How search user in Active Directory and then find it in a table's Aplication with c# ?

Hi everyone:

I need to make a login that must make the follow:
1- When the user is connected to the INTRANET, is in Active Directory where they need to put the user name and password.
2.- Once the user is inside the INTRANET, he can go to his explorer, open a Web Page and he'll never enter the username and Password BUT:
a) Behind in the background the user send to the server his username and password from the sesion when he entered to the INTRANET, then:
b) If the username and password in the Active Directory are correct, the aplication return a true and then this same username and passworda are:
c) Are searched in the tables of the application that we are development, if this final step are true then:
d) Return true and the username and password are saved in a Session variable


500 Point to this SOLUTION, thank you so much.


In the example below, this code receive the username and password, BUT where I can get the username and password if this are posting where the user are enter to INTRANET?

private bool Authenticate(string userName,
    string password, string domain)
{
    bool authentic = false;
    try
    {
        DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain,
            userName, password);
        object nativeObject = entry.NativeObject;
        authentic = true;
    }
    catch (DirectoryServicesCOMException) { }
    return authentic;
}

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