Erryl

I have revisited the list of bugs that we came across our last project and here’s one I would like to share…

After months of rigorous testing of our new web services we encountered a major showstopper, our web service would not install. We got an “AddressAlreadyInUseException”.

The exception was caused by a port colliding between our web services and some of our applications. We were surprised that it took months for the port collision to surface. Closing all the running applications, we tried to replicate the scenario but tough luck trying for the nth time it was not reproducible.  What we did after was to reopen those applications from our suite and alas we were able to catch the culprit. It turned out that our applications use ephemeral ports intermittently causing port collision with our web services as we install them.

So what is an ephemeral port?

An ephemeral port or “short lived port” is a dynamically allocated port generally used for the client-end of a client-server communication. In a client-server interaction, the server’s application typically listens on a well known port whereas the client’s application queries the operating system for a dynamically allocated TCP or UDP port that is not used by other applications.

What we thought was a safe port range (ports higher than 5000) for our web services has changed.  We're aware that earlier Windows versions (XP and 2003) used 1025 through 5000 as their default port range.  However we found out that for Windows Vista and Windows Server 2008, Microsoft complied with the port range recommended by the Internet Assigned Numbers Authority (IANA) - to use 49152 through 65535 as dynamic and/or private default port range.

So to lessen the chances of port collision at least for our applications and web services what we did (as Microsoft recommends it) was prior to installation of our application suite on a Vista or Server 2008 operating system, we ensure to revert back to the default port range of the old Windows versions, 1025 through 5000 using the netsh command;

netsh int <ipv4|ipv6> set dynamic <tcp|udp> start=number num=range

 in our case we used: netsh int ipv4 set dynamic tcp start=1025 num=3975

And to verify the dynamic tcp port range in Vista or Server 2008 we used

netsh int ipv4 show dynamicport tcp

 Some netsh commands are as well available for different protocols:

• netsh int ipv4 show dynamicport udp
• netsh int ipv6 show dynamicport tcp
• netsh int ipv6 show dynamicport udp 

A few important things I have read about port range:

• The smallest range of ports you can set is 255.
• The lowest starting port that you can set is 1025.
• The highest end port (based on the range you set) cannot exceed 65535.
  

Normal 0 false false false MicrosoftInternetExplorer4

For more info read  Article 929851