but if you use the dns of fortiguard it would not be advantageous since if the fortiguard servers go down, you would have problems browsing the internet. It seems to me that two months ago there was a problem so the dns of fortiguard fell.
While it is not strictly speaking "advised against" using FortiGuard DNS servers for regular DNS queries, there are simply no benefits to using them. The common sense, on the other hand, will tell us that the disadvantages would be:
Unfortunately, Fortinet's infrastructure is a bit smaller, yet, than Google (22.214.171.124) or CloudFlare (126.96.36.199), which means less DNS servers, less geographical spread of those servers, less capacity. All of this may result in high latency of the answers, and lost queries.
Even the local ISP where the FGT is installed will be much faster to answer DNS (4-5 msec) than FTNT DNS located some 40-50 msecs away.
The only benefit/obligation to use FortiGuard DNS servers was for DNS Filtering to work, but it was lifted as well some 5-6 years ago, so even for that there is no benefit to using Fortiguard DNS.
(almost forgot) Yes, you may use them if using DDNS for the FGT WAN IP resolving.
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