Thursday, June 16, 2005

Getting dumps for apps failing on start up using terminal services

In my previous post I discussed the use of gflags to invoke a debugger to start when a process starts and then sending debugger commands to create dumps when exceptions occur. However if your trying to debug against a process run in session 0, e.g. a web application or service, this technique will not work. As most support teams will probably be using terminal services to access production servers clearly we need a technique to get the dumps.

Remote debugging is commonly used to debug across two machines i.e. setting up a debugger on the server and using a debugging client to connect to it using named pipes or tcp. However there is nothing stopping us using the same machine to host the debugging server and the client. We need to set up a debugger server which runs in session 0 and a debugger client which runs in the terminal services session i.e. 3 in our example.

As before open gflags and specify the image module name you need to debug however instead of simply entering the path to the debugger you should specify some command line options to tell windbg to start as a debugging server. Enter the following in the debugger field

C:\Debuggers\windbg.exe -server tcp:port=123 –Q

The server option creates a debugging server and the tcp:port option specifies the port number. The –Q option suppresses the “Save workspace” dialog which is important if the debugger is running in session 0 and can’t interact with the user. Note the command line options are case sensitive so be sure to copy this exactly as it appears. As soon as the process your debugging starts the windbg debugee should be loaded in session 0 and broken in on the LdrpInitializeProcess method.

To control the debugger server start a new instance of windbg as the debugging client. Press Ctrl+R to connect to a remote debugger session and enter the following


When you connect the windbg instance will connect to the remote debugger and allow you full control to enter commands e.g. you can enter

Sxe *

to break on all exceptions and then continue. You can also send the same commands to create dump files as mentioned in my previous post. Or you could simply use .dump to create a dump when a specific exception is raised e.g.

.dump /ma c:\debugger\ExceptionDump.dmp

If you look at task manager you should also see two instances of windbg open. If you select the “Session ID” column you can see the different terminal services sessions used for each instance.

No comments: