Hi Spike,

I have once seen such an issue on RHEL7. It was caused by a wrong SELinux context on /etc/krb5.keytab file. That is, SSSD updated the password in AD, attempted to update /etc/krb5.keytab, and SELinux denied this attempt. Audit log will contain a denied entry if that is the case. Maybe it will help you.

Kind regards,
Grigory Trenin

чт, 7 окт. 2021 г. в 20:02, Spike White <spikewhitetx@gmail.com>:
FYI -- update on this situation.

AD DC logs no help.  They show the exact same response sent back to a good machine account password renewal as for a failed renewal.

One of the AD administrators have identified a particular AD DC NIC teaming configuration that they state has caused problems with Kerberos on the past.  It's on a small percentage of their AD DCs and they will work to correct.  They will keep us apprised as to update.

I'm skeptical that's the underlying root cause -- for two reasons:
1.  If Kerberos was sensitive to this, it should affect all Kerberos operations  (Kerberos auth, etc.) and not just the kpasswd operations.
2. This is not occurring on our older RHEL6 and RHEL7 builds AD integrated via our older commercial AD integration product.  It's occurring only on our sssd-integrated builds.

At this point, we're turned off debug level 7 (it was filling up our /var/log filesystems and we have the verbose adcli update output from at least two failed clients).   We're going to take the alternate suggestion of setting ad_maximum_machine_account_password_age to 0 (disabling sssd from updating password) and run a cron job to do 'adcli update'.  

We're wrapping this adcli_update with tcpdump to get the exact kpasswd request/response packets, as well as wrapping with KRB5_TRACE.  

We want to call adcli update exactly as sssd calls it.  From SOURCES/sssd-2.4.0/src/providers/ad/ad_machine_pw_renewal.c, this appears to be how sssd calls external program /usr/sbin/adcli to do its adcli update:

      /usr/sbin/adcli update --verbose --domain=$AD_DOMAIN --host-keytab=/etc/krb5.keytab --host-fqdn=$FQDN --computer-password-lifetime=30

because we aren't doing any Samba stuff.  Is that the correct invocation?   We'll set computer-password-lifetime lower, say to 7.  Because we want to see examples more frequently, to find failed updates.

BTW, the packet capture on a successful machine account password renewal is only 8K, so that very targeted debug will not swamp our /var/log or /tmp filesystems.


On Wed, Aug 25, 2021 at 10:32 AM Spike White <spikewhitetx@gmail.com> wrote:

Sssd experts,

Short summary:  How can we troubleshoot sssd’s ‘Automatic Kerberos Host Keytab Renewal’ process?    We have ~0.4%  of our Linux servers dropping off the AD domain monthly.

Longer explanation:

Over the past two years, we have on-boarded sssd as our Linux AD integration component.  Largely displacing a former commercial product that did the same.

We have about ~20K Linux servers that are sssd-enabled.  A mix of RHEL6, RHEL7, RHEL8, Oracle Linux 6, 7 and 8.   We have ~7K Linux servers still on the old commercial product.  (For certain edge-case scenarios, such as DMZs, the commercial product works better.)

Our AD forest is a single AD forest, with 4 regional child domains.  All with transitive trust.  Sssd auto-discovers parent domain and all 4 child domains, no problem – whenever it’s adcli joined to its regional local domain.

Why are I writing this?

Because we are researching an ongoing problem reported by L1 server ops.  About 70 – 80 sssd-enabled Linux servers / month drop off the domain.  Out of our current sssd-enabled population of ~20K server, that’s not horrible.  But still it should be better.  (Our former commercial product did better.)

It’s not limited to one particular OS, OS version, build location or region.  We have surveyed; it seems to occur randomly among all OS versions, regions and locations.

To be clear, it’s extremely likely that this behavior arising from some subtle misconfiguration on our part – not from any sssd or adcli or Kerberos bug.  We have a couple of configuration improvements we’re pursuing.  (Kerberos max ticket lifetime mismatch between AD and /etc/krb5.conf file for instance.)

We are taking sssd’s default settings for ad_maximum_machine_account_password_age and ad_machine_account_password_renewal_opts.   So after 30 days, sssd will attempt daily to renew the host Kerberos keytab file.  It should re-attempt daily if not renewed.  By company policy, our AD disables any machine accounts that have not renewed their credentials in 40 days.   So when we find servers that have dropped off the domain, it’s because they have not renewed their AD machine accounts in 40 days.

We have SR’s open with our OS vendors (Redhat and Oracle respectively) for months now.  To no great help.  (They gave a few suggestions, but none panned out.)

We thought we were hitting this bug:


But packet captures proved that adcli update is using TCP on RHEL7/8.  Thus, this might be a potential problem, but only on RHEL6.  (BTW ‘udp_preference_limit = 0’ doesn’t force use of TCP for the kpasswd invocation in RHEL6 – it still uses UDP.  Thus, the recommended work-around for this bug doesn’t work.)

So that isn’t our underlying problem. 

We’re at a loss now – as you can see, we’re grasping at straws.

How can we troubleshoot sssd’s ‘automatic Kerberos Host keytab renewal’ process?  Whenever we inspect a particular server it works.  We can’t run all sssd clients at debug level 9;  it fills up /var/log filesystem after a few days of that.  We’re interested in troubleshooting that one particular sssd process on all clients;  not all parts of sssd.

Other than a steep learning curve (on our part), obscure situations (like DMZ auto-discovery of AD controllers) and exotic scenarios (like above), we’re quite happy with our 2 yr journey of direct AD integration with sssd.    Obviously, the troubleshooting tools on RHEL6 are very minimal.  But certainly, overall the quality of sssd on RHEL7/8 is excellent.  AD integration has innumerable devils in the details; I’m amazed that sssd performs as well as it does against our multi-domain forest.


PS the problem with sssd auto-discovery of AD controllers in DMZs has been fixed in a recent sssd release.  The better discovery algorithm was implemented – same one used by Windows clients and commercial products.  It’s just that recent sssd version is not on RHEL7 or 8.


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