Proxy CAS Walkthrough

A manual walkthrough of CAS proxy tickets.
This walkthrough was provided by David Spencer on the CAS Mailman list.

Introduction

When I was trying to understand the mechanisms involved in writingproxying applications using CAS, I found it very helpful to manuallywalkthrough the aquisition of a proxy ticket. The CAS server playeditself in this exercise and I played all the other roles - user,proxying application and proxied application - simply by constructingURLs and feeding them into a web browser.

The only part of the exercise that can't be done with just a webbrowser and careful URL construction is the part where CAS makes it'sown callback to the proxying application. For this, I chose a proxycallback url on a machine for which I had access to the log files andscanned through the HTTP requests to find the information I wanted.

Step One: login

To start with, log in to CAS with some invented service:

https://foo.bar.com/is/cas/login?service=http://localhost/bling

On successful login, CAS will redirect you to the service with aticket appended (it doesn't matter that the service is made up as theticket you're after is part of the url and will appear in the locationbar even if your browser can't find the resource):

http://localhost/bling?ticket=ST-956-Lyg0BdLkgdrBO9W17bXS

Step Two

(a): verify the ticket and be done

So, playing the role of the first application (not a proxyingapplication at this stage - lets just see if we can get our applicationauthenticated without proxying for now), you need to take the ticketand turn it into a username:

https://foo.bar.com/is/cas/serviceValidate?ticket=ST-956-Lyg0BdLkgdrBO9W17bXS&service=http://localhost/bling

which will produce a result like:

<cas:serviceResponse xmlns:cas

='http://www.yale.edu/tp/cas'>




<cas:authenticationSuccess>




<cas:user>

endjs</cas:user>




</cas:authenticationSuccess>




</cas:serviceResponse>

This is the end of the road for normal applications that don't need to proxy other applications.

Step Two (b): verify the ticket and enable further proxying

If instead you do want to be able to proxy other applications youneed to also supply a pgtUrl to your validation request so that CAS cancallback with the Proxy Granting Ticket. This is where life getscomplicated, especially if you forget that service tickets areone-time-only tickets and that once you've used them withserviceValidate, you have to go back to CAS and get a new one (so ifyou've done Step One and Step Two (a) you'll need to do Step One againbefore you can do Step Two (b)).

The choice of pgtUrl here is fairly arbitrary except that it needsto be an https url and it needs to be on a server on which you canaccess the log files.

https://foo.bar.com/is/cas/serviceValidate?ticket=ST-956-Lyg0BdLkgdrBO9W17bXS

&service=http://localhost/bling&pgtUrl=https://foo.bar.com/pgtCallback

results in:

<cas:serviceResponse xmlns:cas

='http://www.yale.edu/tp/cas'>




<cas:authenticationSuccess>




<cas:user>

endjs</cas:user>




<cas:proxyGrantingTicket>

PGTIOU-85-8PFx8qipjkWYDbuBbNJ1roVu4yeb9WJIRdngg7fzl523Eti2td</cas:proxyGrantingTicket>




</cas:authenticationSuccess>




</cas:serviceResponse>

Step Three: dig out the PGT

Now our first application knows who the user is and has a ProxyGranting Ticket IOU. To find the real PGT we look in the apache accesslog for foo.bar.com and hunt out the request made by CAS to deliver thePGT:

foo.bar.com - - [10/Dec/2003:09:28:15 +0000] "GET


/pgtCallback?pgtIou=PGTIOU-85-8PFx8qipjkWYDbuBbNJ1roVu4yeb9WJIRdngg7fzl523Eti2td


&pgtId=PGT-330-CSdUc5fCBz3g8KDDiSgO5osXfLMj9sRDAI0xDLg7jPn8gZaDqS HTTP/1.1" 200 13079

(Editor's note: linebreaks introduced for page formatting.)

Step Four: get a proxy ticket

With the PGT in our grasp we can make a call on CAS to give us a proxy ticket for some other service we wish to proxy:

https://foo.bar.com/is/cas/proxy?targetService=http://localhost/bongo&pgt=PGT-330-CSdUc5fCBz3g8KDDiSgO5osXfLMj9sRDAI0xDLg7jPn8gZaDqS

resulting in:

<cas:serviceResponse>




<cas:proxySuccess>




<cas:proxyTicket>

PT-957-ZuucXqTZ1YcJw81T3dxf</cas:proxyTicket>




</cas:proxySuccess>




</cas:serviceResponse>

Step Five: verify the proxy ticket

Now we take on our final role for the exercise - the proxiedapplication. The proxying application has invoked our service url andhas passed in the proxy ticket it's got. We take that ticket andvalidate it to find out both who the user is and which applications arein the proxy chain:

https://foo.bar.com/is/cas/proxyValidate?service=http://localhost/bongo&ticket=PT-957-ZuucXqTZ1YcJw81T3dxf

resulting in:

<cas:serviceResponse xmlns:cas

='http://www.yale.edu/tp/cas'>




<cas:authenticationSuccess>




<cas:user>

endjs</cas:user>




<cas:proxies>




<cas:proxy>

https://foo.bar.com/pgtCallback</cas:proxy>




</cas:proxies>




</cas:authenticationSuccess>




</cas:serviceResponse>

Obviously, this walkthrough doesn't help with acquiring and pluggingin good proxying code for your application but it does help to see whatthe proxying code needs to be doing and makes it easier to write yourown.

Originally provided by: David Spencer on the CAS mailing list.

Useful links:

Cas installation:

http://www.ja-sig.org/wiki/display/CASUM/Demo

Java Client:

http://www.ja-sig.org/wiki/display/CASC/Using+the+basic+Java+CAS+Client+objects