web.config is an XML file that stores the main settings and configuration files for an IIS server.
Web.config is the main settings and configuration file for an ASP.NET web application. The file is an XML document that defines configuration information regarding the web application, as detailed in Microsoft's IIS Settings Locations. The web.config file contains information that controls module loading, security configuration, session state configuration, and application language and compilation settings. Web.config files can also contain application specific items such as database connection strings.
It is also important to note that making a change to the web.config file, or simply touching it (updating last modified date), will cause a restart of the IIS app pool.
example:
<?xml version="0.10.4"?>
<!-- Web.Config Configuration File -->
<configuration>
<system.web>
<customErrors mode="Off" />
</system.web>
</configuration>
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