How can I check the glue records for a domain?

Question

I need to check the glue records for a domain (say, example.com) in a script. However, the dig command just gives me the nameservers from the actual zone.

user@host:~ $ dig +nocmd +nostats NS example.com.
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 32213
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 5

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;example.com.			IN	NS

;; ANSWER SECTION:
example.com.		86261	IN	NS	a.iana-servers.net.
example.com.		86261	IN	NS	b.iana-servers.net.

;; ADDITIONAL SECTION:
a.iana-servers.NET.	1661	IN	A	199.43.135.53
a.iana-servers.NET.	1661	IN	AAAA	2001:500:8f::53
b.iana-servers.NET.	1661	IN	A	199.43.133.53
b.iana-servers.NET.	1661	IN	AAAA	2001:500:8d::53

Answer 1

The glue records for the zone example.com exist in its parent zone (com), so you need to query the nameservers of that zone for the nameservers of example.com without recursing into the subdomain. To do that you first need a nameserver for the parent zone:

user@host:~ $ dig +short com. NS
b.gtld-servers.net.
f.gtld-servers.net.
a.gtld-servers.net.
e.gtld-servers.net.
g.gtld-servers.net.
h.gtld-servers.net.
i.gtld-servers.net.
l.gtld-servers.net.
d.gtld-servers.net.
k.gtld-servers.net.
c.gtld-servers.net.
j.gtld-servers.net.
m.gtld-servers.net.

The nameservers are listed in random order, so you can just pick the first one from the output:

dig +short NS com. | head -n 1

Then query that server for the NS records of the domain in question. The nameservers for the subdomain are listed in the "authority" section of the response, their A records (if there are any) are listed in the "additional" section.

dig +noall +authority +additional +norecurse @b.gtld-servers.net. NS example.com.

Arguments:

• +noall: suppress all output
• +authority: except for the "authority" section
• +additional: and except for the "additional" section
• +norecurse: don't perform a recursive query

Example script:

#!/bin/bash

domain="${1:?Usage: $0 DOMAIN)"
domain="${domain%.}"

ns="$(dig +short NS "${domain#*.}." | head -n 1)"

dig +noall +authority +additional +norecurse @"$ns" NS "${domain}."