04 Feb 2019
DANE (RFC 6698) basically allows to publish the hash of server certificate as a TLSA record in DNS, signed with DNSSEC. A client can validate that a server's TLS certificate is owned by the same entity which controls DNSSEC signed zone.
The idea is to add a new DNS record of type TLSA to the zone, in particular a Certificate Usage DANE-EE (3).
25._tcp.mail IN TLSA 3 1 1 2c788de8eaf09f6b1f5a704a5e0718206c668f00dfca8f8112608dc25571553cIn the example, 3 is the usage (DANE-EE), 1 the selector (subject public key), 1 the matching type (SHA-256). This for my MX record,
mail.pmeerw.net, port 25 (wildcard port number * would also be possible). The hash can be conveniently generated (taken from Viktor Dukhovni's tlsagen script):
openssl x509 -in /etc/letsencrypt/live/pmeerw.net/fullchain.pem -noout -pubkey | \ openssl pkey -pubin -outform DER | \ openssl dgst -sha256 -binary | \ hexdump -ve '/1 "%02x"'
Since my server certificate is issued by Let's Encrypt, the certificate is renewed every 90 days or so.
Because DANE essentially puts the certificate's public key into DNS, the DNS record needs to be updated whenever the key changes.
letsencrypt scripts (since >= 0.25.0, June 2018) support the argument
renew --reuse-key, so the same keys are reused. The certificate is renewed, but not the underlying keys.
Here is my weekly cron job script
!/bin/sh letsencrypt renew --reuse-key --pre-hook "systemctl stop apache2" --post-hook "systemctl start apache2" res=$(find /etc/letsencrypt/live/ -type l -mtime -1) if [ -n "$res" ]; then echo "letsencrypt: new cert" systemctl restart apache2 systemctl restart postfix systemctl restart dovecot else echo "letsencrypt: nothing to do" fiWeirdly, the documentation is to be found with
certbot -h automation, not
certbot -h renew.