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# `sso.py` - A Tornado Kerberos Single Sign-On Module

## About The SSO Module

sso.py is a Tornado Single Sign-On (SSO) authentication module that implements GSSAPI authentication via python-kerberos (import kerberos). If "Negotiate" authentication (GSSAPI SSO) fails it will gracefully fall back to "Basic" auth (authenticating a given username/password against your Kerberos realm).

For this module to work you must add 'sso_realm' and 'sso_service' to your Tornado application's settings. See the docstring of the KerberosAuthMixin for how to do this.

This module should work with regular MIT Kerberos implementations as well as Active Directory (Heimdal is untested but should work fine). If you're experiencing trouble it is recommended that you set debug=True in your application settings. This will enable printing of Kerberos exception messages.

### Troubleshooting

If your browser asks you for a password (i.e. SSO failed) there's probably something wrong with your Kerberos configuration on either the client or the server (usually it's a problem with forward/reverse DNS resolution or an incorrect or missing service principal in your keytab).

If you're using Active Directory, make sure that there's an HTTP servicePrincipalName (SPN) matching the FQDN of the host running your Tornado server. For example: HTTP/somehost.somedomain.com@CORP.MYCOMPANY.COM You may also want a short hostname SPN: HTTP/somehost@CORP.MYCOMPANY.COM

Also make sure that the service principal is in upper case as most clients ( web browsers) will auto-capitalize the principal when verifying the server.

Here's some things to test in order to find problems with your Kerberos config:

Try these from both the client and the server (NOTE: Assuming both are Unix): kinit -p <user@REALM> # To verify you can authenticate via Kerberos (at all) nslookup <server FQDN> # To verify the IP address reverse maps properly (below) nslookup <IP address that 'server FQDN' resolves to> kvno HTTP/somehost.somedomain.com # To verify your service principal

Remember: Kerberos is heavily dependent on DNS to verify the server and client are who they claim to be.

I find that it is useful to get GSSAPI authentication working with OpenSSH first before I attempt to get a custom service principal working with other applications. This is because SSH uses the HOST/ prinicipal which is often taken care of automatically via most Kerberos management tools (including AD). If you can get SSO working with SSH you can get SSO working with anything else.

## Class Docstrings

class `sso.``KerberosAuthMixin`(application, request, **kwargs)

  Authenticates users via Kerberos-based Single Sign-On. Requires that you define 'sso_realm' and 'sso_service' in your Tornado Application settings. For example:

      settings = dict(
          cookie_secret="iYR123qg4UUdsgf4CRung6BFUBhizAciid8oq1YfJR3gN",
          static_path=os.path.join(os.path.dirname(__file__), "static"),
          gzip=True,
          login_url="/auth",
          debug=True,
          sso_realm="EXAMPLE.COM",
          sso_service="HTTP" # Should pretty much always be HTTP
      )

  NOTE: If you're using 'HTTP' as the service it must be in all caps or it might not work with some browsers/clients (which auto-capitalize all services).

  To implement this mixin:

      from sso import KerberosAuthMixin
      class KerberosAuthHandler(tornado.web.RequestHandler, KerberosAuthMixin):

          def get(self):
              auth_header = self.request.headers.get('Authorization')
              if auth_header:
                  self.get_authenticated_user(self._on_auth)
                  return
              self.authenticate_redirect()

          def _on_auth(self, user):
              if not user:
                  raise tornado.web.HTTPError(500, "Kerberos auth failed")
              self.set_secure_cookie("user", tornado.escape.json_encode(user))
              print("KerberosAuthHandler user: %s" % user) # To see what you get
              next_url = self.get_argument("next", None) # To redirect properly
              if next_url:
                  self.redirect(next_url)
              else:
                  self.redirect("/")

  `initialize`()

    Print out helpful error messages if the requisite settings aren't configured.

    NOTE: It won't hurt anything to override this method in your RequestHandler.

  `get_authenticated_user`(callback)

    Processes the client's Authorization header and calls self.auth_negotiate() or self.auth_basic() depending on what headers were provided by the client.

  `auth_negotiate`(auth_header, callback)

    Perform Negotiate (GSSAPI/SSO) authentication via Kerberos.

  `auth_basic`(auth_header, callback)

    Perform Basic authentication using Kerberos against self.settings['sso_realm'].

  `authenticate_redirect`()

    Informs the browser that this resource requires authentication (status code 401) which should prompt the browser to reply with credentials.

    The browser will be informed that we support both Negotiate (GSSAPI/SSO) and Basic auth.

    NOTE: I know this isn't technically a redirect but I wanted to make this process as close as possible to how things work in tornado.auth.

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