Red Hat Bugzilla – Bug 462394
nscd ignores configured caching time
Last modified: 2016-11-24 07:35:22 EST
Description of problem:
The nscd only caches for a few minutes, although /etc/nscd.conf says
positive-time-to-live hosts 3600
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. run tcpdump -n in the background
2. run date(1) to timestamp
3. run "host ipv6.google.com"
4. check output of step 1, it has DNS requiest for AAAA ipv6.google.com
5. re-run steps 1-3, see that there is no DNS traffic -- this is good
6. wait 20 minutes
7. repeat steps 1-3
8. observe that there's DNS traffic again
Short lifetime of cache
Setting bug component to glibc because nscd has no its own component.
The replied TTL from Google is rather goofy, only 35s for AAAA:
ipv6.google.com. 6340 IN CNAME ipv6.l.google.com.
ipv6.l.google.com. 35 IN AAAA 2001:4860:0:2001::68
However, nscd caches for more than 35 seconds, so I think it obeys
wrong part of nscd.conf or something.
Created attachment 316788 [details]
This bug appears to have been reported against 'rawhide' during the Fedora 10 development cycle.
Changing version to '10'.
More information and reason for this action is here:
nscd respects the TTL of the DNS reply. This overwrites the value in the nscd.conf file which is only used when no TTL information is available. The entry gets automatically reloaded a number of times and therefore will be again in the cache. This might be why you think the entry is cached longer than the specified number of seconds. Run nscd in the foreground by adding -d -d -d to the command line to observe the behavior. Unless you see in the debug messages that the entry is cached significantly longer than the number of seconds specified in the TTL close the BZ since there is no problem.
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