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Web Server Rescue

Read-only

Compare A and AAAA Records

A site works for some users but fails for IPv6-capable clients, so you need to compare IPv4 and IPv6 DNS.

Command

printf 'A '; dig +short edge.test A; printf 'AAAA '; dig +short edge.test AAAA

Before you run this

System impact: Read-only. Low when scoped to the shown target.

When not to use it: Do not remove IPv6 records without confirming the intended edge and provider behavior.

Expected output

The IPv4 and IPv6 DNS answers for the same hostname.

System impact

Read-only. Nothing changes. The command reads DNS and compares address families.

Recovery / rollback: no state is changed.

When to use it

Use when mobile networks, certain ISPs, or IPv6 clients report a different failure.

When not to use it

Do not remove IPv6 records without confirming the intended edge and provider behavior.

Explanation-only example

Illustrated output, not a live lab run

This example is intentionally illustrative. It shows the command shape without killing real processes or changing your machine.

demo@lab:~$

$ printf 'A '; dig +short example.com A; printf 'AAAA '; dig +short example.com AAAA

A 203.0.113.10
AAAA 2001:db8:44::10

$ curl -4 -I https://example.com && curl -6 -I https://example.com

HTTP/2 200
server: edge-fixture
content-type: text/html

ipv6_path=connect_failed status=000
View commands shown

These are the commands shown in the sanitized transcript.

Commands shown

  1. printf 'A '; dig +short edge.test A; printf 'AAAA '; dig +short edge.test AAAA
  2. curl -4 -I https://edge.test && curl -6 -I https://edge.test

next steps

Related commands

Web Server Rescue Read-only

Compare DNS Answers Across Resolvers

One resolver can still have the old edge IP while another has the new one.

for r in 1.1.1.1 8.8.8.8 9.9.9.9; do printf '%s ' "$r"; dig @"$r" +short edge.test A; done
Web Server Rescue Read-only

Compare Authoritative Nameserver Answers

The recursive resolver was not the problem. One nameserver disagreed.

for ns in $(dig +short NS edge.test); do printf '%s ' "$ns"; dig @"$ns" +short edge.test A; done
Web Server Rescue Read-only

Check CAA Certificate Issuers

The certificate request failed because DNS allowed the wrong issuer.

dig +short edge.test CAA
Web Server Rescue Read-only

Check the WWW CNAME Target

The apex was right. The www name pointed through a different path.

dig +short www.edge.test CNAME
Web Server Rescue Read-only

Show the DNS Answer TTL

The fix was correct. The TTL explained why users still saw the old edge.

dig +noall +answer edge.test A
Study mapping

Use this as independent command practice: read the notes, predict the output, then compare it with the example before using a real shell.

  • lpic1:109-networking
  • lfcs:networking
  • lfcs:services-logs
  • linuxplus:provisional
  • linuxplus:troubleshooting
  • risk:read-only

Useful for

  • LPIC-1 style command-line practice
  • LFCS style performance tasks
  • Linux+ style troubleshooting review

Independent study support only. No affiliation, endorsement, exam dumps, or real exam questions.