Problems with HE's DNS resolver

[#1]

Posting this more for general info as there's not much you (Plusnet) can do about it other than handing out more IPv6 resolvers in your DHCPv6 assignment, and maybe including a Plsusnet one in there...? 
I've noticed over the past couple of days that HE's DNS resolver at 2001:470:20::2 is not responding and, as a result, is causing noticeable delays with web browsing given the timeout and failover to other entries in the client resolver list (on Linux at least resolv.conf is parsed in list order for every lookup).

$ dig @2001:470:20::2 www.newtonnet.co.uk
; <<>> DiG 9.6-ESV-R4 <<>> @2001:470:20::2 www.newtonnet.co.uk
; (1 server found)
;; global options: +cmd
;; connection timed out; no servers could be reached

The server does at least seem reachable at the network level:

$ ping6 2001:470:20::2
PING 2001:470:20::2(2001:470:20::2) 56 data bytes
64 bytes from 2001:470:20::2: icmp_seq=1 ttl=58 time=36.2 ms
64 bytes from 2001:470:20::2: icmp_seq=2 ttl=58 time=33.2 ms
64 bytes from 2001:470:20::2: icmp_seq=3 ttl=58 time=34.7 ms
^C
--- 2001:470:20::2 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 33.245/34.749/36.274/1.245 ms
$ traceroute6 2001:470:20::2
traceroute to 2001:470:20::2 (2001:470:20::2), 30 hops max, 80 byte packets
 1  2a02:16c8:2000:600:b6a4:e3ff:fe11:a57a (2a02:16c8:2000:600:b6a4:e3ff:fe11:a57a)  2.390 ms  2.565 ms  2.785 ms
 2  2a02:16c8:0:1::1 (2a02:16c8:0:1::1)  36.780 ms  39.496 ms  39.871 ms
 3  2a02:16c8:1:8000::1 (2a02:16c8:1:8000::1)  39.071 ms  40.047 ms  40.801 ms
 4  2a02:16c8:1:8001::1 (2a02:16c8:1:8001::1)  42.293 ms  42.789 ms  44.211 ms
 5  2a02:16c8:1:8019::1 (2a02:16c8:1:8019::1)  45.175 ms  46.455 ms  47.666 ms
 6  10gigabitethernet1-1.core1.lon1.he.net (2001:7f8:4::1b1b:1)  57.742 ms  42.941 ms  36.251 ms
 7  ordns.he.net (2001:470:20::2)  38.090 ms  38.653 ms  39.422 ms

I have tried queries from an alternative location with the same results, still in the UK however so as these servers are apparently anycasted they might not be having issues elsewhere.
I'm sure it'll likely be fixed soon enough, but I've not got the patience what with the effect on browsing so I've replaced it with those from Google (further info😞

2001:4860:4860::8888
2001:4860:4860::8844

(There are some performance and security details too which might be of interest)
Mathew

[#2]

Hi MJN.
One thing that was picked up by one of my colleagues in the office was that for some reason (best known to Google) their IPv6 DNS servers dont provide IPv6 addresses for Google content.  HEs DNS although sometimes a little flaky (could be a load balanced issue) does provide v6 addresses for Google servers.

[#3]

Hi Paul,
Thanks for that - I hadn't considered their whitelisting. Looking into it it seems that they'll still only add DNS server addresses to their whitelist (and give out AAAA records for Google services) but not dual-stack client addresses/ranges.
I was going to say that any response however is better than none at all but I see that HE's resolver is back online now.
Cheers,
Mathew