Default libkrb5 ccache location

[Lennart Poettering]

On Mon, 29.07.13 21:11, Simo Sorce (simo at redhat.com) wrote:

> On Tue, 2013-07-30 at 02:08 +0200, Lennart Poettering wrote:
> > On Mon, 29.07.13 23:56, David Woodhouse (dwmw2 at infradead.org) wrote:
> > 
> > > On Tue, 2013-07-30 at 00:50 +0200, Lennart Poettering wrote:
> > > > So, why don't you revert to using /tmp then?
> > > 
> > > The problem with /tmp is that if you want predictable filenames for the
> > > storage, you open yourself to a denial-of-service attack where another
> > > user can create a file with the same name.
> > 
> > Well, but that's not unsurmountable, just pick a randomly named
> > directory in /tmp and make sure to have a symlink:
> > 
> >    ln -s /tmp/krb.XXXXXX "$HOME/.krb-`cat /etc/machine-id`"
> 
> What would create this directory ?

The same component that creates the temporary directory?

In pseudo code:
   
   char temp[] = "/tmp/krb.XXXXXX", link[PATH_MAX];
   char *machine_id, *home;

   mkdtemp(temp);
   machine_id = get_file_contents("/etc/machine_id");
   *strchrnul(machined_id, '\n') = 0;
   home = getenv("HOME);
   snprintf(link, "%s/.krb-%s", home, machine_id);
   symlink(temp, link);

Of course, you should skip this if the symlink already exists and points
to a valid directory...
   

> > to give it a stable, machine-local name.
> 
> in what case /tmp contains non-'machine-local' files ?

/tmp doesn't. But /home does. Hence you include the machine ID in the
symlink name.

> Also I need one directory per-user and not per-machine.

Well, you want it per-user *and* per-machine.

> And how is this different than /run/kerberos in the end ?

That there's a sane cleanup scheme done via /tmp and not yet another
place where we have unrestricted runtime objects of the user.

Lennart

-- 
Lennart Poettering - Red Hat, Inc.