On 12/10/2012 05:31 PM, Ben Morrow wrote:
At 3PM -0500 on 10/12/12 you (Maura Dailey) wrote:
I'm in a situation here at work where I'm trying to support a mixed
network of OS X and RHEL desktop machines with a Postfix/Dovecot
combination.
- user account information is stored in LDAP
- user credentials are in MIT Kerberos
- server is running RHEL 6/Dovecot 2.0.9/Postfix 2.6.6
I am currently using the PAM passdb module to authenticate my users (I
began to have trouble with using GSSAPI directly). After I implemented
it, a few weeks later, I noticed that some users were no longer getting
their mail if they hadn't logged in during the past day. Postfix's mailq
showed that hundreds of messages were backing up in the queue. I
eventually tracked it down to leftover Kerberos credential cache files
(/tmp/krb5cc_????) sitting in /tmp on the mail server. The presence of
expired credential files was preventing Postfix from delivering mail to
those users' mail spools. If I delete the credential files manually,
Postfix immediately delivers the queued emails.
This is rather odd. Is krb5-authenticated NFS involved here, or does
Postfix's delivery make any other use of Kerberos? The only other thing
I can think of is that so many expired ccaches are accumulating that the
user goes over their inode quota.
Each user has one credential cache file in /tmp on the mail server after
logging into Dovecot. We aren't using randomized names, so everything is
in the standard format /tmp/krb5cc_uid. We do use KRB5 authenticated
(and encrypted) NFS, but we don't deliver mail to home directories.
Since all users are "real" users, and our office size is small, everyone
has a mail spool directory on the mail server.
Postfix is configured to use Dovecot for Kerberos. The relevant lines in
its /etc/postfix/main.cf are as follows:
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_auth_enable = yes
smtpd_recipient_restrictions = permit_mynetworks,
permit_sasl_authenticated, reject_unauth_destination
smtpd_client_restrictions = permit_mynetworks,
permit_sasl_authenticated, reject
smtpd_sasl_security_options = noanonymous
Currently, I have a cron job deleting the files manually every night.
Obviously, this is a cruddy solution.
Well, I don't know about that: obviously, it would be a good idea to
work out what's going on here, in case it causes anything else to go
wrong, but a cronjob clearing out /tmp is a good idea in any case.
Fair enough. And of course, I get a cache file every time I use sudo or
log in through a GUI. Those aren't cleaned up either! Maybe I should be
deleting credential cache files on all the machines every night. Those
cache files have never given me a problem, though.
I have Dovecot configured on a
RHEL 6 box. The Pam stack on a RHEL 6 machine uses sssd (pam_sss.so) for
authentication with Kerberos, not pam_krb5.so. I'm trying to track down
which piece of the puzzle is responsible for cleaning up leftover
credential caches. Is there a configuration option I can pass to
Dovecot's passdb directly to clean up these cache files?
There are two relevant Dovecot settings: passing setcred=yes to the pam
passdb will make Dovecot call pam_setcred, and passing session=yes will
make it open and immediately close a PAM session. (Probably it ought to
wait until the IMAP user logs out to close the session, but currently it
can't do that because of the way the passdb lookups are done.) Changing
either of these may have an effect, depending on when exactly your PAM
module creates and destroys ccaches.
I've tried session=yes by itself, but I don't think I've tried it with
setcred=yes. I'll throw it in there and give it a try after I send this
email.
Do others
generally have more success using a custom PAM stack with pam_krb5.so
instead of pam_sss.so?
Well, I don't use RH (I use FreeBSD), but I use and would recommend Russ
Allbery's pam_krb5.so, which may or may not be the same as the normal
pam_krb5.so provided by your system. It has options to control whether
and where ccaches are created; assuming Dovecot doesn't need krb5 creds
(say, for NFS), you would probably be better off telling it not to
create a permanent ccache at all.
http://www.eyrie.org/~eagle/software/pam-krb5/
I haven't had to configure pam_krb5.so directly before (we use the Red
Hat/Fedora configured default, pam_sss.so, which claims to be a one stop
shop for LDAP/Kerberos/NIS,etc.), but it does seem to have more options.
We certainly don't need credential caches to stick around for email
users. All the mail spools are stored on locally mounted storage on the
mail server.
Poring over sssd's configuration options didn't
reveal anything useful. I'm still not sure why Postfix even cares if
there are expired credential cache files in /tmp at all.
I'm back to trying GSSAPI directly again as well in the meantime on a
few test clients. When I used that in the past, users were getting
issued duplicate Kerberos tickets and users were forced to log into the
mail server directly using SSH after a day in order to get their mail to
work (seemingly a related issue). If I get those same errors again, I'll
start another thread.
This certainly does all sound related. What are the ccache files called:
are they just /tmp/krb5cc_UID or is there a random portion as well? Are
they being created with the correct permissions, and are there any
security policies (SELinux or ACLs of some kind) set up which might
interfere with their creation or destruction?
No random portion (the only application we have configured with random
ccache names is SSH), just the usual /tmp/krb5cc_uid. Permissions appear
correct, they belong to the appropriate user and group accounts. The
SELinux permissions are set to: system_u:object_r:user_tmp_t:s0. This is
different from the machine's credential cache
(system_u:object_r:gssd_tmp_t:s0), but I've disabled and re-enabled
SELinux during different parts of my testing and didn't notice any errors.
Using GSSAPI directly doesn't create any cached credential files on the
server. However, I just verified that I'm still sporadically getting the
duplicate tickets (two lines for imap/[email protected],
with identical expiration dates). It comes and goes. I have a few users
testing it for me, and they'll let me know if their logins break
tomorrow like they have in the past once the normal log in period
elapses. Since the "fix" for that problem is to log into the mail
server, I have a hard time testing for the problem myself (being logged
into the mail server nearly continuously just now).
What happens if you log in as an ordinary user (preferably using the
same PAM stack as Dovecot uses), use klist to find the ccache name and
'ls -i' to find its inode number, then manually kinit again? Does the
kinit succeed, and does the new ccache have a different inode number
from before? Does the ccache file get removed when you log out?
I've noticed that the ccache files for regular users do not get deleted
when they log out. On another user's workstation, I found a ccache file
for my account from a week ago (my current session was using a randomly
generated SSH ccache file). I ran a command with sudo to force it to
reauthenticate with pam_sss, and the timestamp on the ccache file was
updated and the inode changed. My best guess is that pam_sss.so just
doesn't do ccache cleanup.
Ben
Thanks for taking a look, I appreciate all the suggestions. This
problem's been driving me up a wall for weeks now. I only just managed
to narrow down the cause in the last few days. If I can't get a fix
working in Dovecot, I'll try tackling it from a different perspective. I
understand why Dovecot might not be cleaning up credential files
(especially if it's just calling PAM normally), but I definitely do not
understand why Postfix is behaving the way it does. If I can just get
GSSAPI to work, I could probably bypass the problem as well.
- Maura Dailey
[email protected]
