# WHATPORT(1023)

## NAME

port 1023 — Reserved — top of the system port range — IANA-reserved, and the last port of the privileged 0–1023 range that only root may bind on Unix.

Protocol: TCP  
IANA registry

## DESCRIPTION

RFC 6335 defines 0–1023 as the System (well-known) ports, which on Unix-like systems only a privileged process may bind, and IANA holds 1023 itself Reserved rather than assigning it. So no standard service lives here. Where 1023 does show up is as a source port: rsh, rlogin and NFS-era clients deliberately chose a reserved source port in 512–1023 as proof the caller was root on a trusted host. A listener on 1023 is a deliberate choice by something running with privilege.

## EXPOSURE

**Expose with care.** No assigned service means nothing to allow by default. The privileged-source-port convention it belongs to was host-based trust, which is not a security control on a modern network.

## CHECK

```sh
sudo lsof -nP -iTCP:1023
```

## SEE ALSO

- spec — [RFC 6335 — Port ranges: System (0–1023), User, and Dynamic](https://www.rfc-editor.org/rfc/rfc6335.html) (rfc-editor.org)
- iana — [IANA Service Name and Transport Protocol Port Number Registry (1023 Reserved)](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml) (iana.org)

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HTML: <https://whatport.net/1023/> · JSON: <https://whatport.net/1023.json> · full dataset: <https://whatport.net/ports.json>