diff --git a/DANE-How-to.md b/DANE-How-to.md index abb60ba..1606e45 100644 --- a/DANE-How-to.md +++ b/DANE-How-to.md @@ -7,8 +7,81 @@ # Implementing DANE for outbound e-mail traffic This part of the How-to describes the steps that should be taken with regard to your outbound e-mail traffic. This enables other parties to use DANE for validating the certificate offered by your e-mail server. +## Debian Stretch +**Specifics for this setup** +* Linux Debian 9.7 (Stretch) +* SpamAssassin version 3.4.2 (running on Perl version 5.24.1) +* Postfix 3.1.8 +* BIND 9.10.3-P4-Debian +* Two certficates for two mailservers were purchased from Comodo/Sectigo + +**Assumptions** +* DNSSEC is used +* Mailserver is operational + ## Generating DANE records +**primairy mailserver (mail1.example.com)** +Command +`openssl x509 -in /path/to/primairy-mailserver.crt -noout -pubkey | openssl pkey -pubin -outform DER | openssl sha256` +Output +`(stdin)= 29c8601cb562d00aa7190003b5c17e61a93dcbed3f61fd2f86bd35fbb461d084` + +**secundairy mailserver (mail2.example.com)** +Command +`openssl x509 -in /path/to/secundairy-mailserver.crt -noout -pubkey | openssl pkey -pubin -outform DER | openssl sha256` +Output +`(stdin)= 22c635348256dc53a2ba6efe56abfbe2f0ae70be2238a53472fef5064d9cf437` + ## Publishing DANE records +Configuration options +* Selector field is "1" because we used the certificates' public key to generate DANE hash/signature +* Usage is "3". In this case we generated a DANE hash of the leaf certificate itself. Therefore we use usage field "3" (DANE-EE: Domain Issued Certificate) +* Matching-type is "1" because we use SHA-256. + +With this information we can create a DNS record for DANE: +`_25._tcp.mail.example.com. IN TLSA 3 1 1 29c8601cb562d00aa7190003b5c17e61a93dcbed3f61fd2f86bd35fbb461d084` +`_25._tcp.mail2.example.com. IN TLSA 3 1 1 22c635348256dc53a2ba6efe56abfbe2f0ae70be2238a53472fef5064d9cf437` + +## Generating DANE roll-over records +First we split the bundle file into multiple certificates. +`cat ca-bundle-file.crt | awk 'BEGIN {c=0;} /BEGIN CERT/{c++} { print > "bundlecert." c ".crt"}'` +In this case this results in two files: _bundlecert.1.crt_ and _bundlecert.2.crt_. + +For each file we view the subject and the issuer. +Command +`openssl x509 -in bundlecert.1.crt -noout -subject -issuer` + +Output +`subject=C = GB, ST = Greater Manchester, L = Salford, O = Sectigo Limited, CN = Sectigo RSA Domain Validation Secure Server CA` +`issuer=C = US, ST = New Jersey, L = Jersey City, O = The USERTRUST Network, CN = USERTrust RSA Certification Authority` + +Command +openssl x509 -in bundlecert.2.crt -noout -subject -issuer +Output +subject=C = US, ST = New Jersey, L = Jersey City, O = The USERTRUST Network, CN = USERTrust RSA Certification Authority +issuer=C = US, ST = New Jersey, L = Jersey City, O = The USERTRUST Network, CN = USERTrust RSA Certification Authority + +The second certificate (bundlecert.2.crt) is the root certficate as the subject and the issuer are the same. Root certificates are self-signed, while intermediate certificates are signed by another certificate (being a root certificate, of another intermediate certificate). + +## Publishing DANE roll-over records +In this case we select the root certificate as a roll-over anchor. + +Command +`openssl x509 -in bundlecert.2.crt -noout -pubkey | openssl pkey -pubin -outform DER | openssl sha256` + +Output +`(stdin)= c784333d20bcd742b9fdc3236f4e509b8937070e73067e254dd3bf9c45bf4dde` + +Configuration options +* Selector field is "1" because we use the certificate public key to generate DANE hash/signature +* Usage is "2". In this case I generated a DANE hash of a certificate in the chain the chain of trust, instead of the certificate itself. Therefore we use usage field "2" (DANE-TA: Trust Anchor Assertion) +* Matching-type is "1" because I use SHA-256. + +With this information we can create a rollover DNS record for DANE: +`_25._tcp.mail.traxotic.net. IN TLSA 2 1 1 c784333d20bcd742b9fdc3236f4e509b8937070e73067e254dd3bf9c45bf4dde` +`_25._tcp.mail2.traxotic.net. IN TLSA 2 1 1 c784333d20bcd742b9fdc3236f4e509b8937070e73067e254dd3bf9c45bf4dde` + + ## Configuring mailserver # Implementing DANE for inbound e-mail traffic