Weird DNS issue....

[Cr00zng]

Hello, I have an Asus RT-AX86U broadband router with Merlin v.388.1 The router gets the DNS server assignment via the cable modem, has been using the ISP's DNS server, that is used by the internal client. Couple of days ego, the internal, wired clients (W10 boxes and MacBookPro) could not access the internet. After checking the physical connections and devices, the client's could access each other and the AX86U, but still no internet access. A quick DNS lookup showed that the the name resultion did not work. Strangly enough, the wireless clients, smartphones, Roku, cable box and laptops had no issues with accessing the internet. Their DNS connection and name resolution worked just fine.

Both, the LAN and wireless devices get their IP assigned dynamicall via DHCP. Some of the devices have reservered IPs, but their network configuration is still DHCP assign. I am stump by this issue and don't have a clue as to why. Yes, I assigned the DNS server address manually and the internet access had been restored for the LAN clients. Maybe updating the Marlin firmware to v.388.2 would reset the DNS settings, but I am not certain. I don't recall how old is the AX-86U, around 3 - 5 years. I can jjust get the same router new, but rather keep this one working. Any suggestions?

TIA...

 

[joegreat]

Sounds strange to me too...!?

Did not get the point in your description: The router has the DNS addresses from the cable modem (your internet provider/ISP) and the clients get which DNS entries?
I assume the router is referenced as DNS server on the clienst, or (would be strange!) the internet provider DNS directly?

To find the culprit, I would start to debug the thing like:
1. Reboot the router - also the cable modem (just in case...) - maybe the 2 reboots fixes it already
2. Reboot the clients (good to do anyway from time to time)
3. Use manual DNS settings in the router (not the ISP ones) like 1.1.1.1 & 1.0.0.1 and/or for IPV6 2606:4700:4700::64 & 2606:4700:4700::6400
4. ...scratch head...

 

[Cr00zng]

I did #4 first and couple of times later on. Kidding aside...

The router used to get the DNS addresses from the ISP via DHCP; currently, the the manually configured DNS addresses point to the Cloudflare DNS servers.

Just checked on the client side, where the DNS server used to be the internal IP of the router. Since manually assigning the DNS server addresses, the client gets these IPs as the primary/secondary and third IP is the router's internal IP. Yes, directly doing the DNS lookup with Cloudflare server.

Rebooted a number of times, both the wirelesss cable modem, the RT-AX86U router and clients., but no use, the issue had been the same for LAN connected devices. The wireless clients did have the router as the DNS server and had no issues with connecting to the internet during this time, or now directly querying Cloudflare DNS server.

Yeah, I am still scratching my head.... Why did the clients get the Cloudfare DNS addresses now, instead of using the router IP as before?

 

[paul0363]

I did #4 first and couple of times later on. Kidding aside...

The router used to get the DNS addresses from the ISP via DHCP; currently, the the manually configured DNS addresses point to the Cloudflare DNS servers.

Just checked on the client side, where the DNS server used to be the internal IP of the router. Since manually assigning the DNS server addresses, the client gets these IPs as the primary/secondary and third IP is the router's internal IP. Yes, directly doing the DNS lookup with Cloudflare server.

Rebooted a number of times, both the wirelesss cable modem, the RT-AX86U router and clients., but no use, the issue had been the same for LAN connected devices. The wireless clients did have the router as the DNS server and had no issues with connecting to the internet during this time, or now directly querying Cloudflare DNS server.

Yeah, I am still scratching my head.... Why did the clients get the Cloudfare DNS addresses now, instead of using the router IP as before?

You've likely added the Cloudflare settings on the LAN DHCP page here:

You need to change it under the WAN settings and leave the boxes above in the LAN settings blank if you don't wan't the Cloudflare IPs to be pushed to your clients (the router's IP will be set instead).

If that's what you've already done then I'm scratching my head too

 

[Cr00zng]

Thx @paul0363...

That's what I've done in the WAN settings, but the devices on the LAN still could not connect the internet, but the wireless client didn't have issues before or after adding the IPs. Adding the DNS servers in the LAN DHCP page resolved the issue.

I'll retest this weekend by blanking out the the LAN DHCP settings to see, if this issue had been just a glitch in the firmware/clients with all of the reboots. If not, I'll update the firmware and check if the glitch goes away. If that does not resolve it either, I do have a new RT-86U on hand. While the current router has backups, I'll probably won't restore it to the new one, just start new configuration after updating to Merlin's latest version.

While there's no issue with the clients directly doing their DNS lookup if the issue remains, the question is what else might break down the road on the current router? I'd rather address this during the weekend, if additional issues come may come up with the router on working days.