Re: ldap_sasl_interactive_bind_s

[Simon Wilkinson <simon@sxw.org.uk>]

On 29 May 2007, at 23:33, Howard Chu wrote:

> Quanah Gibson-Mount wrote:
> > I'm working on a patch to add LDAP SASL support to Postfix 2.4 (I  
> > made one for 2.2/2.3 a long time ago), and this time I want it to  
> > be accepted upstream, so I'm working on what they feel the issues  
> > are.
> > Right now, they
> > (a) always want LDAP_SASL_QUIET enabled (makes perfect sense to me)
> > and
> > (b) want the SASL mechanism to be a list of mechanisms the client  
> > supports, that should be tried when connecting to the server.
> > I think (b) is rather non-sensical, given the configurations are  
> > rather different between things like DIGEST-MD5, EXTERNAL, and  
> > GSSAPI just to start, but...
> > I assume to support this I should use the  
> > ldap_sasl_interactive_bind_s function, which takes as a parameter  
> > a list of mechanisms, if I'm reading it right.  The question to me  
> > comes up with mixing LDAP_SASL_QUIET in, because part of the  
> > routine involved with multiple mechansisms seems to want  
> > interaction with the client.
> > My assumption is that if I use ldap_sasl_interactive_bind_s, with  
> > LDAP_SASL_QUIET, and pass in a list of mechanisms, the client will  
> > just use the first mechanism in its list.  Is that correct?
>
> No. The list of mechanisms is passed directly to the SASL library.  
> The SASL library will choose a mechanism from that list based on  
> the security properties that were set. And obviously, since it is a  
> separate library that has no knowledge of the LDAP_SASL_ flags,  
> LDAP_SASL_QUIET doesn't affect it at all.

I've done something similar to this with other SASL clients. I assume  
that what you (they?) want is to be able to provide a list along the  
lines of 'try GSSAPI, then if that fails, try DIGEST-MD5', etc. You  
can drive Cyrus SASL in this way, but I suspect you need a closer  
relationship to the SASL code than ldap_sasl_bind_interactive_s gives  
you. Roughly, what works is to take the list of mechanisms that the  
server gives you, and start a loop. Call into the SASL library with  
this list, and do the SASL handshake as normal. If it fails at any  
point, ask it what mechanism just failed, and remove it from your  
list of permitted mechanisms, and go round again. You're done when  
you've either run out of permitted mechanisms, or the authentication  
succeeds. This model means that you can try GSSAPI first, and then  
fall back to password based mechanisms when that fails, without  
having to involve the user in that process.

Cheers,

Simon.