19 KiB
Table of contents
- Executive Summary
- Introduction
- What is DANE?
- Why use DANE for SMTP?
- Guaranteeing a valid TLSA record
- Tips, tricks and notices for implementation
- Outbound e-mail traffic (DNS records)
- Implementing DANE for SMTP on Postfix (inbound e-mail traffic)
- Implementing DANE for SMTP on Exim (inbound & outbound e-mail traffic)
- Implementing DANE for SMTP using Halon (inbound & outbound e-mail traffic)
Table of contents generated with markdown-toc
Executive Summary
- DANE is a best-practice technology for securing the transfer of email (SMTP) between organizations across the public Internet.
- Successful DANE deployments require additional operational discipline.
- Automated monitoring of your own email servers and related DNS records is is a must.
- Robust automation of coördinated DNS and email server certificate chain updates.
- These topics will be covered in more detail below.
- Please deploy DANE for your email servers, but plan carefully, botched deployments not not only harm the domain in question, but also have a deterrent effect on adoption by others.
Introduction
This how-to is created by the Dutch Internet Standards Platform (the organization behind internet.nl) and is meant to provide practical information and guidance on implementing DANE for SMTP.
What is DANE?
DANE is short for "DNS-based Authentication of Named Entities" and is described in RFC 6698 and RFC 7671. DANE enables publication in secure DNS of keys and certificates for use with TLS. DANE TLSA records published in the server operator's signed DNS zone provide a downgrade-resistant means to discover support for STARTTLS and to validate the server's certificate chain without relying on additional trusted parties outside the delegation chain in DNS.
DANE is designed to work with any TLS service, not just email, but DANE for HTTP is not presently supported by the major browsers and so has seen little deployment. DANE for SMTP (which is described in RFC 7672 on the other hand, is used increasingly and adds active attack (man-in-the-middle) resistance to SMTP transport encryption RFC 7672 Section 1.3. DANE for SMTP uses the presence of DANE TLSA records to securely signal TLS support and to publish the means by which SMTP clients can successfully authenticate legitimate SMTP servers. The result is called "opportunistic DANE TLS", and resists downgrade and man-in-the-middle (MITM) attacks when the destination domain and its MX hosts are DNSSEC signed, and TLSA records are published for each MX host.
Why use DANE for SMTP?
The use of opportunistic TLS (via STARTTLS) is not without risks:
- Because forcing the use of TLS for all mail servers would break backwards compatibility, SMTP uses opportunistic TLS (via STARTTLS) as a mechanism to enable secure transport between mail servers. However, the fact that STARTTLS is opportunistic means that the initial connection from one mail server to another always starts unencrypted making it vulnerable to man in the middle attacks. If a mail server does not offer the 'STARTTLS capability' during the SMTP handshake (because it was stripped by an attacker), transport of mail occurs over an unencrypted connection.
- By default mail servers do not validate the authenticity of another mail server's certificate; any random certificate is accepted (see RFC 2487. This was probably done because there is no user who can act on errors in case they occur. Unfortunately this default behavior introduces the risk of a man in the middle attack; an attacker can offer a false certificate enabling the attacker to decrypt encrypted traffic.
DANE addresses these shortcomings because:
- Sending mail servers can deduce a receiving mail server's ability to use TLS, by the presence of a TLSA record. This means that a connection does not have to start unencrypted (awaiting the STARTTLS capability) but can be encrypted from the start.
- TLSA records can be used to validate the certificate provided by the receiving mail server. This implies that the administrator of a domain 'guarantees' that the TLSA record is always correct and can be used to validate the certificate. Because of this guarantee the sending mail server does not have to fallback to unencrypted mail transport when the offered certificate does not match a single TLSA record. If this is the case the sending mail server can abort the transport and not send the e-mail.
Guaranteeing a valid TLSA record
Because it is important that there is always a valid TLSA record to make sure mail transport can occur, DANE offers a roll-over mechanism. A roll-over is useful when certificates expire and need to be replaced. Since distributing information via DNS can be a bit slow (depending on the TTL settings), it's important to anticipate a certificate change from a DANE perspective. This can be done by applying a roll-over scheme of which there are two:
- Current + next. This roll-over scheme provides two TLSA records per mail server. One with the fingerprint of the current mail server's certificate (usage type 3), and another with the fingerprint of the future mail server's certificate (usage type 3). The latter can, for example, be determined by using a Certificate Signing Request (CSR).
- Current + issuer. This roll-over scheme provides two TLSA records per mail server. One with the fingerprint of the current mail server's certificate (usage type 3), and another with the fingerprint of a certificate within the current mail server's certificate chain of trust; an intermediate or root certificate.
Tips, tricks and notices for implementation
This section describes several pionts for attention when implementing DANE for SMTP.
- Purchasing of expensive certificates for mail server has no to little added value for the confidentiality since mail server don't validate certificates by default. Depending on the context there can be other advantages which makes organizations decide to use specific certificates.
- It is recommended to use a certificates public key for generating a TLSA signature (selector type "1") instead of the full certificate (selector type "0"), because this enables the reuse of key materials. Notice that the use of Forward Secrecy decreases the need to use a new key-pair on every occasion.
- An issuer certificate (usage type "2") validates only when the full certificate chain is offered by the receiving mail server.
- Mail servers don't validate certificates and therefore don't have their own certificate store. That's why DANE for SMTP only supports usage type "2" (DANE-TA) and usage type "3" (DANE-EE). Usage type "0" (PKIX-TA) and usage type "1" (PKIX-EE) are not supported.
- Make sure the TTL (time-to-live) of your TLSA records is not too high. This makes it possible to apply changes relatively fast in case of problems. A TTL between 30 minutes (1800) and 1 hour (3600) is recommended.
- The refresh value of your full DNS zone should be in accordance with the TTL setting of your TLSA record, to make sure all name servers give the same information when (after expiration of the TLSA TTL) being queried.
- In case of roll-over scheme "current + issuer", the use of the root certificate is preferred because in some contexts (PKIoverheid) this makes it easier to switch supplier / certficate without impacting DANE. (Remember DigiNotar).
- Roll-over scheme "current + next" gives less flexibility but the highest form of certainty, because of "tight pinning".
- Implement monitoring of your DANE records to be able to detect problems as soon as possible.
- Make sure your implementation supports the usage of a CNAME in your MX record. There are some inconsistencies between multiple RFC's. According to RFC 2181 a CNAME in MX records is not allowed, while RFC 7671 and RFC 5321 imply that the usage of a CNAME in MX records is allowed.
Outbound e-mail traffic (DNS records)
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 certificates offered by your e-mail servers.
Generating DANE records
Primary mail server (mail1.example.com)
Generate the DANE SHA-256 hash with the following command:
openssl x509 -in /path/to/primary-mailserver.crt -noout -pubkey | openssl pkey -pubin -outform DER | openssl sha256
This command results in the following output:
(stdin)= 29c8601cb562d00aa7190003b5c17e61a93dcbed3f61fd2f86bd35fbb461d084
Secondary mail server (mail2.example.com)
For the secondary mail server we generate the DANE SHA-256 hash using the command:
openssl x509 -in /path/to/secondary-mailserver.crt -noout -pubkey | openssl pkey -pubin -outform DER | openssl sha256
This command results in the following output:
(stdin)= 22c635348256dc53a2ba6efe56abfbe2f0ae70be2238a53472fef5064d9cf437
Publishing DANE records
Now that we have the SHA-256 hashes, we can construct the DNS records. We make the following configuration choices:
- Usage field is "3"; we generated a DANE hash of the leaf certificate itself (DANE-EE: Domain Issued Certificate).
- Selector field is "1"; we used the certificates' public key to generate DANE hash/signature.
- Matching-type field is "1"; we use SHA-256.
With this information we can create the 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
We use the provided bundle file for generating the DANE hashes belonging to the root certificate. In order to do that, we first split the bundle file into multiple certificates using cat ca-bundle-file.crt | awk 'BEGIN {c=0;} /BEGIN CERT/{c++} { print > "bundlecert." c ".crt"}'. In this specific case this results in two files: bundlecert.1.crt and bundlecert.2.crt.
For each file we view the subject and the issuer. We start with the first file using the following command:
openssl x509 -in bundlecert.1.crt -noout -subject -issuer
This results in the following 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
For the second file we use the command:
openssl x509 -in bundlecert.2.crt -noout -subject -issuer
This results in the following 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
Based on the information of these two certificates, we can conclude that the second certificate (bundlecert.2.crt) is the root certificate; since root certificates are self-signed the subject and the issuer are the same. The other certificate (bundlecert.1.crt) is an intermediate certificate which is (in this case) signed by the root certificate.
Publishing DANE roll-over records
Because we prefer the root certificate to be our roll-over anchor, we generate the DANE SHA-256 hash using the command:
openssl x509 -in bundlecert.2.crt -noout -pubkey | openssl pkey -pubin -outform DER | openssl sha256
This results in the following output:
(stdin)= c784333d20bcd742b9fdc3236f4e509b8937070e73067e254dd3bf9c45bf4dde
Since both certificates for the primary and secondary come from the same Certificate Authority, they both have the same root certificate. So we don't have to repeat this with a different bundle file.
Now that we have the SHA-256 hash, we can construct the DANE roll-over DNS records. We make the following configuration choices:
- Usage field is "2"; we generated a DANE hash of the root certificate which is in the chain the chain of trust of the actual leaf certificate (DANE-TA: Trust Anchor Assertion).
- Selector field is "1"; because we use the root certificate's public key to generate a DANE hash.
- Matching-type field is "1"; because we use SHA-256.
With this information we can create a rollover DNS record for DANE:
_25._tcp.mail.example.com. IN TLSA 2 1 1 c784333d20bcd742b9fdc3236f4e509b8937070e73067e254dd3bf9c45bf4dde
_25._tcp.mail2.example.com. IN TLSA 2 1 1 c784333d20bcd742b9fdc3236f4e509b8937070e73067e254dd3bf9c45bf4dde
Implementing DANE for SMTP on Postfix (inbound e-mail traffic)
Specifics for this setup
- Linux Debian 9.8 (Stretch)
- SpamAssassin version 3.4.2 (running on Perl version 5.28.1)
- Postfix 3.4.5
- BIND 9.10.3-P4-Debian
- Two certificates (for two mail servers) from Comodo / Sectigo
Assumptions
- DNSSEC is used
- Mail server is operational
- Software packages are already installed
Configuring Postfix
Postfix plays an important role in using DANE for validating the when available.
Make sure the following entries are present in /etc/postfix/main.cf
smtp_dns_support_level = dnssec
This setting tells Postfix to perform DNS lookups using DNSSEC. This is an important prerequisite for DANE to be effective, since regular DNS lookups can be manipulated. Without DNSSEC support, Postfix cannot use DANE.
smtp_tls_security_level = dane
By default Postfix uses opportunistic TLS (smtp_tls_security_level = may) which is susceptible to man in the middle attacks. You could tell Postfix to use mandatory TLS (smtp_tls_security_level = encrypt) but this breaks backwards compatibility with mail servers that don't support TLS (and only work with plaintext delivery). However, when Postfix is configured to use the "dane" security level (smtp_tls_security_level = dane) it becomes resistant to man in the middle attacks, since Postfix will connect to other mail servers using "mandatory TLS" when TLSA records are found. If TLSA records are found but are unusable, Postfix won't fallback to plaintext or unauthenticated delivery.
smtp_host_lookup = dns
This tells Postfix to perform lookups using DNS. Although this is default behavior it is important to make sure this is configured, since DANE won't be enabled if lookups are performed using a different mechanism.
smtpd_tls_CAfile = /path/to/ca-bundle-file.crt
When applying a DANE roll-over scheme using an "issuer certificate" (an intermediate or root certificate), Postfix must be able to provide the certificates of the used issuer in the chain of trust. Hence this setting.
Implementing DANE for SMTP on Exim (inbound & outbound e-mail traffic)
Specifics for this setup
- Ubuntu 18.10 ‘Cosmic Cuttlefish’
- Exim 4.92 (DANE support is non-experimental since version 4.91)
Assumptions
- DNSSEC is used
- Mail server is operational
Configuration for inbound e-mail traffic
Install or generate key pair
You can use a commercial or Let's Encrypt certificate, but you can also generate your own key pair by using the provided Exim tools. Use the following command to generate a key pair.
sudo bash /usr/share/doc/exim4-base/examples/exim-gencert
Configure TLS
In Exim you should configure TLS by adding the following to main/03_exim4-config_tlsoptions
MAIN_TLS_ENABLE = yes
tls_advertise_hosts = *
tls_certificate = /path/to/certificate.crt
tls_privatekey = /path/to/private.key
Configuration 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 certificates offered by your e-mail servers.
DNSSEC validating resolvers
Make sure to configure DNSSEC validating resolvers on the mail server. When using the locale systemd resolver, make sure to add the following to /etc/systemd/resolved.conf.
DNSSEC = yes
Configure DNSSEC validation in Exim
In Exim you explicitly need to configure DNSSEC validation by adding the following to main/02_exim4-config_options since some resolvers only validate DNSSEC on request.
dns_dnssec_ok = 1
Configure DANE
In order to use DANE, you should tell Exim to check for DANE records when sending e-mail. You can configure DANE validation to be mandatory by adding the following to transport/30_exim4-config_remote_smtp.
hosts_require_dane = *
This means that TLS connections are not accepted when the domain you are trying to send mail to does not have a valid TLSA record. Since this is rather strict and not recommended to be the default, you are probably better of by configuring DANE validation to be additional. This can be done by adding the following to transport/30_exim4-config_remote_smtp.
hosts_try_dane = *
Notice that depending on the way you configured Exim, you need to apply DANE for all SMTP transports.
Implementing DANE for SMTP using Halon (inbound & outbound e-mail traffic)
Serveral Dutch hosting providers use Halon (a scriptable SMTP server who's virtual appliances are based on FreeBSD) as the internet facing e-mail server. The actual mail boxes reside on Direct Admin (which uses Exim) within the internal network. In this specific setup you could say that all security features are applied at the internet facing mail server which is Halon.
Halon has built-in support for DANE and can be configured by using the information provided on the website: https://halon.io/dane and https://wiki.halon.io/DANE.