§2024-11-07

What is the DNS-01 Challenge?

In the context of the ACME protocol (which is used by Let's Encrypt and other CAs), the DNS-01 challenge is a way for you to demonstrate that you control a specific domain (e.g., example.com) by creating a specific DNS record in your domain's DNS zone.

The DNS-01 challenge involves adding a TXT record to the domain's DNS settings that contains a unique value provided by the CA. Once this record is created and the DNS changes have propagated, the CA can verify that you control the domain and will then issue the SSL/TLS certificate. How Does the DNS-01 Challenge Work?

Request: You request a certificate for your domain from the CA (e.g., Let's Encrypt).

Challenge Issued: The CA issues a DNS-01 challenge. This challenge asks you to create a specific TXT record in the DNS zone of the domain you're requesting a certificate for. The record typically looks like this:

_acme-challenge.example.com.  300 IN TXT "random_string_from_CA"

Where:
    random_string_from_CA is a unique string (or token) generated by the CA to verify domain ownership.

Add TXT Record: You log into your DNS provider (like Cloudflare, AWS Route 53, Google DNS, etc.) and add the provided TXT record to the DNS configuration for the domain.

DNS Propagation: Once the TXT record has been added, you wait for DNS propagation to complete. This can take anywhere from a few seconds to several minutes, depending on the TTL (Time To Live) value set on your DNS records.

Validation: The CA then checks the DNS for the TXT record at the specified domain (e.g., _acme-challenge.example.com). If the record is found and the value matches the one issued in the challenge, the CA can confirm you control the domain.

Certificate Issuance: After successful validation, the CA issues the SSL/TLS certificate for your domain, which you can use on your web server or other services.

Why Use the DNS-01 Challenge?

The DNS-01 challenge is particularly useful in situations where:

You do not have a web server running on the domain you are requesting the certificate for (since the other common challenge, HTTP-01, requires serving a file over HTTP).
You need to automate certificate issuance for many domains, as it can be easier to automate DNS updates (with API support from DNS providers like Cloudflare, Route 53, etc.) than managing HTTP servers.
You are using wildcard certificates (e.g., *.example.com). Wildcard certificates can only be obtained through the DNS-01 challenge, as the CA needs to validate the entire domain.