CIFS mounts and Kerberos - permissions on access or best practice

Solution 1:

Use automount + multiuser option of mount.cifs

You can achieve this using automount and the multiuser option for mount.cifs. Install the required packages:

sudo apt install autofs keyutils cifs-utils

The following example assumes that the cifs server exports a share that is named after the user that is accessing it. Normally that would be suitable for home directories.

Add this to your /etc/auto.master:

/cifs /etc/auto.cifs

In /etc/auto.cifs put this:

*   -fstype=cifs,multiuser,cruid=${UID},sec=krb5    ://server.domain/&

Make sure to replace server.domain by your file server. You could also use a fixed share this way. Just replace the * by a fixed name and also the &.

An important detail in the above configuration is the cruid=${UID}. It will make the kernel look for a kerberos ticket in the context of the user accessing the share. Otherwise it would be trying roots ticket cache.

Finally reload automount:

sudo service autofs reload

If you have a kerberos ticket, it will mount the file system /cifs/$USER on first access. That means you need to explicitly type e. g. cd /cifs/myuser or a similar action in a GUI file browser. To avoid this you could place symbolic links pointing to this from somewhere else and tell users to access those.

If you are using a fixed share (not using * and &) of course you would have to type cd /cifs/sharename.

Subsequent access by other users to the same share will be using their permissions, made possible by the multiuser option. No additional mount will be made but the existing one reused.

From mount.cifs(8):

   multiuser
       Map user accesses to individual credentials when accessing
       the server. By default, CIFS mounts only use a single set of
       user credentials (the mount credentials) when accessing a
       share. With this option, the client instead creates a new
       session with the server using the user's credentials whenever
       a new user accesses the mount. Further accesses by that user
       will also use those credentials. Because the kernel cannot
       prompt for passwords, multiuser mounts are limited to mounts
       using sec= options that don't require passwords.

It is also possible to add the required automount maps to an LDAP server for central management, but this is probably beyond the scope of this answer.

In your question you asked for the mount to be mounted as root on boot. Technically this is done here in form of a place holder mount for autofs. Practically the real mount is only done on first access by a user.

We are using this setup for ~100 clients at my workplace for accessing quite a big lustre file system and it works reliably.