TCP port 9 is primarily used for the Discard Protocol.
The Discard Protocol
The Discard Protocol, defined in RFC 863, is a very simple network service. Its sole function is to receive any data sent to it and then immediately discard it, without generating any response.
- Purpose: It's often used for network debugging and testing. For instance, a network administrator might send data to TCP port 9 on a remote host to verify basic network connectivity and throughput to that host without requiring any complex application-level response. It confirms that data can reach the destination port.
- Simplicity: Because it provides no response, it's one of the simplest diagnostic tools for checking if a port is open and reachable.
Port 9 and Wake-on-LAN
While port 9 is also associated with Wake-on-LAN (WoL), it's important to differentiate its use between TCP and UDP:
- TCP Port 9: Specifically serves the Discard Protocol.
- UDP Port 9: Wake-on-LAN typically utilizes UDP (User Datagram Protocol) on port 9 (or sometimes port 7) to send a "magic packet" that can remotely power on a computer. It's important to note that TCP is not used for Wake-on-LAN on port 9.
Well-Known Ports Context
Port 9 is classified as a "well-known port," which are ports reserved for specific services by the Internet Assigned Numbers Authority (IANA). These ports range from 0 to 1023.
Here's an excerpt of well-known ports, including port 9, highlighting their common uses:
| Port | TCP | Description |
|---|---|---|
| 9 | Yes | Discard Protocol |
| 9 | No | Wake-on-LAN (typically UDP) |
| 11 | Yes | Active Users (systat service) |
| 13 | Yes | Daytime Protocol |
Practical Insights
- Network Diagnostics: The Discard Protocol can be a quick way to test if a network path is open and functional to a specific port on a remote device. It helps confirm basic network reachability.
- Security Considerations: While the Discard Protocol itself is harmless as it simply discards data, any unnecessary open port can still be a potential entry point for network reconnaissance by attackers. As a general security best practice, it's advisable to close or filter ports that are not actively required.
