Multiple listeners F5-DNS
Hi,
This is a basic question about the behavior of the F5 DNS which I am trying to understand and want to know how exactly it works.
I have configured two tcp/udp listeners in F5 as List 1 - 192.168.20.1 and List 2 - 192.168.20.2
List 1 is configured with a pool for forwarding the dns request, pool member is the external bind server List 2 is not configured with any pool.
I have configured two test wide ips in gtm, as test1.abc.com - 100.100.100.1 test2.abc.com - 200.200.200.1
Wide ip pools are configured as Preferred method : Global Availability Alternate : Round Robin Fail Back : Return to DNS
Now the interesting part:
- when I do a nslookup from a machine for both the wide ips above by pointing to List 1 ip address then I get the dns reply almost immediately.
- when I do the same thing for List 2 ip address then I get dns timeouts and then finally get the answer. Surely this is not right.
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when I configure the List 2 with the same pool which was used for List 1 then I don't see dns timing out for List 2 ip address, which is very strange, because I don't want any dns forwarding for listener 2.
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I checked the named config under the F5 zonerunner and saw that recursion was set to yes for 10.0.0.0/8 and 192.168.0.0/16, since we are not using the F5 binds I thought of disabling it and disabled recursion and removed the 10 and 192.168 subnet.
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I ran the nslookup test again for the List 2 ip and this time dns reply was almost instant.
This clearly means that when a dns query was sent to List 2 ip it was forwarding this to F5 bind, but the question is why it was sending the request to bind as F5 always considers wide ips first and since both the wide ips were configured on the GTM so it should have given the reply without any time outs.
Second thing is what was changed after I disabled the recursion, both the listeners are configured exactly the same with only one exception which is List 1 had pool and List 2 doesn't.
Last thing is how to check/troubleshoot if the dns query is handled by GTM(wide ip) or F5 bind.
I can check the statistics for wide ips but how to check if the bind is replying the query.
Could anyone explain this behavior to me or there is something which I have misunderstood.
Thanks, Pankaj