ASP.NET supports forms authentication in a distributed environment, either across applications on a single server or in a Web farm. Several configuration attributes in the <forms> element configuration section must match for all applications participating in shared forms authentication. The directives name, protection, path, validationKey, decryptionKey, and validation in the following example must be identical (unless otherwise noted) across the applications.

<configuration>
    <system.web>
        <authentication>
            <forms name=".ASPXAUTH" 
                   loginUrl="logon.aspx"     
                   protection="all"  <!-- Protection must be identical.-->
                   timeout="30" 
                   path="/" >   <!-- Path must have a compatible scope.-->
        </authentication>

        <!-- Validation and decryption keys must exactly match and cannot
             be set to "AutoGenerate". The validation algorithm must also 
             be the same. -->
        <machineKey>
            validationKey= "C50B3C89CB21F4F1422FF158A5B42D0E8DB8CB5CDA1742572A487D9401E3400267682B202B746511891C1BAF47F8D25C07F6C39A104696DB51F17C529AD3CABE" 
            decryptionKey= "8A9BE8FD67AF6979E7D20198CFEA50DD3D3799C77AF2B72F" 
            validation="SHA1"
            isolateApplications="false"
        </machineKey>
    </system.web>
</configuration>

The encryption and validation keys and the encryption scheme used for cookie data must be exactly the same. If the settings do not match, cookies cannot be shared.

After a cookie has been issued, expiration of the cookie is tracked based on the Expires value in the cookie itself. This means that if two applications have different timeout attributes, the expiration date and time that was set when each cookie was originally issued is retained throughout the lifetime of the cookie. When a cookie is updated, the cookie's original expiration is used to compute the new expiration. The only time the configuration timeout value is used is when the cookie is initially created.

See Also

ASP.NET Web Application Security | Simple Forms Authentication