Tuesday, September 29, 2015

Hitting Shibboleth from a Cloud Service

Say you use a mobile browser to hit a web resource which is protected as an SP (service provider), authenticated by a Single Sign-On service's IdP (Identity Provider). Say you authenticate, get your resource, and then switch from wifi to your cellular data provider. If you then refresh the resource, this will hide your SP session (so, it will send you back to the IdP login page), because your IP has changed -- although your original session will probably still work if you switch back to your wifi network. This is the system behavior if your SSO uses the default settings.

But you can configure Shibboleth SP to ignore the changing IP address, by adding this property to the Sessions element in /etc/shibboleth/shibboleth2.xml:


so that it looks something like:



... and then restart shibd.

What does this have to do with a cloud service? Well, if you tried to turn your Google App Engine cloud-based application, on appspot.com or some configured domain, into a User Agent (UA), in the UA/SP/IdP trinity, and you check the logs on the SP or IdP, you will notice that as a UA Google changes IP addresses constantly. So, as said above, you need to turn off the SP's hidden default 'true' value for consistentAddress.

If you want your SSO system to work in this scenario, you'll probably also need to ignore different IP's on the IdP. This is a little harder: you need to re-install shibboleth (without overwriting the existing configuration). But first -- go to the directory you installed the Shibboleth IdP from, then go to src/main/webapp/WEB-INF, and edit the web.xml file. Add the following within the filter section, right after the end of the definition of the filter class:


Then you need to: 
  • stop tomcat
  • run the install script again -- make sure to answer "no" to "Would you like to overwrite this Shibboleth configuration?"
  • start tomcat
Then you can use HTTPClient or your favorite user-agent framework to interact with the Shibboleth IdP and SP from Google App Engine or Google Cloud Platform.


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