Using the Python Kerberos Module

I’m currently integrating Kerberos authentication support into a custom Pulp client and have completely failed to find any good documentation on how to use the kerberos module.

I managed to find a basic example, which makes reference to “another example in the python-kerberos package”, which I assume is a reference to the final test case in the package. I also looked at the XML-RPC wrapper implemented in kobo.

Rather than just documenting this for my own use, I decided to write up and publish what I figured out. Besides, it gives me an excuse to try out Kenneth Reitz’s famous requests module :)

All examples in this document are from a Python 2 interactive session. As far as I know, there is not yet a Python 3 compatible version of the kerberos module available.

Kerberos Basics

When setting up Kerberos authentication on a server, there are two basic modes of operation. The simplest from a client implementation point of view just uses Basic Auth to pass a username and password to the server, which then checks them with the Kerberos realm. That’s not the case I’m interested in, since it just looks like ordinary Basic Auth from the client side.

The case I am interested in is the one where the client has a preexisting Kerberos ticket and we want to pass that to the server automatically without the user needing to reenter their password. The relevant HTTP authorization protocol is called “Negotiate”.

The basic flow of a typical Kerberos authentication is as follows:

  • Client sends an unauthenticated request to the server
  • Server sends back a 401 response with a WWW-Authenticate: Negotiate header with no authentication details
  • Client sends a new request with an Authorization: Negotiate header
  • Server checks the Authorization header against the Kerberos infrastructure and either allows or denies access accordingly. If access is allowed, it should include a WWW-Authenticate: Negotiate header with authentication details in the reply.
  • Client checks the authentication details in the reply to ensure that the request came from the server

This article doesn’t cover server side authentication, as I just use mod_auth_kerb to handle that side of things and set up the application to accept the REMOTE_USER setting from Apache. One useful undocumented trick that mod_auth_kerb supports is the KrbLocalUserMapping option (which strips the realm details from the value stored in REMOTE_USER).

The Role of the Python Kerberos Module

From a client point of view, the kerberos module handles two tasks:

  • Figuring out the value to send in the Authorization field
  • Checking Kerberos level authentication of the response provided by the server

The kerberos module does this by exposing the GSS API - this is an ugly interface, but it does work.

The Initial Request and Response

This part doesn’t involve the kerberos module at all, just a basic HTTP request:

>>> import requests
>>> r = requests.get("https://krbhost.example.com/krb")
>>> r.status_code
401
>>> r.headers["www-authenticate"]
'Negotiate, Basic realm="Example Realm"'

This example uses a fictional host and realm. This fictional host accepts either Negotiate (i.e. Kerberos tickets) or direct username/password authentication.

As the same header occurs multiple times in the response, requests reports it as a comma separated list. This isn’t very convenient, so we’ll write a helper to split out the auth headers more cleanly:

>>> def www_auth(response):
...     auth_fields = {}
...     for field in response.headers.get("www-authenticate", "").split(","):
...         kind, __, details = field.strip().partition(" ")
...         auth_fields[kind.lower()] = details.strip()
...     return auth_fields
...
>>> www_auth(r)
{'negotiate': '', 'basic': 'realm="GlobalSync: Kerberos Login"'}

That means we can now easily detect when the client should reply with a Kerberos authenticated connection. For example, a host may provide two entry points, one configured to use mod_auth_kerb for preauthentication of users, while the other handles authentication entirely at the application level:

>>> r = requests.get("https://krbhost.example.com/krb/")
>>> r.status_code == 401 and www_auth(r).get('negotiate') == ''
True
>>> r = requests.get("https://krbhost.example.com/api/")
>>> r.status_code == 401 and www_auth(r).get('negotiate') == ''
False

If we accessed the "https://krbhost.example.com/krb/" URL with a web browser, it would forward the Kerberos ticket if available (and the browser is configured to do so), otherwise it would pop up a password dialog, using the realm info from the WWW-Authenticate: Basic header as the dialog title (at least, that’s what Firefox does - I assume other browsers are similar)

The Kerberos Authenticated Request

Now we know we want to send a Kerberos authenticated request to the server, the kerberos module comes into play. While this is a very thin wrapper around a C API, it does at least turn failures into exceptions (rather than setting the return code) so we’ll ignore that value:

>>> __, krb_context = kerberos.authGSSClientInit("HTTP@krbhost.example.com")
>>> kerberos.authGSSClientStep(krb_context, "")
0
>>> negotiate_details = kerberos.authGSSClientResponse(krb_context)
>>> headers = {"Authorization": "Negotiate " + negotiate_details}
>>> r = requests.get("https://krbhost.example.com/krb/", headers=headers)
>>> r.status_code
200
>>> r.json
["example_data"]

You can set additional GSS flags in the call to authGSSClientInit but I haven’t found any need to for simple client authentication via Kerberos.

Authenticating the reply from the server

While we can just trust SSL to ensure the integrity of the response from the server, we can also complete the Kerberos handshake and use it to further authenticate the reply from the server:

>>> kerberos.authGSSClientStep(krb_context, www_auth(r)["negotiate"])
1
>>> kerberos.authGSSClientClean(krb_context)
1

As with other calls, these should throw an exception if they fail, so even though the return code is passed through from C, it should never be anything other than 1 at the Python level.

Wrapping this up in a helper class

Here’s a simple class that can help make this a bit easier to use in a client without making any assumptions about the HTTP interface being used:

class KerberosTicket:
    def __init__(self, service):
        __, krb_context = kerberos.authGSSClientInit(service)
        kerberos.authGSSClientStep(krb_context, "")
        self._krb_context = krb_context
        self.auth_header = ("Negotiate " +
                            kerberos.authGSSClientResponse(krb_context))
    def verify_response(self, auth_header):
        # Handle comma-separated lists of authentication fields
        for field in auth_header.split(","):
            kind, __, details = field.strip().partition(" ")
            if kind.lower() == "negotiate":
                auth_details = details.strip()
                break
        else:
            raise ValueError("Negotiate not found in %s" % auth_header)
        # Finish the Kerberos handshake
        krb_context = self._krb_context
        if krb_context is None:
            raise RuntimeError("Ticket already used for verification")
        self._krb_context = None
        kerberos.authGSSClientStep(krb_context, auth_details)
        kerberos.authGSSClientClean(krb_context)

And an example of using it with requests:

>>> krb = KerberosTicket("HTTP@krbhost.example.com")
>>> headers = {"Authorization": krb.auth_header}
>>> r = requests.get("https://krbhost.example.com/krb/", headers=headers)
>>> r.status_code
200
>>> krb.verify_response(r.headers["www-authenticate"])
>>>
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