A colleague of mine shared with me a issue they were having with LDAP properties such as group memberships not being updated in a timely manner on some RHEL7 hosts that were running NSCD.
Not having any experience with NSCD, only SSSD for integrating Linux hosts in a enterprise LDAP environment for identity management, I madly started my google-fu engine and got to work.
Fortunately for you the reader, we can save you some time and get you the details you need.
In our example of group memberships, the first thing we are going to do is making a modification externally to the Linux host to a LDAP user’s group memberships. Use whatever tool you typically use.
Next, we are going to run the following command on the Linux host to see if it get’s the changes:
getent group GROUPNAMEHERE
getent allows us to query Name service switch databases for our LDAP group that we just modified. Hopefully the output displays the group membership prior to your change and verifies the issue.
Now that we have verified the issue lies within the path of the Name server > LDAP, one of the intermediate points within this path is the Name service cache daemon or NSCD. Potentially, if the time-to-live on the NCSD cache is quite long then our host maybe query old and invalid data. Let’s validate this by clearing said cache with:
sudo nscd --invalidate=group
Now that we have cleared the group-specific cache, lets verify again what our host can see with a:
getent group GROUPNAMEHERE
Hopefully we now see the group membership updated to include the changes we made. It did? Great. Now we have our root cause.
Okay, we have a stale cache problem. How do we go about preventing this in the future? Let’s open up, with your favourite text editor, the NSCD configuration file with:
sudo nano /etc/nscd.conf
You should be presented with something like the following:
# This is a comment. logfile /var/log/nscd.log threads 6 server-user nobody debug-level 0 enable-cache passwd yes positive-time-to-live passwd 600 negative-time-to-live passwd 20 suggested-size passwd 211 check-files passwd yes enable-cache group yes positive-time-to-live group 3600 negative-time-to-live group 60 suggested-size group 211 check-files group yes enable-cache hosts yes positive-time-to-live hosts 3600 negative-time-to-live hosts 20 suggested-size hosts 211 check-files hosts yes
In this example, there are two values we are interested in the most, the positive TTL for group and the negative TTL:
positive-time-to-live group 3600 negative-time-to-live group 60
Positive TTL is the number of seconds after which a cached entry is removed from the cache and Negative TTL is the number of seconds after which entry marked as “not existent” is removed from the cache.
While not recommended, if it’s preferable to turn off caching altogether, adjust the following value to false:
enable-cache group yes
Tune those values to something that matches your environments needs, save the file and restart the daemon to bring them into effect with:
systemctl restart nscd
or if you are not using systemd:
service nscd reload
With the daemon restarted and your changes in affect, repeat the verification step above and then adjust config as needed until you hit that cache sweet spot.
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