Web.Config

Configuration File Format

ASP.NET configuration files are XML-based text files--each named web.config--that can appear in any directory on an ASP.NET Web application server. Each web.config file applies configuration settings to the directory it is located in and to all virtual child directories beneath it. Settings in child directories can optionally override or modify settings specified in parent directories. The root configuration file--WinNT\Microsoft.NET\Framework\\config\machine.config--provides default configuration settings for the entire machine. ASP.NET configures IIS to prevent direct browser access to web.config files to ensure that their values cannot become public (attempts to access them will cause ASP.NET to return 403: Access Forbidden).

At run time ASP.NET uses these web.config configuration files to hierarchically compute a unique collection of settings for each incoming URL target request (these settings are calculated only once and then cached across subsequent requests; ASP.NET automatically watches for file changes and will invalidate the cache if any of the configuration files change).

For example, the configuration settings for the URL http://myserver/myapplication/mydir/page.aspx would be computed by applying web.config file settings in the following order:

Base configuration settings for machine.
C:\WinNT\Microsoft.NET\Framework\v.1.00\config\machine.config

Overridden by the configuration settings for the site (or the root application).
C:\inetpub\wwwroot\web.config

Overridden by application configuration settings.
D:\MyApplication\web.config

Overridden by subdirectory configuration settings.
D:\MyApplication\MyDir\web.config

If a web.config file is present at the root directory for a site, for example "Inetpub\wwwroot", its configuration settings will apply to every application in that site. Note that the presence of a web.config file within a given directory or application root is completely optional. If a web.config file is not present, all configuration settings for the directory are automatically inherited from the parent directory.