A. How to check if nameserver is valid
$ dig ns com
;; ANSWER SECTION:
com. 93009 IN NS a.gtld-servers.net.
com. 93009 IN NS b.gtld-servers.net.
com. 93009 IN NS c.gtld-servers.net.
com. 93009 IN NS d.gtld-servers.net.
com. 93009 IN NS e.gtld-servers.net.
com. 93009 IN NS f.gtld-servers.net.
com. 93009 IN NS g.gtld-servers.net.
com. 93009 IN NS h.gtld-servers.net.
com. 93009 IN NS i.gtld-servers.net.
com. 93009 IN NS j.gtld-servers.net.
com. 93009 IN NS k.gtld-servers.net.
com. 93009 IN NS l.gtld-servers.net.
com. 93009 IN NS m.gtld-servers.net.
$ host NS1.example.com i.gtld-servers.net
Using domain server:
Name: i.gtld-servers.net
Address: 192.10.10.10#53
Aliases:NS1.example.com has address 69.10.10.180
If the nameserver is invalid, you will not have the last line above.
B. To check nameserver configuration of a site, do this:
$ nslookup -q=any example.com
