DANE is an extension to certificate validation allowing DNSSEC to protect SSL fingerprints, reducing the overall reliance on the public CA infrastructure. It does this by creating a new DNS record type 'TLSA' for storing the raw value or hash of either the complete certificate or the public key.

RFC 6698 defines four usages of DANE:

  • Usage 0: Used to specify a CA certificate, or public key of a certificate, that MUST be found in the certification chain for the service's certificate. This allows whitelisting what CAs are allowed to issue certificates. This mode is also referred to as PKIX-TA.
  • Usage 1: Specify the service's public key or certificate. The service must present this, and pass the normal CA validation steps to be accepted. This mode is also referred to as PKIX-EE.
  • Usage 2: This specifies a certificate or public key of a CA certificate that must be used as the trust anchor when validating the service certificate. Certificate validation then occurs and is valid as long as this CA is in the path. This allows a domain to provide it's own CA certificate for the service. This mode is also referred to as DANE-TA.
  • Usage 3: Directly specify the public key or certificate of the service. No additional validation needs to occur. This mode is also referred to as DANE-EE.

TLSA records are ignored unless the response has been authenticated with DNSSEC.

I find usage 0 interesting to protect against alternative, compromised, or state controlled CAs issueing certificates for my services and have them validate successfully. Usage 1 offers similar protections but is more specific, when used with the publickey I imagine this has very low overhead and protects against malicious certificates issued even by your own CA. Both of these still rely on the public CA system to validate the chain itself.

Usage 2 seems to have limited value to me. It seems impractical in all of these usages to include the entirety of the CA certificate. It will always require the slower TCP lookups due to size and will happy often even with caching. Fingerprints are very manageable and they may be able to fit in a faster UDP lookup.

To validate the service, the CA certificate will have to be presented to the client as part of the SSL handshake to perform the verification. Presenting certificate chains is already common with many of the public CAs but does incur a measurable performance impact on every new connection. Ultimately I think this is only valuable to simplify the overall management and distribution of an in house certificate authority as wildcards may be able to be used to limit the number of records managed.

The most interesting in my opinion is usage 3. Usage 3 has no reliance on any CA public or private, and if a hash of the public key is used record maintenance is only required when a new TLS service/server is setup, or when a key gets rolled due to lifetime or breach. You can re-sign a new certificate without updating the records as long as the key material doesn't change.

I find a certain amount of irony in the last one as the contents of the certificate are almost exclusively used as a means to distribute the public key. TLS certificate and name validation is done to ensure we were provided with the public key of the service we actually want to talk to.

With DNSSEC we know the answer we got back to our name request is accurate. We validate ownership over the name by being able to produce globally authenticated results. With DANE we have a strong hash (minimum SHA256) to validate the public key is the correct one for the service running on the port we're trying to reach.

To break either DNSSEC or DANE would require breaking at least one of RSA, ECDSA, SHA1, or SHA256. Breaking those enough to provide an alternate key would invalidate the guarantees the normal CA validation provides as well.

TLSA Record Definition

TLSA records are broken up into four sections. The usage (which we've already defined), the selector, the match type, and the certificate data.

The selector defines whether the certificate data will have the entire certificate (value of 0) or just the value of subjectPublicKeyInfo (value 1).

The matching type defines any operations that need to be done on the service provided data before comparing it to what is in the TLSA record. A value of 0 indicates the data is raw, (perform no operations, just compare the bytes directly). SHA256, and SHA512 applied over the raw bytes are values 1 and 2 respectively.

The final field is the actual certification data and it's contents are effectively defined by the selector and the matching type fields.


I'm preferential to choosing a usage type of 3 (check the service certificate directly), checking the public key (selector 1), after applying SHA256 to the data (matching type 1).

Mozilla has a script available for generating raw records available here but generating the above is pretty easily done quickly with the following couple lines of bash:


RAW_RRDATA=$( openssl x509 -in ${CERT_FILE} -pubkey -noout | openssl rsa -pubin \
  -outform der | sha256sum | sed 's/ .*//' )

echo "${DOMAIN}   IN    TLSA        3 1 1   ${RAW_RRDATA}"

If the certificate is an ECDSA one instead of RSA you'll need to replace the RAW_RRDATA line with the following one:

RAW_RRDATA=$( openssl x509 -in ${CERT_FILE} -pubkey -noout | openssl ec \
  -pubin -outform der 2> /dev/null | sha256sum | sed 's/ .*//' )

When both ECDSA and RSA certificates are used by a service, both records can be published under the same key.

If Bind / NSD doesn't yet support TLSA you can instead use the 'raw form' just replace the last line with:

echo "${DOMAIN}   IN    TYPE65468   \\#     $(echo ${ENC_RRDATA} | wc -c)   ${ENC_RRDATA}"

There is also an online service for generating these records individually.

Simple Tricks

If one certificate / key pair is used by a particular machine management of the TLSA records for that machine can be simplified by using CNAME records. The following shows a mail server sharing one TLSA record for all it's mail related services:

_dane.mail.stelfox.lab.     IN    TLSA    3 1 1   a06b4224c68f79aa710b445d94263d0ebbeaf1a9df6dcb62d72feee9bdeaeb00

_25._tcp.mail.stelfox.lab.  IN    CNAME   _dane.mail.stelfox.lab.
_587._tcp.mail.stelfox.lab. IN    CNAME   _dane.mail.stelfox.lab.
_993._tcp.mail.stelfox.lab. IN    CNAME   _dane.mail.stelfox.lab.

I tend to make my services available based on CNAME tied to their function. A bonus of using the public key method of TLSA allows me use the private key for the host, while generating a unique certificate tied to that key for each service.

"Common Mistakes"

Take from this page.

  • Failure to automate DNS zone signing
  • Failure to update TLSA record before transitioning to new certificates (see maintenance section)
  • SMTP/TLSA usage MUST be either DANE-TA or DANE-EE
  • Failure to include issuing CA in server provided certificate chain file.
  • Incorrect TLSA selector (ex: using the certificate selector and generating the RRDATA using the public key)
  • Incorrect TLSA digest (ex: indicating RAW but using SHA256)
  • Selective availability of STARTTLS on SMTP servers (must be always available)
  • DNS filtering firewalls that block TLSA queries
  • Nameservers that don't handle the denial of existance for missing TLSA records correctly.
  • Partial implementation (All MX records should be covered by DNSSEC and have published TLSA records)

Maintenance / Key Rollover

Fixed DANE Parameters

This is very straightforward and will likely be the most common type. If you are aware of this happening, plan ahead by reducing the TTL on the TLSA records affected to a short interval (say five minutes), do this at least one TTL in advance of the rollover preferably two to prevent caching issues.

First generate the new material (cert or key, whatever is applicable to your record). Generate a TLSA record for the new material and add it to the zone (leave the old TLSA record in place).

The new and old record should be published for twice the length of the TTL of the records. Once this time has elapsed, the service can begin using the new key material.

Verify the server certificate is still validating with DANE. Once confirmed it is safe to remove the old record from your zone.

Transitioning from Self-Signed DANE-EE

This is a tad bit more complicated as you need to perform two transitions. Because the certificate is self-signed it can not generally be valid in a certificate authority chain.

First ensure you have the new certificate and key material for the certificate that is valid under the new parameters. Perform a DANE-EE -> DANE-EE transition as described in the Fixed DANE Parameters section.

With the new certificate validating, add a TLSA record with the new parameters, After twice the TTL has expired verify, remove the old entry. Wait for the TTL to expire again and ensure the certificate is still validating with DANE.