[OpenSIPS-Devel] [OpenSER-Devel] SF.net SVN: openser: [4294] trunk/modules/auth

[Dan Pascu]

On Tuesday 16 December 2008, Alex Hermann wrote:
> On Monday 02 June 2008, Anca Vamanu wrote:
> > Revision: 4294
> >          
> > http://openser.svn.sourceforge.net/openser/?rev=4294&view=rev Author:
> >   anca_vamanu
> > Date:     2008-06-02 08:18:46 -0700 (Mon, 02 Jun 2008)
> >
> > the auth module keeps state for each nonce - to validate it only on
> > the first usage. A binary array (which can by default accomodate 100K
> > nonces) is used to keep the state. An index in this array is
> > allocated when the challenge is generated; this index in kept for the
> > whole life duration of the nonce. After the first auth result (for
> > the nonce), the following auth results for that nonce are discarded
> > and re-challenged.
>
> Although I like the added security, I think this solution is
> fundamentally wrong. The nonce does not necessarily have been created
> by the same proxy. In a SRV-loadbalanced cluster, the UAC may choose to
> send the authenticated request to another server as from which it
> obtained the nonce
> (Unfortunately, this nasty behaviour is present in the wild).
>
> The proxy should also accept (once!) a nonce it doesn't yet know about.
> When one assumes the nonce has been created with a Kamailio server, it
> may be possible to encode the lifetime into the nonce and extract it on
> first usage.
>
> I know this means every nonce can be used once at every proxy, but
> that's hard to prevent unless all proxies communicate with each other.

I have to agree with this and ask that we add a modparam to control if 
nonce reuse is allowed or not.

I have this case that is broken by the new behavior:

I have a farm of proxies that are advertised in DNS. A UAC will do a DNS 
lookup and send a request to one of the IP addresses that were returned. 
However the internal structure of the network dictates that a certain AOR 
is handled by a certain proxy that is always the same for a given 
collection of proxies, but may change if the proxy collection changes.

This means that if if I send a request to proxy P1, it will detect that 
the proxy that handles my account is Ph, and it will forward the request 
there. Ph will generate an auth reply and a nonce with it.

Now assume that the proxy collection changes while it sends me back the 
401 auth challenge. I will resend the request with an auth response this 
time, and even assuming that I still send it to P1 where I sent the 
original request, P1 will this time see that Px is the new proxy that 
handles my account and will forward the request there. This proxy will 
not recognize the nonce and will issue a new auth challenge with a new 
nonce and send back the 401. This time my UAC will simply give up, 
because most of them when they receive 2 auth challenges in a row 
consider that they have the wrong password and stop trying.

I have seen this in real life and the solution (with openser 1.3 at least) 
was to set the same shared secret on all proxies to allow them to share 
the nonces. But with the new algorithm, even with the secret sharing is 
still unable to work as nonces cannot be shared anymore.

In my case the effect is that every network transition that adds/removes a 
proxy to my proxy collection will leave me with phones that will not 
register anymore until manually reset.

-- 
Dan