[Pki-devel] SPNEGO for Dogtag

[Endi Sukma Dewata]

On 11/12/2015 12:40 AM, Fraser Tweedale wrote:
> Hi all,
>
> Just an update on my investigations of doing SPNEGO authn to
> Dogtag/Tomcat and a summary of what I think I'll need to do to get
> it going.
>
> Even for a trivial app configuring Tomcat to do SPNEGO is pretty
> awful.  I made some notes[1] on this and I'll turn that into a blog
> post later, once I have a Realm story to go with it (I'm thinking
> mod_lookup_identity for Tomcat; more on that below).
>
> [1] https://github.com/frasertweedale/notes-redhat/blob/master/tomcat.rst
>
>
> The unfortunate case is that the `Authenticator' interface does not
> chain, hence we already have our own `SSLAuthenticatorWithFallback'
> authenticator valve implementation which we use to fall back to
> Basic authentication when there is no client certificate.
>
> I see two general approaches to the "multiple authentication
> options" problem.
>
> a) a "parallel deployment" in a separate <Context> where we host the
> app in parallel at '/<subsystem>/spnego/...' or
> '/spnego/<subsystem>/...' and use the `SpnegoAuthenticator' valve.
> This is a pretty "heavyweight" approach and I believe sessions from
> one do not flow to the other, and a whole separate CMS application
> is running.  So it does not seem viable.

This would be the cleanest approach since the client will only talk to 
the web application that supports the same authentication. So IPA and 
Barbican will be talking to /ipa/kra and /barbican/kra that is 
configured with SPNEGO, and CA will be talking to /pki/kra that is 
configured with basic/client cert authentication.

As you know the issue is currently we do not support multiple PKI 
subsystems of the same type running on the same Tomcat instance, and 
adding that support is a huge task.

However, there might be a relatively simpler alternative. The PKI 
subsystems probably can be moved out of the web application 
(WEB-INF/lib) into the Tomcat server itself (common/lib). The web 
applications will remain exists, but it's only used as a façade to the 
PKI subsystems running in the background. This way a single PKI 
subsystem can have multiple web applications customized for different 
clients.

> b) `SSLAuthenticatorWithFallback' becomes "SSL or SPNEGO or Basic"
> valve.  The problem here is that protocol-wise there is no
> indication on initial request whether client can do SPNEGO or not,
> so it is not clear how server should respond.  An example of how
> this could be resolved is the client providing a '?spnego=1' query
> param; ugly but it should work, and w.r.t. REST API the client need
> only do it on /rest/account/login and use their session cookie
> thereafter, so I don't think it is too onerous.
>
> So it looks like (b) but if someone can suggest another approach I'm
> listening!

The issue with (b) is it's mixing the users, groups, authenticators, 
realms, and subsystem-specific objects such as KRA secrets for different 
clients (IPA, Barbican, and PKI itself) into a single pool, plus the 
ugly URL (your word, not mine :) ).

If the option (a) is doable within the timeframe I'd rather pick that 
one. It can avoid a number of problems and solve other issues as well.

> As for the Realm, the current implementation is a bit unorthodox -
> we use our own ProxyRealm class and "inject" a PKIRealm into it.
> I'm not sure why this approach is needed becaues PKIRealm does not
> need any configuration either on instantiation or afterwards.

The ProxyRealm is needed to resolve class dependency issue (which can be 
solved with option (a)):

http://pki.fedoraproject.org/wiki/Authentication

Tomcat requires that the classes for the authenticator and the realm 
must be stored in common/lib. The problem is the PKI classes that do the 
authentication are stored in the WEB-INF/lib, which is not accessible 
directly by the realm. To address this problem, a ProxyRealm is created 
in common/lib to satisfy Tomcat's requirement and a PKIRealm is created 
in WEB-INF/lib to execute the actual authentication. When the Web 
application is started, the ProxyRealm will be linked to the PKIRealm 
(see CMSStartServlet.java below). The ProxyRealm will then forward any 
incoming authentication request to the PKIRealm.

> I think the best approach here would be to use CombinedRealm[2] and
> add PKIRealm as well as whatever other Realms are needed.  For IPA
> usage I am leaning toward the mod_lookup_identity approach; given
> principal name look up roles (groups) via D-Bus.

The CombinedRealm will not be needed with option (a).

> [2] https://tomcat.apache.org/tomcat-7.0-doc/config/realm.html#Combined_Realm_-_org.apache.catalina.realm.CombinedRealm
>
> Jan: does this approach make sense to you and do you know of any
> projects to do this with Tomcat already?
>
> Ade: what sort of behaviour do you need in the Realm for the Barbian
> use case?
>
>
> Any and all feedback welcome.
>
> Thanks,
> Fraser
>


-- 
Endi S. Dewata