eap-ttls/mschapv2 versus eap-peap/mschapv2 behaviour

Phil Mayers p.mayers at imperial.ac.uk
Thu Jul 18 10:53:02 CEST 2013

On 07/18/2013 07:25 AM, Olivier Beytrison wrote:
> Hello,
> We're in the middle of migrating all of our radius server to FR3.
> This is the opportunity to discuss a the difference of behaviour between
> EAP-TTLS/MSCHAPv2 and EPA-PEAP/MSCHAPv2 which is bothersome.
> With EAP-TTLS/MSCHAPv2 we have the following flow

> With EAP-PEAP/MSCHAPv2 the flow is somewhat different

FWIW I don't think either of those are complete or accurate.

For starters, TTLS/MSCHAPv2 and TTLS/EAP-MSCHAPv2 are different, and 

Comparing TTLS/MSCHAP and PEAP/EAP-MSCHAPv2 is comparing two different 
things, and of course they may behave differently.

> This mean that with EAP-PEAP/MSCHAPv2, if the ldap/sql/xxx module in
> authorize{} add attributes to the reply, they will be sent during the
> last challenge/response in authenticate{}, and will not be present in
> post-auth or the final access-accept.

You're not being very precise here. authorize in the inner or outer tunnel?

> On our side, we are doing the policying in the inner-tunnel post-auth
> section, based on values directly mapped from ldap attributes to our
> private dictionary through the ldap module called in authz{}
> With EAP-TTLS we don't need to do anything special to make this work,
> but for PEAP actually we need to use the rlm_cache to cache and restore
> the attributes in post-auth.

Why don't you just set them in post-auth, and skip setting them in 
authorize completely?

You can do this, remember:

post-auth {

> It would be really nice if eap would cache automatically all attributes
> present in the reply list if it has to do another round of
> challenge/response and restore them once the final access-accept (or
> reject) is reached.

I'm not sure about that.

You definitely have to take care with reply attributes, but if you 
always set them in post-auth, you should always be safe.

Setting them in authorize is only safe if you set them on *every pass* 
through authorize. You're not doing this, because you have:

eap {
   ok = return

In 3.0, the "ok = return" will match on EAP-identity packets for the 
inner tunnel but *also* EAP-MSCHAPv2 success/failure packets. So, the 
final pass through the tunnel will be skipped.

This should be logged in the debug with a quite specific/verbose 
message, for exactly this reason:


Maybe this is the issue you're facing?

> Now this is a behaviour I always encountered in our configs (based on
> the default shipped config). Still it's possible that it is due to
> what've done. For this purpose you'll find the inner-tunnel config below
> [1].
> 		# that's where we need the attributes added
> 		# by the ldap module in authz{}
>                  wireless-policy
>          }

What does "wireless-policy" do, and where does it get its data from?

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