Port 870: Unassigned — A Ghost in the RPC Range

Port 870 sits in an interesting gray area of Internet infrastructure. Officially, it's unassigned. IANA—the organization that decides which services get which port numbers—has never registered a protocol here. But that doesn't mean nothing ever ran on it.

The Well-Known Port Range

Port 870 falls within the well-known port range (0-1023), also called system ports. These are the ports that operating systems reserve for system-level services—the fundamental protocols that make networks work.

Getting a well-known port assigned requires going through IANA's formal process, usually backed by an RFC (Request for Comments) that documents the protocol. Port 870 never went through that process. It remains officially unassigned for both TCP and UDP.¹

The Mac OS X Connection

Here's where it gets interesting. Port 870 falls within a range (600-1023) that Mac OS X historically used for RPC-based services.² These ports weren't assigned by IANA—Apple just used them internally for system services.

One of those services was NetInfo, a hierarchical distributed database that stored administrative data: user accounts, group memberships, email configurations, NFS settings, printer configurations. Think of it as a precursor to modern directory services.³

NetInfo is gone now. Apple removed it completely in Mac OS X 10.5 (Leopard), released in 2007. It was replaced by Open Directory, which has been the standard since then.³

But the port numbers remain. Documentation still mentions them. Security scanners still check them. The institutional memory of the Internet moves slowly.

What "Unassigned" Actually Means

When IANA says a port is unassigned, it means:

• No official protocol is registered — There's no RFC defining what should run here
• Available for future allocation — IANA could assign this port to a new protocol tomorrow
• Used informally — Applications can and do use unassigned ports for private purposes

The state of a port can be Assigned, Unassigned, or Reserved. Only ports with Unassigned state are free to use. When a port is registered for one transport protocol (say, TCP), it's marked as Reserved in the other protocol's registry (UDP)—preventing conflicts but leaving it technically unavailable.⁴

Why Unassigned Ports Matter

The existence of unassigned ports in the well-known range represents breathing room in the Internet's addressing system. Not every protocol needs a port number. Not every service deserves to be in the system range.

IANA guards these allocations carefully because there are only 1,024 well-known ports, and they can never be reclaimed. Once a port is assigned and services depend on it, that assignment is permanent. The HTTP on port 80 is forever.

Unassigned ports like 870 are the unused seats at a sold-out concert. They might fill up someday. Or they might stay empty, waiting for a protocol important enough to deserve them.

How to Check What's Listening

If you want to see whether anything is actually using port 870 on your system, you can check:

On macOS/Linux:

# See what's listening on port 870
sudo lsof -i :870

# Or use netstat
netstat -an | grep 870

On Windows:

# See what's listening on port 870
netstat -ano | findstr :870

If something appears, you've found either:

• An application using port 870 for private purposes
• Residual system service from older software
• Potentially malicious software (rare, but unassigned ports can be exploited)

The Ghost Ports

Port 870 is what you might call a ghost port—not officially assigned, not actively used, but not quite forgotten either. It exists in the space between formal allocation and complete absence.

The Internet is full of these ghosts. Protocols that were planned but never deployed. Services that ran for years on borrowed port numbers. Systems that disappeared but left their documentation behind.

They're reminders that the Internet's addressing system isn't just technical—it's archaeological. Every port number is a layer of history, and sometimes the only thing left is the number itself.