Description of problem:

The CoreDNS instances that provide cluster DNS service are configured with a cache timeout of 30 seconds.  This can cause DNS lookups to take an excessive amount of time if forwarded queries overload upstream resolvers.  


Version-Release number of selected component (if applicable):

All versions of OCP 4 so far.


Steps to Reproduce:

1. oc -n openshift-dns get configmaps/dns-default -o yaml


Actual results:

The maximum cache time is 30 seconds:

            cache 30


Expected results:

The maximum cache time should be higher (at least 900 seconds).


Additional info:

OpenShift 3 used dnsmasq (along with SkyDNS) for the cluster DNS service, and the dnsmasq configuration was largely unmanaged, so cluster administrators could configure customize its configuration with little or no restrictions.  OpenShift 4 replaces dnsmasq and SkyDNS with CoreDNS, and the DNS operator manages the CoreDNS configuration, which prevents administrators from making arbitrary modifications to it.  

The cache timeout effectively puts an upper limit on DNS records' TTLs.  When DNS records are properly configured, capping their TTLs provides no advantage but does have the disadvantage of causing more queries to be forwarded to the upstream resolver.  Therefore we should increase this upper limit to avoid performance problems.