weird DNS resolution behavior from AC68U

[texashoser]

I found another issue as well with the Dual WAN function, in particular DNS issues when secondary WAN connection is connected to the Asus router, but the secondary WAN device has no actual internet access.

Particularly, my secondary WAN connection is a ATT Uverse modem. It's powered on and hooked up the Asus router, but ATT is currently upgrading my service so the Uverse modem has no internet connection. Secondary WAN shows as connected per Network Map web GUI (as it should be). Both primary and secondary WAN connections are configured to connect to the ISP's DNS servers automatically.

I have DHCP set up on the Asus router such that 8.8.8.8 is the primary DNS resolver. When clients get assigned DHCP addresses, DNS servers are assigned as 8.8.8.8 with 192.168.1.1 (the Asus router) as secondary.

When querying 192.168.1.1 (the Asus router) for DNS resolution from clients behind the router, every hostname resolves to 192.168.2.254. This is the LAN IP address of the ATT Uverse modem. It seems as if the Asus router is using the DNS server assigned by the secondary WAN connection (ATT Uverse modem) to resolve hostnames and since it can't connect to ATT's name server (either by the secondary WAN connection or because ATT's DNS servers don't support off-network recursion), it just spits back the LAN IP of the ATT modem.

When I unplug the ATT secondary connection from the Asus router, querying the Asus router for DNS resolution works properly.

Here's the /etc/resolv.conf entry from the Asus router:

nameserver 209.18.47.61 (Timer Warner DNS server from primary WAN)
nameserver 209.18.47.62 (Timer Warner DNS server from primary WAN)
nameserver 104.49.x.x (ATT Uverse modem public IP - note: x'd out last part of public IP address)

When connected directly to the Asus router via telnet session, DNS queries work correctly and I am assuming queries are properly using the order of name servers listed in /etc/resolv.conf. But it seems like DNS queries from clients behind the Asus router aren't using the proper order of the listed resolvers in /etc/resolv.conf.

What theoretically should happen is that /etc/resolv.conf is dynamically updated as the status of the primary and secondary WAN interfaces change. Ie, if the primary WAN interface goes down, the primary WAN DNS servers should be removed from /etc/resolv.conf or placed at the bottom of the order. Perhaps this is supposed to happen. But even if it is, something funky is going on where the Asus router is trying to use DNS servers assigned from the secondary WAN connection.

Yes, I know I can fix this problem by hardcoding DNS servers on my clients, but...

 

[texashoser]

Quick update. I fixed the behavior of the Asus modem querying name servers assigned by the secondary WAN connection by statically assigning DNS servers in the secondary WAN connection settings.

I know this fix seems to have changed the behavior because as a test I input two DNS servers into the config that don't support off-network recursion (ie, these name servers will only answer DNS queries for clients/networks specifically allowed by their BIND access lists defined in their named.conf file. In other words, they won't answer queries coming in from my primary WAN connection.

Now when querying hostnames using the Asus router as the resolver, DNS queries are being answered properly because, I assume, the Asus router isn't trying the use the DNS resolvers configured into the secondary WAN configuration.