fortigate custom external DNS Server not reachable or high response time

[fabs]

Hello all,
I had generally entered 1.1.1.1 as the primary DNS server and 8.8.8.8 as the secondar DNS Server.
Since yesterday morning I had the problem that no more external addresses could be resolved, or resolved very slowly. No matter which external DNS servers I specify, I have the same problem. Server hostname I also enter. There was no change in the FortiGate.
If i ping the external DNS servers, no issues <4ms
If I use the FortiGuard servers, I do not have this problem.

Any ideas? Thanks in advance

fabs

[srajeswaran]

Can you take a pcap of DNS traffic towards one of the servers and check the time taken? This will help us to confirm if it is external issue or Fortigate issue.

[fabs]

Hi srajeswaran,

i hope this information is helpful.
custom DNS Servers are 81.90.33.11, 81.90.33.12

Here I use the custom DNS server directly on the Windows 10 client:

Here I use on Windows 10 Client the Fortigate as DNS server, which in turn uses the FortiGuard DNS servers.

Here I use the Fortigate as DNS server on the Windows 10 client, which in turn uses the custom DNS server.

[srajeswaran]

Third one shows there is delay, but with this capture we don't know if the delay is between Fortigate and DNS server. Can you do the capture on fortigate interface connecting to the custom DNS server