RT-AC68U - 384.19 - DNS (DoT) requests start failing after a few days/weeks

[digitalxruin]

A few months ago I noticed my ISP (Optimum) started blocking my custom DNS settings for Cloudflare that I've been using for a long time on this router. I'm assuming their starting to block external DNS and want everyone to use their internal DNS for injection/tracking/etc, so I switched to the Cloudfare DoT in the "WAN DNS Setting" area and everything started working again.

One day a week or two later I noticed that some of my clients, like my iPhone, suddenly were acting as if domains were unavailable when clicking on links or opening apps that pull from the web, such as news apps. My AC68U seemed to still be running fine and internet connectivity was there, but it was almost like any non-cached DNS requests would fail. Restarting the router instantly fixed the problem, until it happened again a week or two later. Rebooted again.

I've seen this many times now so I setup a schedule to reboot the router once a week, thinking maybe there is a memory leak or an issue with the DNS service (dnsmasq?), however it still seems to happen randomly. All clients are set to use 192.168.1.1 for DNS and I have APs I use for wifi, so the AC68U is really just functioning as a router and not even serving wifi.

Anyone else seen this or have a possible solution? I'd rather not have to set the router to reboot daily, really seems like a bug somewhere. I'm a Linux engineer by trade so happy to do any debugging if someone points me in the right direction. Unfortunately I haven't thought to grab the logs before the most recent reboot (which was last night), but can do so the next time if it helps. I've had Merlin firmware on this router for years, so it's not a new setup.

 

[digitalxruin]

Attached are the settings I'm using for DNS.

 

[ColinTaylor]

You said that your ISP was blocking standard DNS queries to Cloudflare but you still have the router's WAN configured to use them. If what you say is true then you should change that to Connect to DNS Server automatically = Yes.

The next time you experience the problem examine the router's syslog. You might also want to specify another DoT provider in addition to Cloudflare in case Cloudflare has a problem.

 

[digitalxruin]

They're blocking standard DNS, which is why I switched to DoT and seems to work fine. "Connect to DNS server automatically" is for using your ISPs automatically provided DHCP DNS, not for a manual Cloudflare config. I'll check syslog but it doesn't seem to be a Cloudflare issue, the router just stops making new DNS requests until I reboot it.

 

[ColinTaylor]

"Connect to DNS server automatically" is for using your ISPs automatically provided DHCP DNS...

That was exactly my point. You say they are blocking standard DNS in which case putting Cloudflare's IP addresses in there could cause problems for the router when it boots up. So you should let the router use your ISP's DNS servers at boot and let your LAN client's use DoT.

 

[digitalxruin]

Ah, sorry I misunderstood what you were getting at. I didn't realize that it would still using DoT for clients if I have automatic configured at the same time as the DoT settings below. I'll see if that makes any difference.