Regmon by Sysinternals.com

Regmon
By Sysinternals.com

Introduction
Regmon is a Registry monitoring utility that will show you which applications are accessing your Registry, which keys they are accessing, and the Registry data that they are reading and writing. All of this is presented in real-time. This is an advanced utility, and it takes you one step beyond what static registry tools are able to do by letting you see and understand exactly how programs use the Registry. With static tools you are able to see what Registry values and keys changed, but that's all! With Regmon you'll see how these values and keys change.

Regmon works on NT 4.0, Win2K, Windows 95, Windows 98, Windows ME.

Sample Screen Shot

Installation and Use
Install Regmon by copying the files to your hard drive, and start it by running Regmon.exe. Menu items and tool bar buttons can be used to toggle on and off monitoring, disable event capturing, control the scrolling of the listview, and save the listview contents to an ASCII file.

Use the Filter dialog, which is accessed with a toolbar button or the Edit|Filter/Highlight menu selection, to select what data will be shown in the list view. The '*' wildcard matches arbitrary strings, and the filters are case-insensitive. Only matches shown in the include filter, but that are not excluded with the exclude filter, are displayed. Use ';' to separate multiple strings in a filter (e.g. "regmon;software").

For example, if the include filter is HKLM", and the exclude filter is "HKLM\Software", all references to keys and values under HKLM, except to those under HKLM\Software will be monitored.

Wildcards allow for complex pattern matching, making it possible to match specific Registry accesses by specific applications, for example. The include filter “Winword*Windows” would have Regmon only show accesses by Microsoft Word to keys and values that include the word “Windows”.

Use the highlight filter specify output that you want to have highlighted in the listview output. Select highlighting colors with Edit|Highlight Colors.

Regmon can either timestamp events or show the time elapsed from the last time you cleared the output window (or since you started Regmon). The Options menu and the clock toolbar button let you toggle between the two modes. The button on the toolbar shows the current mode with a clock or a stopwatch. When showing duration the Time field in the output shows the number of seconds it took for the underlying file system to service particular requests.

Regmon v4.1 introduces a powerful new feature. When you see a Registry value or key in Regmon's output that you want to edit, simply double click on the line that includes the reference (or use the Regedit toolbar button) and Regmon will take you directly to the specific value using Regedit.

Click here to learn about Regmon's boot monitoring capability, which is available on Windows NT.

How Regmon Works
The heart of Regmon on Windows 9x is in the virtual device driver, Regvxd.vxd. It is dynamically loaded, and in its initialization it uses VxD service hooking (See the May 1996 Dr. Dobb's Journal article on VxD service hooking for more information) to insert itself onto the call chain of 16 registry access functions in the Windows 95 kernel (Virtual Machine Manager). All registry activity, be it from 16-bit programs, Win32 applications, or device drivers, are directed at these routines, so Regmon catches all registry activity taking place on a machine.

On Windows NT the Regmon loads a device driver that uses a technique pioneered by Mark Russinovich and Bryce Cogswell for NT called system-call hooking. When a user-mode component makes a privileged system call, control is transfered to a software interrupt handler in NTOSKRNL.EXE (the core of the Windows NT operating system). This handler takes a system call number, which is passed in a machine register, and indexes into a system service table to find the address of the NT function that will handle the request. By replacing entries in this table with pointers to hooking functions, it is possible to intercept and replace, augment, or monitor NT system services. Regmon, which obviously hooks just the Registry-related services, is merely one example of this capability in action.

When Regmon sees an open, create or close call, it updates an internal hash table that serves as the mapping between key handles and registry path names. Whenever it sees calls that are handle based, it looks up the handle in the hash table to obtain the full name for display. If a handle-based access references a key opened before Regmon started, Regmon will fail to find the mapping in it hash table and will simply present the key's value instead.

Information on accesses is dumped into an ASCII buffer that is periodically copied up to the GUI for it to print in its listbox. For more detailed information on how Regmon works on Windows NT, we recommend that you read:

Windows NT System Call Hooking, by Mark Russinovich and Bryce Cogswell, Dr. Dobb's Journal, January 1997
Inside NT Utilities, Windows NT Magazine, February 1999.