Binding Process to TCP/UDP Port Failure on Windows

Windows has the concept of reserved TCP/UDP ports. These ports can nonetheless be used by any other application. These can be annoying because the reserved ports would not shown be used when we query used ports using netstat. For instance, if we want to bind TCP port 23806 to an application, we determine the availability using the netstat command, such as

C:> netstat -anp tcp | find ":23806"

C:>

The output is blank, which means that the port is unused. However, when we attempt to bind the port to a process of our choice, we encounter an error, such as

bind [127.0.0.1]:23806: Permission denied

This is annoying. The reason is that the port somehow becomes a reserved port. To see this, we can query reserved ports, e.g.,

C:> netsh int ipv4 show excludedportrange protocol=tcp

Protocol tcp Port Exclusion Ranges

Start Port    End Port
----------    --------
      1155        1254
      ...          ...
     23733       23832
     23833       23932
     50000       50059     *

* - Administered port exclusions.


C:>
  

which shows that 23806 is now a served port. What is really annoying is that the range can be updated by Windows dynamically. There are several methods to deal with this.

1. Method 1. Stop and start the Windows NAT Driver service.

   
     net stop winnat
     net start winnat
     

   After this, query the reserved the ports again. It is often the reserved ports are much limited when compared to before, e.g.,

   
   C:>netsh int ipv4 show excludedportrange protocol=tcp
   
   Protocol tcp Port Exclusion Ranges
   
   Start Port    End Port
   ----------    --------
         2869        2869
         5357        5357
        50000       50059     *
   
   * - Administered port exclusions.
   
   C:>
     

2. Method 2. If you don't wish to use this feature of Windows, we can disable reserved ports.

   
   reg add HKLM\SYSTEM\CurrentControlSet\Services\hns\State /v EnableExcludedPortRange /d 0 /f