Cloudflare DNS - `nslookup` fails for IPv6 DNS sometimes, but always works for IPv4 DNS

I’m on an FTTH connection that has both IPv4 and IPv6 assigned to it. My ISP is Airtel and my closest Cloudflare datacentre is BLR. I realised that querying for DNS records to any of the 3 Cloudflare DNS services over IPv4 works perfectly fine, but it often fails on IPv6. By often, I mean 95% of the time. On the other hand, it works perfectly fine Google DNS be it IPv4 or IPv6.

By IPv4 and IPv6, I mean the IPv4 / IPv6 DNS server I am connecting to. My network is dual-stack itself throughout the test. Here’s some output from my tests:

Google DNS IPv4

[email protected]:~$ time nslookup cloudflare.com 8.8.8.8
Server:         8.8.8.8
Address:        8.8.8.8#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 2606:4700::6810:85e5
Name:   cloudflare.com
Address: 2606:4700::6810:84e5


real    0m0.113s
user    0m0.004s
sys     0m0.017s
[email protected]:~$ time nslookup cloudflare.com 8.8.4.4
Server:         8.8.4.4
Address:        8.8.4.4#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 2606:4700::6810:85e5
Name:   cloudflare.com
Address: 2606:4700::6810:84e5


real    0m0.107s
user    0m0.013s
sys     0m0.009s

Cloudflare DNS IPv4

[email protected]:~$ time nslookup cloudflare.com 1.1.1.1
Server:         1.1.1.1
Address:        1.1.1.1#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 2606:4700::6810:84e5
Name:   cloudflare.com
Address: 2606:4700::6810:85e5


real    0m0.030s
user    0m0.017s
sys     0m0.004s
[email protected]:~$ time nslookup cloudflare.com 1.0.0.1
Server:         1.0.0.1
Address:        1.0.0.1#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 2606:4700::6810:84e5
Name:   cloudflare.com
Address: 2606:4700::6810:85e5


real    0m0.029s
user    0m0.013s
sys     0m0.008s
[email protected]:~$ time nslookup cloudflare.com 1.1.1.2
Server:         1.1.1.2
Address:        1.1.1.2#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 2606:4700::6810:85e5
Name:   cloudflare.com
Address: 2606:4700::6810:84e5


real    0m0.032s
user    0m0.017s
sys     0m0.005s
[email protected]:~$ time nslookup cloudflare.com 1.0.0.2
Server:         1.0.0.2
Address:        1.0.0.2#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 2606:4700::6810:85e5
Name:   cloudflare.com
Address: 2606:4700::6810:84e5


real    0m0.032s
user    0m0.005s
sys     0m0.017s
[email protected]:~$ time nslookup cloudflare.com 1.1.1.3
Server:         1.1.1.3
Address:        1.1.1.3#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 2606:4700::6810:85e5
Name:   cloudflare.com
Address: 2606:4700::6810:84e5


real    0m0.033s
user    0m0.007s
sys     0m0.014s
[email protected]:~$ time nslookup cloudflare.com 1.0.0.3
Server:         1.0.0.3
Address:        1.0.0.3#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 2606:4700::6810:84e5
Name:   cloudflare.com
Address: 2606:4700::6810:85e5


real    0m0.034s
user    0m0.010s
sys     0m0.011s

Google DNS IPv6

[email protected]:~$ time nslookup cloudflare.com 2001:4860:4860::8888
Server:         2001:4860:4860::8888
Address:        2001:4860:4860::8888#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 2606:4700::6810:84e5
Name:   cloudflare.com
Address: 2606:4700::6810:85e5


real    0m0.094s
user    0m0.014s
sys     0m0.007s
[email protected]:~$ time nslookup cloudflare.com 2001:4860:4860::8844
Server:         2001:4860:4860::8844
Address:        2001:4860:4860::8844#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 2606:4700::6810:84e5
Name:   cloudflare.com
Address: 2606:4700::6810:85e5


real    0m0.102s
user    0m0.000s
sys     0m0.021s

Cloudflare DNS IPv6

[email protected]:~$ time nslookup cloudflare.com 2606:4700:4700::1111
;; connection timed out; no servers could be reached



real    0m15.018s
user    0m0.009s
sys     0m0.014s
[email protected]:~$ time nslookup cloudflare.com 2606:4700:4700::1001
;; connection timed out; no servers could be reached



real    0m15.019s
user    0m0.009s
sys     0m0.014s
[email protected]:~$ time nslookup cloudflare.com 2606:4700:4700::1112
Server:         2606:4700:4700::1112
Address:        2606:4700:4700::1112#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 104.16.133.229
;; connection timed out; no servers could be reached



real    0m15.032s
user    0m0.010s
sys     0m0.013s
[email protected]:~$ time nslookup cloudflare.com 2606:4700:4700::1002
Server:         2606:4700:4700::1002
Address:        2606:4700:4700::1002#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 2606:4700::6810:84e5
Name:   cloudflare.com
Address: 2606:4700::6810:85e5


real    0m0.050s
user    0m0.013s
sys     0m0.009s
[email protected]:~$ time nslookup cloudflare.com 2606:4700:4700::1113
Server:         2606:4700:4700::1113
Address:        2606:4700:4700::1113#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 104.16.133.229
;; connection timed out; no servers could be reached



real    0m15.037s
user    0m0.017s
sys     0m0.011s
[email protected]:~$ time nslookup cloudflare.com 2606:4700:4700::1003
;; connection timed out; no servers could be reached



real    0m15.016s
user    0m0.012s
sys     0m0.016s

Google DNS via hostname

[email protected]:~$ time nslookup cloudflare.com dns.google
Server:         dns.google
Address:        2001:4860:4860::8888#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 2606:4700::6810:85e5
Name:   cloudflare.com
Address: 2606:4700::6810:84e5


real    0m0.094s
user    0m0.018s
sys     0m0.006s

Cloudflare DNS via hostname

[email protected]:~$ time nslookup cloudflare.com one.one.one.one
Server:         one.one.one.one
Address:        2606:4700:4700::1111#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 2606:4700::6810:84e5
Name:   cloudflare.com
Address: 2606:4700::6810:85e5


real    0m1.096s
user    0m0.013s
sys     0m0.013s
[email protected]:~$ time nslookup cloudflare.com 1dot1dot1dot1.cloudflare-dns.com
Server:         1dot1dot1dot1.cloudflare-dns.com
Address:        2606:4700:4700::1001#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 2606:4700::6810:84e5
Name:   cloudflare.com
Address: 2606:4700::6810:85e5


real    0m0.078s
user    0m0.014s
sys     0m0.010s
[email protected]:~$ time nslookup cloudflare.com security.cloudflare-dns.com
Server:         security.cloudflare-dns.com
Address:        2606:4700:4700::1112#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 2606:4700::6810:84e5
Name:   cloudflare.com
Address: 2606:4700::6810:85e5


real    0m1.112s
user    0m0.026s
sys     0m0.000s
[email protected]:~$ time nslookup cloudflare.com family.cloudflare-dns.com
Server:         family.cloudflare-dns.com
Address:        1.0.0.3#53

Non-authoritative answer:
Name:   cloudflare.com
Address: 104.16.132.229
Name:   cloudflare.com
Address: 104.16.133.229
Name:   cloudflare.com
Address: 2606:4700::6810:84e5
Name:   cloudflare.com
Address: 2606:4700::6810:85e5


real    0m4.072s
user    0m0.022s
sys     0m0.009s

Sometimes it works fine, but 95% of the time I see delays or no response. Sometimes I get partial responses. I’m unable to figure out what exactly is the issue.

Duplicate of Cloudlare DNS - `nslookup` fails for IPv6 DNS sometimes, but always works for IPv4 DNS

Hi! Could you add a traceroute / mtr to the v6 endpoints that are not working for you? The v4 and v6 should be routed to the same PoP, so I suspect it might be some issue on the path.

Sure @mvavrusa! Will add mtr results. Would it be possible for you to combine this thread and Cloudlare DNS - `nslookup` fails for IPv6 DNS sometimes, but always works for IPv4 DNS? Because @milk, @user21517 and I have had some discussions in that thread. Akismet had marked this thread as spam and hid it initially and that’s when I made a new thread.

Hey @mvavrusa!

Any updates?

I don’t know how to combine a thread, maybe you can close this and continue in the other one?

I don’t know how to combine a thread, maybe you can close this and continue in the other one?

Sure @mvavrusa! I don’t have an option to close this thread. But let’s continue on Cloudlare DNS - `nslookup` fails for IPv6 DNS sometimes, but always works for IPv4 DNS.

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