watchNS/cdn.gitbook.comについて、ここに記述してください。

{{{
$ dnsq ns cdn.gitbook.com a.gtld-servers.net
2 cdn.gitbook.com:
175 bytes, 1+0+2+4 records, response, noerror
query: 2 cdn.gitbook.com
authority: gitbook.com 172800 NS hugh.ns.cloudflare.com
authority: gitbook.com 172800 NS dahlia.ns.cloudflare.com
}}}

$ dnsq ns cdn.gitbook.com a.gtld-servers.net
{{{
2 cdn.gitbook.com:
175 bytes, 1+0+2+4 records, response, noerror
query: 2 cdn.gitbook.com
authority: gitbook.com 172800 NS hugh.ns.cloudflare.com
authority: gitbook.com 172800 NS dahlia.ns.cloudflare.com
}}}

$ dnsq ns cdn.gitbook.com ns1.gitbook.me
{{{
2 cdn.gitbook.com:
64 bytes, 1+1+0+0 records, response, noerror
query: 2 cdn.gitbook.com
answer: cdn.gitbook.com 3600 A 128.199.158.79
}}}

なにを訊いても、Aを返すようだ。

{{{
$ dnsq ns qmail.cdn.gitbook.com ns1.gitbook.me
2 qmail.cdn.gitbook.com:
76 bytes, 1+1+0+0 records, response, noerror
query: 2 qmail.cdn.gitbook.com
answer: qmail.cdn.gitbook.com 3600 A 128.199.158.79
}}}