A Chat with ChatGPT on Lets Encrypt

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A key point for dealing with AI as of 2025 is to lower expectations on facts. What's that? Well some people refer to AI 'models' as having hallucinations. This is 'code speak' for lies and making stuff up.

Below is a general enough of a 'conversation' with ChatGPT on a subject that allows it enough wiggle room on making stuff up VS fact where one can check the details of configuration necessary to take the below abstract discussing into functional reality.

·  What are the requirements of a DNS server in terms of certificates and Let's Encrypt? Don't need any configuration or commands, just requirements of what is needed in terms of functionality.

· ChatGPT said:

For a DNS server to work with Let's Encrypt and certificate issuance, the following requirements must be met:

1. DNS Record Requirements

  • A (IPv4) / AAAA (IPv6) Records:
    • The domain name must resolve to a public IP address of the server requesting the certificate.
  • CNAME Records (Optional):
    • If using a subdomain that points elsewhere, ensure proper resolution.
  • TXT Records (For DNS-01 Challenge):
    • If using DNS-01 validation, the server must be able to create/update TXT records dynamically.

2. DNS Server Capabilities

  • Authoritative DNS Server:
    • If managing domain validation via DNS-01, the DNS server must be authoritative for the domain.
  • API Support (For Automated DNS Challenges):
    • If using acme.sh or certbot DNS plugins, the DNS provider must support API access for adding/removing TXT records.

3. Certificate Requirements

  • Root and Intermediate Certificates:
    • The DNS server must trust Let's Encrypt’s CA (ISRG Root X1 and intermediates).
  • TLS Support (For DNS over HTTPS/TLS):
    • If supporting DoH (DNS over HTTPS) or DoT (DNS over TLS), a valid certificate is needed.

4. ACME Protocol & Validation Requirements

  • HTTP-01 Challenge:
    • If using web-based validation, the DNS server must ensure proper name resolution so the requesting web server can respond correctly.
  • DNS-01 Challenge:
    • The DNS server must allow dynamic updates (via API, scripts, or manual intervention) to place the required TXT record.

5. Security & Compliance

  • Certificate Renewal & Automation:
    • Automatic renewal should be supported via DNS API access or scheduled scripts.
  • CAA Records (Optional but Recommended):
    • A CAA (Certification Authority Authorization) record should allow Let's Encrypt to issue certificates for the domain.

Summary of Required Features

Requirement Purpose
A/AAAA Records Resolves domain to the correct server
TXT Record Support Required for DNS-01 challenge
API for DNS Updates Automates TXT record management
DNSSEC (Optional) Enhances security for DNS records
CAA Record (Optional) Authorizes Let's Encrypt as the CA
TLS Certificate Needed for DNS over HTTPS/TLS

Would you like additional details on any of these?

4o

· You said:

OK, just had an idea where I could make this work. Here's the issue, all public facing DNS servers in my infrastructure only host 'slave' zones, and therefore cannot modify the DNS files. But if I write a BASH script to insert a TXT file temporarily, then that would work. So let's focus on BIND / NAMED right now. Is there any capability to modify records, albeit temporarily, in a slave zone (like adding a TXT record)?

· ChatGPT said:

In BIND/named, a slave (secondary) DNS server cannot modify DNS records because it gets its zone data from the master (primary) DNS server. The slave zone is read-only and updated only through zone transfers (AXFR/IXFR) from the master.

However, there are a few possible workarounds to temporarily add a TXT record for Let’s Encrypt’s DNS-01 challenge:

1. Using a Separate Zone for _acme-challenge

How It Works:

  • You delegate _acme-challenge.yourdomain.com to a small local master zone on the slave server.
  • This allows the slave to be authoritative only for _acme-challenge.yourdomain.com, making it writable.

Steps:

  1. On the Primary DNS Server (yourdomain.com), Delegate _acme-challenge to the Slave

dns

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_acme-challenge IN NS acme-dns.yourdomain.com.

acme-dns IN A <IP-of-Slave-DNS>

  1. On the Slave (Configure as Master for _acme-challenge.yourdomain.com)
    • Add a new master zone in named.conf:

conf

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zone "_acme-challenge.yourdomain.com" {

  type master;

  file "/var/named/acme-challenge.db";

  allow-update { localhost; };

};

  1. Your Bash Script Can Temporarily Insert TXT Records

sh

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nsupdate <<EOF

server 127.0.0.1

zone _acme-challenge.yourdomain.com

update add _acme-challenge.yourdomain.com. 60 TXT "your-LE-token"

send

EOF

    • This adds a TXT record for Let’s Encrypt validation.
    • The record auto-expires after 60 seconds.
  2. Restart named or Use rndc reload

sh

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rndc reload _acme-challenge.yourdomain.com

2. Use allow-update-forwarding to Let Slaves Accept Dynamic Updates

If you cannot create a separate _acme-challenge zone, BIND 9.10+ supports allow-update-forwarding, which lets slaves relay dynamic updates to the master.

How It Works:

  • Slave servers temporarily accept updates but forward them to the master.
  • This requires allow-update-forwarding on the slave and allow-update on the master.

Slave Configuration (named.conf)

conf

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zone "yourdomain.com" {

  type slave;

  masters { <IP-of-master>; };

  file "/var/named/slaves/yourdomain.db";

  allow-update-forwarding { localhost; };

};

Master Configuration (named.conf)

conf

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zone "yourdomain.com" {

  type master;

  file "/var/named/yourdomain.db";

  allow-update { <IP-of-slave>; };

};

Bash Script on Slave (Dynamic Update)

sh

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nsupdate <<EOF

server 127.0.0.1

zone yourdomain.com

update add _acme-challenge.yourdomain.com. 60 TXT "your-LE-token"

send

EOF

  • The slave relays the request to the master.
  • If the master allows it (allow-update), the TXT record is added temporarily.

3. Use a Reverse Proxy for DNS-01 Validation

If modifying DNS records on the slave is impractical, you could:

  • Set up a small authoritative DNS server (like acme-dns) on a separate machine.
  • Proxy _acme-challenge.yourdomain.com requests to this temporary DNS service.

This is a cleaner solution but requires an additional service.

Which Method Is Best?

Method Pros Cons
Separate _acme-challenge zone Allows full control over TXT records on slaves Requires delegation from the primary
allow-update-forwarding Allows temporary updates to the master Requires the master to allow updates
Proxy DNS-01 validation Completely separate from the existing DNS structure Requires an additional service

If you control the master, allow-update-forwarding is a solid option. If not, delegating _acme-challenge to a separate zone gives full control.

Would you like specific examples for implementation?

4o

· You said:

The backend DNS server is Windows based, so idea 2 won't work, but the 'zone delegation' idea seems like it would work. So for Let's Encrypt, where does whatever process creates, not how to do it, but just what it does on say a DNS server that hosts the master file, what gets created by Certbot or acme.sh, etc? Again, not any configuration, just what happens on the DNS server / in the dns zone file.

· ChatGPT said:

For Let’s Encrypt DNS-01 validation, when using Certbot, acme.sh, or another ACME client, the process involves creating a TXT record in the DNS zone. Here’s exactly what happens on the DNS server:

1. A Temporary TXT Record Is Created

  • Let’s Encrypt requires a TXT record under _acme-challenge.<yourdomain> to validate domain control.
  • The TXT record contains a randomly generated token.

Example DNS zone entry (for example.com):

dns

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_acme-challenge.example.com. 60 IN TXT "random-LE-token"

  • The TTL is usually set low (e.g., 60 seconds) for fast propagation.

2. Let’s Encrypt Queries the TXT Record

  • The Let’s Encrypt validation servers (e.g., r3.o.lencr.org) check if the TXT record exists and matches the expected token.
  • If the response is correct, domain ownership is confirmed.

Example query:

sh

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dig _acme-challenge.example.com TXT

Response:

sh

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_acme-challenge.example.com. 60 IN TXT "random-LE-token"

3. The TXT Record Is Removed (Optional)

  • Once validation is successful, the ACME client removes the TXT record.
  • Some setups leave the record until it expires based on TTL.

4. (Optional) If Using Wildcard Certificates (*.example.com)

  • Wildcard certificates require DNS-01 (HTTP-01 won’t work).
  • The process is identical, but the challenge is for _acme-challenge.example.com instead of a subdomain.

What This Means for a Slave DNS Server

  • Since slaves can’t modify records, the _acme-challenge TXT record must be added to a master zone.
  • Delegating _acme-challenge.example.com to a master allows Certbot or acme.sh to write the TXT record dynamically.

Summary of What Happens on the DNS Server

  1. A TXT record is added (_acme-challenge.<yourdomain> IN TXT "random-LE-token")
  2. Let’s Encrypt queries the TXT record
  3. If validated, the TXT record is removed (or left until TTL expires)

Would you like any additional details?

4o

· You said:

What is added configuration or change in configuration for the syntax of the command line for acme.sh to request a certificate via DNS (don't need the zone stuff for BIND / NAMED, assume that's configured). Below is the command I currently use to get a certificate via the 'web server' method. It works and it is perfect for that method. acme.sh --issue -d files.WhatEver.com --cert-home /etc/acme/certs --config-home /etc/acme/config --webroot usr/share/apache2/htdocs/WhatEver.com/Files.WhatEver.com

· ChatGPT said:

To switch from the webroot (HTTP-01) method to the DNS-01 method, you need to remove the --webroot option and instead specify a DNS provider or manual DNS mode.

Command Syntax for DNS-01 Challenge

1. Using a Supported DNS Provider API

If your DNS provider has an API that acme.sh supports, the command structure is:

sh

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acme.sh --issue -d files.WhatEver.com --cert-home /etc/acme/certs --config-home /etc/acme/config --dns <dns_provider>

  • Replace <dns_provider> with the name of your DNS provider.
  • Example for Cloudflare:

sh

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acme.sh --issue -d files.WhatEver.com --cert-home /etc/acme/certs --config-home /etc/acme/config --dns dns_cf

  • You must configure API credentials for the DNS provider (varies by provider).

Reference:

List of supported DNS providers:

https://github.com/acmesh-official/acme.sh/wiki/dnsapi

2. Using Manual DNS Mode

If no API is available and you need to manually update DNS:

sh

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acme.sh --issue -d files.WhatEver.com --cert-home /etc/acme/certs --config-home /etc/acme/config --dns --yes-I-know-dns-manual-mode-enough-go-ahead-please

  • The script will output a TXT record (_acme-challenge.files.WhatEver.com).
  • You must add this TXT record manually to your DNS server.
  • Once added and propagated, run the validation step:

sh

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acme.sh --renew --dns --yes-I-know-dns-manual-mode-enough-go-ahead-please

3. Example Command for Delegated _acme-challenge Zone

If _acme-challenge.files.WhatEver.com is delegated to a local BIND server that allows updates:

sh

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acme.sh --issue -d files.WhatEver.com --cert-home /etc/acme/certs --config-home /etc/acme/config --dns dns_nsupdate

  • Requires configuring NSUPDATE_KEY, NSUPDATE_SERVER, etc.

Summary of Changes

Method Old Command (Webroot) New Command (DNS-01)
Webroot --webroot usr/share/apache2/htdocs/WhatEver.com/Files.WhatEver.com Remove this option
DNS API Not applicable --dns <dns_provider>
Manual DNS Not applicable --dns --yes-I-know-dns-manual-mode-enough-go-ahead-please

Would you like an example for a specific DNS provider?

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