For one of our F5 GTM setup DNS resolution issue is happening.
Setup is as below:
While exploring application through browser from end user Nslookup does not provide server information for some users with specific ISP and for some users it is working fine.
F5GTM no issues while we bypass vendor waf and vendor waf team confirmed no issues at their end as well. If we point ip address instead of cname for vendor waf it is working fine only issue with the cname entry at f5 gtm.
Same vendor is hosting multiple url via cname but no issues on that url.
It was concluded that there is some issues with ISP recursive dns records but we do not have visibility on ISP DNS.
While using google DNS in the browser there is no issues for the end user. Issue only happen when they user ISP DNS.
So the question is what could be reason gor this intermittent issues with the users for application which some time is reachable and some time not ?