In the past I have written about SPF, DKIM and DMARC for email authentication in Exchange server to improve e-mail security. Additional topics to improve security are secure DNS (DNSSEC) and DNS-based Authentication of Named Entities (DANE). Where SPF, DKIM and DMARC are focusing more on the email messages and the sending hosts they are coming from, is DANE more focusing on setting up the TLS connection between mail servers. DANE has a dependency on DNSSEC but since my focus is on Exchange and Exchange Online, I don’t discuss DNSSEC and leave that to my provider (DANE only works when DNSSEC is enabled).

When two mailservers setup a connection, they negotiate a common TLS protocol and this handshake as it is called is unencrypted. As such, this handshake is vulnerable for man-in-the-middle or downgrade attacks.

DANE is using a special DNS record (a TLSA record, Transport Layer Security Authentication) where an organization can publish information regarding the certificate on the mail server and thus the TLS options available.

The TLSA record for my own domain is shown in the following example:

_25._tcp.smtphost.exchangelabs.nl IN TLSA 3 1 1 50167c478a2f536a88ee9ec232b14b0d223c2d5bdc837451eee104c153376cbe

When a mailserver wants to send email to my Exchange environment it retrieves the MX record first which is smtphost.exchangelabs.nl (this is an Exchange 2019 Edge transport server, with a Digicert certificate that matches the servername). When DNSSEC is not used at the recipient domain, opportunistic TLS is used. When DNSSEC is used at the recipient domain, a DANE capable mail server will check for a TLSA record.

When a TLSA record is found and retrieved, the sending server sets up a TLS connection with the receiving server, which in turn returns the fingerprint of the certificate. The sending server compares the fingerprint with the information found in DNS and when it matches, the connection is established and communication between the two servers continue, and the email is sent.

So, how do you create the TLSA record? There are publicly available webservers that can create TLSA records. Shumon Huque is a software engineer and technologist based in Washington DC who has a site that can generate TLSA records. As a bonus his site can also check TLSA records.

To generate a new TLSA record, navigate to his website on https://www.huque.com/bin/gen_tlsa and fill out the necessary information as shown in the following screenshot:

The easiest way to create a PEM format X.509 certificate is using the MMC certificate snap-in. Select your certificate and export it. Select the ‘No, do not export the private key’ option, select the ‘Base-64 encoded X.509 (CER)’ option and export the certificate. You can use Notepad to open the certificate export, and copy-and-past this into the PEM format X.509 textbox.

Select the port number (25), the protocol (TCP, not SMTP) and enter the domain name. This is the FQDN of the receiving server, so in my example it is smtphost.exchangelabs.nl and click Generate.

It will show the generated TLSA record, including the certificate information as shown in the following screenshot:

For my domain the following TLSA record is generated:

_25._tcp.smtphost.exchangelabs.nl. IN TLSA 3 1 1 a63d74fc7ec1acad702017d13479a1b60b36f234ccb96b90f97c9619ba2c91ab

How is this TLSA record structured?

TCP port 25 and the FQDN of the server make up the first part of this TLSA record.

The first number after the TLSA text is the certificate verification or the certificate usage on the server and this number can have 4 values:

• 0 – CA Constraint. The certificate provided when establishing the TLS connection must be issued by the listed root-CA or one of its intermediate Certificate Authorities.
• 1 – Service Certificate constraint. – The certificate used must match the TLSA record, and it must also pass the certification path validation to a trusted root-CA.
• 2 – Trust Anchor Assertion. The TLSA record must match the certificate of the root CA, or one of the intermediate CAs of the certificate in use by the mailserver.
• 3 – Domain Issued Certificate. The TLSA record matches the used certificate itself. The certificate does not need to be signed by other parties and as such a self-signed certificate can be used (this is interesting on an Exchange server).

In my TLSA record a value of 3 is used for the certificate usage, so the TLSA record must match the certificate on the Exchange 2019 server.

The second number that is used is the selector. The selector can have two values:

• 0 – The entire certificate is used for matching.
• 1 – Only the public key of the certificate is used for matching.

In my TLSA record the value is 1 for the selector, so only the public key is used.

The third number that is used is the matching type. The matching type can have three values:

• 0 – the entire information select is present in the certificate associated data (the last text string in the TLSA record)
• 1 – The SHA-256 hash of the public key of the certificate must match the certificate associated data.
• 2 – The SHA-512 hash of the public key of the certificate must match the certificate associated data.

In my TLSA record a value of 1 is used for matching, to a SHA-256 has of the public key of the Exchange server certificate must match the data in the TLSA record.

The next step is to add the generated TLSA record in DNS and check the TLSA record once added.

You can use the same site (https://www.huque.com/bin/danecheck-smtp) to check the TLSA record. The site queries the MX record for your domain and checks the accompanying TLSA record. The result is shown in the following screenshot:

But there are more sites that can check your TLSA records. Mailhardener (https://www.mailhardener.com/tools/dane-validator) for example can do the same as shown in the following screenshot:

In this blog I am discussing DANE for mail server, but it can also be used for other services. For a website a TLSA record can be:

_443._tcp.www.exchangelabs.nl. IN TLSA 3 1 1 a63d74fc7ec1acad702017d13479a1b60b36f234ccb96b90f97c9619ba2c91ab

Or for IMAP it can be:

_995._tcp.mail.exchangelabs.nl. IN TLSA 3 1 1 a63d74fc7ec1acad702017d13479a1b60b36f234ccb96b90f97c9619ba2c91ab

The ugly part is that Exchange 2019 does not support DANE out of the box. For incoming mail, you can configure the TLSA record based on the certificate that is installed on the Exchange (Edge Transport) server. A DANE capable mail server automatically checks for a TLSA record and when available, use this to setup the TLS connection.

Exchange Online on the other hand only supports DANE for outbound connections at the time of writing. As an Exchange Online customer, there’s nothing you must configure, it’s available for everyone. DANE for inbound mail will become available in the future, but when is unclear. According to Microsoft it is ‘near future’ but unfortunately this is already the case for quite some time.

One final (and very important) closing note: When you have configured a TLSA record for your (inbound) Exchange server you MUST update your TLSA record when renewing the Exchange certificate. You must also do this when the Exchange server is used in a hybrid configuration, since Exchange Online will check for a TLSA record. Mail flow from Exchange Online to Exchange 2019 will halt and the following error message is shown in Exchange Online message trace:

Reason: [{LED=450 4.7.323 tlsa-invalid: The domain failed DANE validation [Message=450 4.7.323 tlsa-invalid: The domain failed DANE validation] [LastAttemptedServerName=smtphost.exchangelabs.nl] [LastAttemptedIP=185.116.41.45:25] [SmtpSecurity=11;-1] [HE1EUR01FT069.eop-EUR01.prod.protection.outlook.com 2023-06-06T10:23:06.0. OutboundProxyTargetIP: 185.116.41.45. OutboundProxyTargetHostName: smtphost.exchangelabs.nl

This is solved when a new TLSA record is generated and published in DNS.