[Freeipa-users] documentation or example of using S42U for NFS

[Charles Hedrick]

The repo is https://github.com/clhedrick/kerberos.git. There’s a README.md, and man pages for the individual programs. While I’m currently using these programs, we haven’t fully rolled out Kerberos yet. As we do, I expect we’ll want to add more features. (E.g. kgetcred / credserv need to support redundant servers.)

When free ipa implements all planned features, kgetcred / credserv may not longer be needed. Rnewd and skinit still will be (though skinit should be modified to use “kinit -n” rather than “kgetcred -a"

On Mar 1, 2017, at 5:47 PM, Orion Poplawski <orion at cora.nwra.com<mailto:orion at cora.nwra.com>> wrote:

If you ever get this into a repository, I'd love to see it.  Thanks.

On 01/17/2017 07:44 PM, Charles Hedrick wrote:
Instructions like that are several places. But NFS is different, and I believe the configuration would be different from other services.

I’ve given up on this approach, and have written my own utilities. I’ve actually got three. The first two assume that users who want to do cron jobs on a machine register a keytab for that machine on a secure system. I don’t want to put the actual keytab on the machine where the user is running, because if someone can become root, they can take the keytab and use it anywhere. (I’m in a computer science dept with systems run by faculty and grad students; also systems in public labs.)

So instead, I allow users to register that they want to do cron on a specific machine by putting a keytab on a secure server, associated with their username and the hostname. I expect to write a web app to set that up.

Credserv runs on the machine with the keytabs. It accepts a request from a server, authenticated using the host’s host key (i.e. /etc/krb5.keytab), with a username in the request. If the user has registered a keytab for that host, credserv returns a credential, locked to that IP address, with forwarding off.

Kgetcred is the client side. It runs setuid root (so it can read /etc/krb5.keytab), though it drops privs as quickly as possible. It creates a credentials cache for the user from the credentials returned from credserv.

This gives the best granularity of control I can come up with.

There’s a third utility in the family: renewd. It gets the uids of all current processes, and renews credentials for all of the users. It’s more complex than you’d expect because a normal renewal (as in kinit -R or kstart) leaves a brief period of time when the credentials cache is unusable. This can result in NFS failing. I use a special approach that depends upon the details of the KEYRING implementation. It should be free of race conditions for NFS. If desired, a different approach to avoid race conditions could be used for caches located in /tmp. I haven’t written that code but there’s a comment in the source outlining it.

I’ll be creating a git repository for the code.


On Jan 17, 2017, at 6:41:13 PM, Orion Poplawski <orion at cora.nwra.com<mailto:orion at cora.nwra.com>> wrote:

On 01/09/2017 09:52 AM, Charles Hedrick wrote:
Various documentation suggests that it is possible for Gssproxy to get tickets for users who need to use NFS. This is a possible way to handle things like cron jobs.

However while a gssproxy.conf example is given, there’s no sign of what needs to be done in freeipa to authorize it. I tried following instructions for LDAP access, but it doesn’t work. NFS seems to use a different, two-stage method for getting credentials, so that’s not a surprise. There are, not surprisingly, no useful error messages even with logging turned all the way up.



I'm interested in this as well.  All I've been able to find so far is:

https://vda.li/en/posts/2013/07/29/Setting-up-S4U2Proxy-with-FreeIPA/

haven't tried anything.

--
Orion Poplawski
Technical Manager                          720-772-5637
NWRA, Boulder/CoRA Office             FAX: 303-415-9702
3380 Mitchell Lane                       orion at nwra.com
Boulder, CO 80301                   http://www.nwra.com



--
Orion Poplawski
Technical Manager                          720-772-5637
NWRA, Boulder/CoRA Office             FAX: 303-415-9702
3380 Mitchell Lane                       orion at nwra.com<mailto:orion at nwra.com>
Boulder, CO 80301                   http://www.nwra.com

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listman.redhat.com/archives/freeipa-users/attachments/20170302/58ab662c/attachment.htm>