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What makes a DNS zone compromise so dangerous is that DNS is what users’ browsers rely on to know what IP address they should contact when trying to reach your domain. If the authoritative DNS servers reply with a DNS record that contains the correct challenge token, ownership over the domain is proven and the certificate issuance process can continue. As an example, if you were trying to validate the domain for *.eff.org, the validation subdomain would be "_." When the token value is added to the DNS zone, the client tells the CA to proceed with validating the challenge, after which the CA will do a DNS query towards the authoritative servers for the domain. More specifically, the CA sends a unique random token to the ACME client, and whoever has control over the domain is expected to put this TXT record into its DNS zone, in the predefined record named "_acme-challenge" under the actual domain the user is trying to prove ownership of. When the client requests a certificate, the CA asks the client to prove ownership over the domain by adding a specific TXT record to its DNS zone.
SIMPLE DNS PLUS SERVICE NOT AUTOMATIC START SOFTWARE
In the DNS challenge, the user requests a certificate from a CA by using ACME client software like Certbot that supports the DNS challenge type. How Does the DNS Challenge Work?Īt a high level, the DNS challenge works like all the other automatic challenges that are part of the ACME protocol-the protocol that a Certificate Authority (CA) like Let's Encrypt and client software like Certbot use to communicate about what certificate a server is requesting, and how the server should prove ownership of the corresponding domain name. In the rest of this post, we'll take a deep dive into the components involved in that process, and what the options are for making it more secure. In many cases, this means that if the machine handling the process gets compromised, so will the DNS credentials, and this is where the real danger lies. the login and password, or a cryptographic token), and those credentials will have to be stored wherever the automation takes place. In order to modify the DNS records, that software will also need to have access to the credentials for the DNS service (e.g. In order to be automatic, though, the software that requests the certificate will also need to be able to modify the DNS records for that domain. From the perspective of a Certificate Authority (CA) like Let's Encrypt, there's no better way to prove that you control a domain than by modifying its DNS records, as controlling the domain is the very essence of DNS.īut one of the key ideas behind Let's Encrypt is that getting a certificate should be an automatic process. In order to issue wildcard certificates, Let's Encrypt is going to require users to prove their control over a domain by using a challenge based on DNS, the domain name system that translates domain names like into IP addresses like 69.50.232.54. So if you examine the certificate for eff.org today, it will say it's valid for *.eff.org.) That way, a system administrator can get a certificate for their entire domain, and use it on new subdomains they hadn't even thought of when they got the certificate.
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(In the certificate, this is indicated by using a wildcard character, indicated by an asterisk. A wildcard certificate is just a certificate that says “I'm valid for all of the subdomains in this domain” instead of explicitly listing them all off. Certificates can also list multiple domains (e.g.,, , etc.) if the owner just wants to use one certificate for all of her domains. For example, a certificate from has to actually list as a valid domain for that certificate.
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In order to validate an HTTPS certificate, a user’s browser checks to make sure that the domain name of the website is actually listed in the certificate. And that number is just going to keep growing, because in a few weeks Let's Encrypt will also start issuing “wildcard” certificates-a feature many system administrators have been asking for. Earlier this month, Let's Encrypt (the free, automated, open Certificate Authority EFF helped launch two years ago) passed a huge milestone: issuing over 50 million active certificates.