I was getting a lot of "server not found" browser messages after the upgrade to 25.7. All would seem OK for a while, then there'd be a period of failures. (Multiple devices, wired and wireless, and multiple browsers on those devices). Also, most devices were laggy at intervals, without any error messages. It would just take a long time before any result would appear (in the case of browsing).
I previously had Unbound enabled (it came that way by default). I did nothing further with it. In System->Settings->General, I had specified three DNS server IPs. Nothing more for DNS. It's been this way for a couple of years, and no trouble here that I'm aware of.
After upgrade to 25.7, the problems came.
In searching the 'net for help, I stumbled on a setup post for Unbound that had me mark Enable DNSSEC Support, Register ISC DHCP4 Leases, and optionally Flush DNS Cache during reload, which I also marked. After applying the changes I rebooted.
It has now been several hours and I have not seen any DNS failures or experienced any lags. Performance is quite snappy again.
I assume what I had was a poorly configured DNS situation that was better tolerated before 25.7.
Most people here are far beyond me in config and expertise, I'm just posting in case it helps someone.
(Cable modem -> Protectli Vault with OPNsense -> Cisco switch -> wired clients and one wireless AP for the rest. Basic install setup plus some reserved DHCP LAN IPs.)
***ETA***: All has been working fine for more than 24 hours, when suddenly again nothing is getting DNS resolved. Unbound DNS reporting showed a sharp drop and 0% of queries resolving.
I tried restarting Unbound service, tried stopping/pause/starting, tried flushing the Unbound cache. Resorted to reboot OPNsense via WebUI menu and all is working again.
If anyone has any ideas on what else I can look at or do, I would really appreciate the help.