raymond blogs…

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