Whether a DNS query is made over IPv4 or IPv6 does not affect whether you will connect to the target site over IPv4 or IPv6. That is, a DNS query made over IPv4 can return an IPv6 address, and vice versa.
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| % dig +short @8.8.8.8 forums.thinkbroadband.com. aaaa
forums.thinkbroadband.com.cdn.cloudflare.net.2606:4700:10::6814:e32
2606:4700:10::6814:f32
% dig +short @2001:4860:4860::8888 forums.thinkbroadband.com. aforums.thinkbroadband.com.cdn.cloudflare.net.
104.20.15.50104.20.14.50 |
For the OP's problem I think there are two main possibilities:
1. EE are pointing at two different DNS servers but the first is broken, so the client is timing out and retrying the DNS request to the second.
2. EE is configuring clients with both IPv4 and IPv6, but the IPv6 path is broken, so the client is trying IPv6 first and then falling back to IPv4 (a.k.a. "happy eyeballs"). There are many causes of "IPv6 path is broken", including things like path MTU discovery is broken, which can be caused via ICMP blocking by the router or the network.
For case 2: to determine whether the site is actually being reached via IPv4 or IPv6, you can use a browser plugin like IPvFoo for Chrome. If you connect to a site which you know implements IPv6, and the client is successful with IPv6, then it rules out this case. But if everything connects over IPv4, even to IPv6-supporting sites, then IPv6 is broken. Disabling IPv6 either in the router or on individual clients should make things run faster.
For case 1: firstly, see what DNS servers your clients are picking up via DHCP (e.g. Windows: "ipconfig /all").
If these DNS servers are the router's own IP address (e.g. DNS server IP matches gateway IP) then the router itself is doing DNS forwarding. You'd then have to look at the router settings to see what DNS servers the router is forwarding to.
Once you know what IP address(es) are being used for DNS queries, it is possible to test them individually.
But this is getting quite technical: really, this is EE's problem to diagnose and fix. And that's where you find out what their customer service is like.