered port range has over 48,000 slots. IANA assigns thousands, but thousands more fill up through informal convention — tools and protocols that never went through official registration but became widespread enough that their port numbers are, in practice, reserved.

Port 2604 is one of those. It's not in the IANA registry, but any network engineer who recognizes it knows immediately: OSPF daemon, routing suite, Linux router. The unofficial standard is the actual standard.