What is port 0 used for?

Port 0 has special meaning in network programming, especially in Unix when it comes to socket programming, where this port is used to request dynamic, system-allocated ports. Port 0 is a wildcard port, telling the system to find a matching port number.

Unlike most port numbers, port 0 is a dedicated port in a TCP / IP network, meaning it is not used in TCP or UDP messages. The network ports in TCP and UDP range from 0 to 65535.

The port number in the range 0 to 1023 is identified as the system port. The Internet Assigned Numbers Authority (IANA) maintains an official list of the intended uses of these port numbers on the Internet, and system port 0 will not be used.

How TCP / UDP port 0 works in network programming

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

IANA pre-allocates designated system ports for basic Internet applications, such as web servers (port 80), but many TCP and UDP network applications do not have their own system ports and must obtain a port from the system. device operator every time they run.

To be assigned the source port number, the application calls TCP / IP network functions such as bind () to request a port. The application can provide a fixed number (hard coded) for bind (), if one wants to request a specific port, but such request may fail in case another application runs on the system. currently using that port.

Additionally, the application can provide port 0 to bind () as the connection parameter. That triggers the operating system to automatically find and return a suitable available port in the range of dynamic TCP / IP port numbers.

The application is not given port 0 but instead is a number of dynamic ports. The advantage of this programming convention is efficiency. Instead of each application having to deploy and run code to test multiple ports until a valid port is available, applications can rely on the operating system.

Unix, Windows, and other operating systems will not be identical in handling port 0, but apply the same common convention.

Port 0 and network security

Network traffic sent over the Internet to servers listening on port 0 may be generated by network attackers, or by incorrectly programmed applications. The messages that the server generates in response to port 0 traffic help attackers learn about that device's behavior and potential network vulnerabilities.

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