DANE https://datatracker.ietf.org/doc/html/rfc7671 stands for DNS-Based Authentication of Named Entities. This protocol allows clients to check the remote certificate used trough TLSA DNS records. DANE requires DNSSEC https://datatracker.ietf.org/doc/html/rfc9364.
This post is not about the client side implementation, it is about the backend. I’ll explain how to created TLSA records that use the public key of the certificate and issuer certificate used. Basically I only use openssl to create the TLSA records. The article is based on https://www.mailhardener.com/kb/how-to-create-a-dane-tlsa-record-with-openssl. Most common use is for mail servers to ensure encrypted mail transfer between MTA’s, so that is what I will use in this example. The mail server that wants to deliver an email to your protected mail server should still respect the TLSA records you have published. The adaption for DANE has increased the last years. even companies as Microsoft start adoption DANE.
Obtaining the certificate and chain from your mail server
With openssl you can easily re-generate the PEM encoded certificate and chain from a mailserver.
echo QUIT | openssl s_client -connect mail.example.com:25 -starttls smtp -showcerts
This opens a connection over port 25 with STARTTLS and prints the certificates and then quits the connection made. To store the output in a file just redirect the output.
The certificate chain in the output probably contains multiple certificates starting with the server certificate.
Lets save the first certificate in the chain as server.crt, and the second as intermediate.crt.
For a mail server we are interested in the server certificate (the first certificate in the chain) and the issuer certificate. We use the schema 3 1 1 (DANE EE) for server certificate and 2 1 1(DANE TA) for the issuing certicate. The TLSA value published in DNS is a SHA256 hash of the public key. The public key will only change if the private key used to create the CSR of the certificate has changed.
Create SHA256 hash from the public key
From the certificate files we have obtained we now can calculate the SHA256 hash.
openssl x509 -in server.crt -pubkey -noout | openssl rsa -pubin -outform der | sha256sum
generates the public key hash for the server certificate. Output could be something similar like:
writing RSA key
4648564dc7c901037f631391d765643e8f8f86622849f59dfc9564838e1e8a76 -We only need the long string here. We can repeat this for the intermediate certificate.
Create and publish TLSA DNS records
DANE authentication checks TLSA records published. For our mail server we want to publish the public key SHA256 hash for server and intermediate certificate for port 25 (you can also publish records for fort 465 or 587 is you want). So lets say we want to publish the server public key for our mail server (mail.example.com) we publish the following record:
Name: _25._tcp.mail.example.com.
Type: TLSA
TTL: 1 day
Value: 3 1 1 4648564dc7c901037f631391d765643e8f8f86622849f59dfc9564838e1e8a76
It is a good practice to also publish a 2 1 1 TLSA record for the intermediate certificate. When your certificate changes (and the private key has changed too) make sure you publish a new TLSA record before installing the new certificate. You can have multiple instances for 3 1 1 xxx and 2 1 1 x x x records. After the new certificate has been installed, the stale records can be removed.