Portal authentication using two login module stacks?

Question

G'day,
I am noticing something odd when I authenticate to the portal: there are two login module stacks used.
Background: I have created a custom logon page, which is basically a form with username/password input as per [this guide|http://help.sap.com/saphelp_nw04/helpdata/en/62/601e1eebf54ca6a97e2873c8c63517/content.htm|Changing the logon screen]. I then modified the authschemes.xml file by defining a new authscheme "mylogon" that uses my own login module stack ("mystack") and uses the new logon page ("mylogonform"). This new authscheme is then made the default reference:
<authscheme name="mylogon">
  <authentication-template>mystack</authentication-template>
  <priority>21</priority>
  <frontendtype>2</frontendtype>
  <frontendtarget>com.foo.bar.mylogonpage</frontendtarget>
</authscheme>
<authscheme-refs>
  <authscheme-ref name="default"><authscheme>mylogon</authscheme></authscheme-ref>
  <authscheme-ref name="UserAdminScheme"><authscheme>mylogon</authscheme></authscheme-ref>
</authscheme-refs>
When I want to access the portal, up pops the "mylogonform" page, and on clicking the "submit" button the portal page for the user is shown.
Now here is the interesting thing: when the "ticket" login module stack is unchanged (ie. it uses the BasicpasswordLoginModule), then the log shows that authentication to the portal uses just my login module.
This can be seen as follows, where I navigate to the portal, logon as one user, then logoff and logon as another user:
Message : LOGIN.OK
User: tu-1
Authentication Stack: mystack
Message : LOGOUT.OK
User: tu-1
Authentication Stack: mystack
Message : LOGIN.OK
User: Administrator
Authentication Stack: mystack
The "mylogonform" page is shown when logon is required in both cases.
However, if I modify the "ticket" login module stack by replacing the BasicPasswordLogonModule with a custom logon module that does automatic authentication, then the following is observed when the "mylogonform" page is displayed:
Message : LOGIN.FAILED
User: N/A
Authentication Stack: ticket
Message : LOGIN.OK
User: tu-1
Authentication Stack: ticket
For some reason, the modified "ticket" login module stack is now being executed, which was not the case when this login module stack was unmodified.
This stack automatically authenticates the current user (the initial failure is because the new login module asks the browser to send authentication data), and this "failure" causes the logon form to be displayed.
I can logon to the portal as the same user, and the logs show that "mystack" login module stack is used:
Message : LOGIN.OK
User: tu-1
Authentication Stack: mystack
Logoff shows that "mystack" is used for the actual logoff, but "ticket" is called again automatically and succeeds:
Message : LOGOUT.OK
User: tu-1
Authentication Stack: mystack
Message : LOGIN.FAILED
User: N/A
Authentication Stack: ticket
Message : LOGIN.OK
User: tu-1
Authentication Stack: ticket
(Again, the initial logon failure is the new login module requesting that the browser send authentication data in the next request).
This brings up the "mylogonform" page, even though it appears that a user has already been authenticated. If I try to logon as another user, the following is shown:
Message : LOGIN.FAILED
User: Administrator
Authentication Stack: mystack
Login Module                                                            Flag        Initialize  Login      Commit     Abort      Details
com.sap.security.core.server.jaas.EvaluateTicketLoginModule             SUFFICIENT  ok          exception  false      true       authscheme not sufficient: basicauthentication<mylogonform
Central Checks                                                                                exception             Call logout before login.
I guess one cannot authenticate as a new user until the current user has been logged out.
So ... why does the "ticket" login module get called in the second case, but not in the first case (or only shows logging in the second case) ?
What is the logic behind portal authentication and showing a logon page?
If I want to use custom authentication and a custom logon page, why is the "ticket" stack called at all?

Answer 1

Jayesh,
there is no such thing like "login module stacks". The <b>do</b> exist on the other hand:
- login module
- logon stacks
Login module and logon stacks are part of the JAAS concept for defining a complex pluggable authentication scheme, original by SUN (see: java.sun.com/products/jaas)
A logon process is defined by a logon stack which itself consists of several login modules. Each login module performs an authentication step. Example:
login module 1: check if valid sap logon ticket provided
if module 1 fails: then login module 2: request user id/password
if module 2 succeeds: then login module 3: create new sap logon ticket for user
You can define multiple logon stacks and configure individual applications to use the one stack or the other.
The logon stack configuration is done using visual administrator. Here select the security provider service for configuring logon stacks.
btw: As logon stacks are "java-only", there are no transaction names (which only exist on Web AS ABAP).
Regards,
Dominik