Port 0 in TCP and UDP

Using Port 0 in TCP and UDP Network Programming

Linksys WRT54GS - Port Forwarding
Port Forwarding (Linksys WRT54GS).

Port 0 is a reserved port in TCP/IP networking, meaning that it should not be used in TCP or UDP messages. However, port 0 carries special significance in network programming, particularly Unix socket programming: for requesting system-allocated, dynamic ports.

Network ports in TCP and UDP range from number zero up to 65535. Port numbers in the range between zero and 1023 are defined as system ports, or well-known ports.

The Internet Assigned Numbers Authority (IANA) maintains an official listing of the intended usage of these port numbers on the internet; system port 0 is not to be used.

How Port 0 Works in Network Programming

Configuring a new network socket connection requires that one port number be allocated on both the source and destination side. TCP or UDP messages sent by the originator (source) contain both port numbers so that the message recipient (destination) can issue response messages to the correct protocol endpoint.

IANA has pre-allocated designated system ports for basic internet applications like web servers (port 80), but many TCP and UDP network applications do not have their own system port and must obtain one from their device's operating system each time they start running.

To allocate its source port number, applications call TCP/IP network functions like bind() to request one. The application can supply a fixed (hard-coded) number to bind() if they prefer to request a specific number, but such a request can fail because some other running application on the system may currently be using it.

Alternatively, it can provide port 0 to bind() as its connection parameter instead. That triggers the operating system to automatically search for and return a suitable available port in the TCP/IP dynamic port number range.

Note that the application will not actually be granted port 0 but rather some other dynamic port.

The advantage of this programming convention is efficiency. Instead of each application having to implement and run code for trying multiple ports until they obtain a valid one, apps can rely on the operating system to do so.

Unix, Windows, and other operating systems vary slightly in their handling of port 0, but the same general convention applies.

Port 0 and Network Security

Network traffic sent across the internet to hosts listening on port 0 might be generated from network attackers or accidentally by applications programmed incorrectly. The response messages that hosts generate in response to port 0 traffic can help attackers learn more about the behavior and potential network vulnerabilities of those devices.

Many internet service providers (ISPs) block traffic on port 0 (both incoming and outgoing messages) to help guard against these exploits.