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Code Editor : gssapi.html

<HTML><HEAD>
<title>Configuring GSSAPI and Cyrus SASL</title>

</HEAD>
<BODY>
<h1>Configuring GSSAPI and Cyrus SASL</h1>

This document was contributed by <a
href="mailto:kenh@cmf.nrl.navy.mil">Ken Hornstein</a> and updated
by <a href="mailto:Alexey.Melnikov@isode.com">Alexey Melnikov</a>.

A couple of people have asked me privately, "Hey, how did you get the
GSSAPI mechanism to work? I tried, but the sample apps kept failing".
(The short answer: I'm a tenacious bastard).

I figured that it couldn't hurt to give a quick explanation as to
how you get GSSAPI working with the sample apps, since it wasn't
obvious to me, and I consider myself not completely ignorant of GSSAPI
and Kerberos.

<ol>
<li> Compile the Cyrus-SASL distribution with the GSSAPI plugin
 for your favorite GSS-API mechanism. I personally use the GSSAPI
 libraries included with the <a href="http://web.mit.edu/kerberos/www/">MIT</a>
 Kerberos 5 distribution; <a href="http://www.pdc.kth.se/heimdal/">Heimdal</a>
 and <a href="http://www.cybersafe.com/">CyberSafe</a> work as well.

<li> Start up the sample-server. The command-line used for
 sample-server needs to specify the GSSAPI service name and the
 location of the plug-ins; your sample command line might look
 something like this:

<pre> ./sample-server -s host -p ../plugins/.libs
</pre>

on UNIX and like

<pre> sample-server -s host -p ..\plugins
</pre>

on Windows.

In this example, I am using "host", which already exists on my
 machine, but only root can read it, so I an running this as root.
 If you want to use an alternate service name, you will need to
 create that service in Kerberos, place it in a keytab readable by
 you, _and_ point your Kerberos library at it.

Unix: both MIT Kerberos and Heimdal, 
 use /etc/krb5.keytab on Unix by default, but this can be changed
 by setting the <tt>KRB5_KTNAME</tt> environment variable; the default
 for CyberSafe Kerberos is /krb5/v5srvtab for UNIX systems and can be
 changed by setting the <tt>CSFC5KTNAME</tt> environment variable.

Windows: the default service key table location for CyberSafe is
  C:\Program Files\CyberSafe\v5srvtab, unless the
  CyberSafe registry setting for the KeyTab key is set to an
  alternate path. MIT Kerberos on Windows uses the keytab filename
  krb5kt.

You should get a response similar to:

<pre> Generating client mechanism list...
 Sending list of 3 mechanism(s)
 S: R1NTQVBJIFBMQUlOIEFOT05ZTU9VUw==
</pre>

Note that later on (assuming everything works) you might need to paste
 in lines that are longer than canonical input processing buffer on your
 system. You can get around that by messing around with stty; while
 the details vary from system to system, on Solaris you can do something
 like:

<pre> ( stty -icanon min 1 time 0 ; ./sample-server -s host -p ../plugins/.libs )
</pre>

<li> Obtain a Kerberos ticket for the user you want to authenticate as.

<pre> kinit kenh
</pre>

<li> Start up the sample client. You need to specify the service
 name, the hostname, and the userid. An example might be

<pre> ./sample-client -s host -n your.fqdn.here -u kenh -p ../plugins/.libs
</pre>

You should get a response similar to this:

<pre> Waiting for mechanism list from server...
</pre>

<li> Cut-and-paste the initial mechanism line from the server process
 (this includes the "<tt>S: </tt>") into the client process. You
 should get something similar to:

<pre> S: R1NTQVBJIFBMQUlOIEFOT05ZTU9VUw==
 Choosing best mechanism from: GSSAPI PLAIN ANONYMOUS
 Using mechanism GSSAPI
 Preparing initial.
 Sending initial response...
 C: <.... lots of base 64 data ...>
 Waiting for server reply...
</pre>

If GSSAPI isn't selected as the mechanism, there is a few things that
 might have gone wrong:

<ul>
<li> The mechanism might not have been offered by the server. The decoded
 mechanism list offered by the server appears in the "<tt>Choosing best
 mechanism</tt>" line. If GSSAPI didn't appear in that list, then
 something is wrong on the server. Make sure that you specified the
 correct plugins directory. If the plugin directory is correct, but
 the library fails to load, you might be running across a bug
 in libtool on some platforms. If you have your Kerberos/gssapi
 libraries not installed in the system library path, those libraries
 are likely not able to be found when the SASL GSSAPI plugin loads.
 The solution varies from system to system; what I did was take
 the linker line generated by libtool and run it by hand, adding
 a <tt>-R/path/to/kerberos/libraries</tt> switch (this was on Solaris).
 You can check with a system call tracer to see exactly what it is
 trying to do.

<li> The client doesn't know about the mechanism. The reasons for this
 happening are the same as the server: check the -p switch, check
 to make sure the correct libraries are being loaded with the GSSAPI
 plugin.
</ul>

You can turn on a healthy amount of debugging information by changing
 the definition in config.h of the VL macro to (and recompiling libsasl):

<pre> #define VL(foo) printf foo;
</pre>

There is a possibility
you might get an error that looks like this:

<pre> sample-client: Starting SASL negotiation: generic failure
</pre>

This can mean that you didn't provide all of the required information
 to the sample-client (did you provide a service name with -s, the
 hostname of the service with -n, and a username with -u ?), or that
 GSSAPI has failed (unfortunately, on the client you cannot find out
 the internal GSSAPI error; you will need to break out the debugger
 for that).

<li> Cut and paste the client response (The _entire_ line that begins
 with C:, including the initial "<tt>C: </tt>") to the server
 process. You should get a response back that starts with "<tt>S:
 </tt>". Cut and paste that to the client, and continue this
 exchange until you either get "<tt>Negotiation complete</tt>", or an error.
 If you get an error on the server you should get a complete error
 message (including the GSSAPI error string); on the client you
 unfortunately will only probably get "<tt>generic failure</tt>", which will
 again require the use of a debugger (but the VL macro should help
 with this).

One common thing that happens is that on your server you might see
 the error:

<pre> sample-server: Performing SASL negotiation: authentication failure
	(Requested identity not authenticated identity)
</pre>

This comes from not having a requested identity (the -u option) that
 matches the identity that you were authenticated to via the GSSAPI.
 This is of course mechanism specific, but if for example you're using
 Kerberos, the Cyrus SASL library strips out the @REALM from your
 identity if you are in the same realm as the server. So if your
 Kerberos identity is user@SOME.REALM and the server is in SOME.REALM,
 you need to specify "user" to the -u flag of the client. If you're
 accessing a server in a foreign realm, you need to pass the full
 principal name via the -u option to make this work correctly.

If you complete the negotiation successfully, you should see something
 that looks like (on both the client and server):

<pre> Negotiation complete
 Username: kenh
 sample-server: realm: can't request info until later in exchange
 SSF: 56
</pre>

If you get to that, then you've done it, and GSSAPI works successfully!
If you have questions about any of this, feel free to drop me a line.

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