In this tutorial, you will find a step by step instruction how to add ELMAH (Error Logging Modules and Handlers for ASP.NET) to your existing ASP.NET MVC application. ELMAH (Error Logging Modules and Handlers) is an application-wide error logging facility that is completely pluggable. It can be dynamically added to a running ASP.NET web application, or even all ASP.NET web applications on a machine, without any need for re-compilation or re-deployment.

I've tested the steps below with ASP.NET MVC 2 and ASP.NET MVC 3 applications, but they may work also for other types of ASP.NET applications.

First you have to download ELMAH from the download page. After adding a reference to Elmah.dll in your project, all you have to do is to add the ELMAH configuration to your Web.config.

The example configuration shown above includes two customizings I found very useful for web applications:

- If the server returns a status code between 400 (included) and 500 (excluded), ELMAH ignores the exceptions.
- A remote access to the /elmah.axd web interface is enabled for specific (admin) users.

<configSections>
  [...]
  <sectionGroup name="elmah">
    <section name="security" requirePermission="false"
type="Elmah.SecuritySectionHandler, Elmah" />
    <section name="errorLog" requirePermission="false" type="Elmah.ErrorLogSectionHandler, Elmah" />
    <section name="errorMail" requirePermission="false" type="Elmah.ErrorMailSectionHandler, Elmah" />
    <section name="errorFilter" requirePermission="false" type="Elmah.ErrorFilterSectionHandler, Elmah" />
  </sectionGroup>
  [...]
</configSections> 

<system.web>
  [...]
  <httpHandlers>
    <add verb="POST,GET,HEAD" path="elmah.axd" type="Elmah.ErrorLogPageFactory, Elmah" />
  </httpHandlers>
  <httpModules>
    <add name="ErrorLog" type="Elmah.ErrorLogModule, Elmah" />
    <add name="ErrorMail" type="Elmah.ErrorMailModule, Elmah" />
    <add name="ErrorFilter" type="Elmah.ErrorFilterModule, Elmah" />
  </httpModules>
  [...]
</system.web> 

<system.webServer>
  <modules runAllManagedModulesForAllRequests="true">
    <add name="ErrorLog" type="Elmah.ErrorLogModule, Elmah" />
  </modules>
  <handlers>
    <add name="Elmah" verb="POST,GET,HEAD" path="elmah.axd" type="Elmah.ErrorLogPageFactory, Elmah" />
  </handlers>
</system.webServer> 

<elmah>
  <security allowRemoteAccess="1" />
  <errorLog type="Elmah.XmlFileErrorLog, Elmah" logPath="~/App_Data" />
  <errorMail from="[FROM_ADDRESS]" to="[TO_ADDRESS]" subject="[SUBJECT]" async="true" smtpPort="25" smtpServer="[SMTP_SERVER]"/>
  <errorFilter>
    <test>
      <and>
        <greater binding="HttpStatusCode" value="399" type="Int32" />
        <lesser binding="HttpStatusCode" value="500" type="Int32" />
      </and>
    </test>
  </errorFilter>
</elmah> 

<location path="elmah.axd">
  <system.web>
    <authorization>
      <allow users="[ALLOWED_ADMIN_USERS]" />
      <deny users="*" />
    </authorization>
  </system.web>
</location>