There are four cases where we've discussed using sessions for
optimizations. During today's phone discussion we analysed them.
1. Avoiding the negotiate round trip.
Requesting a page protected by Kerberos requires two round trips to the
server: 1 for the initializ request, which gets denied with the
negotiate challenge, and one for the negotiate response. mod_auth_kerb
can fall back to userid/password. THe suggestion was t hat we allow
mod_auth_kerb to set a session cookie after a successful negotiate
request. Future requests can use that cookie to bypass the negotiate
handshake. The problem with this approach is NFS home directories, and
root users being able to get access to the session cookies, allowing a
replay attack. Note that this is a problem with the userid/password
fall-back as well. We are not going to pursue this right now.
2. Caching the service ticket. Once the http request has gone through,
the ipa web server needs to request a service ticket for LDAP. If the
session contained the service ticket, the could be bypassed for
additional requests. Since the request has to be validated by Kerberos
for the initial negotiate call, there is no additional loss of security
in caching the ticket.
A potential alternative to server side caching is for the client to
request the service ticket and send it in the negotiate handshake.
There is some question as to whether the web server would be able to
acces this ticket, and also whether the client can somehow request a
ticket that the server can use, and still comply with the Kerberos
3. File Upload. Session time out provides a means to automate the
clean up of files that might otherwise be orphaned.
4. Windowing search results. the 'find' APIs as implemented by LDAP
limit the responses to 200 records by default. One request we've had is
to provide sorting and windowing. Windowing here is defined as, for a
sorted response, return a delimited number of records starting at an
offset greater than 0. The LDAP implementation requires the equivalent
of a cursor from the requester, in this case the Apache server. To
maintain association between the user and the cursor, the cursor
identifier would be stored in the session. Implementing this correctly
will require further design. It will likely be done in the future.
In summary, the caching of the service ticket alone provides a
compelling reason to implement sessions. File upload will take
advantage of them. Other uses may be found over time.
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