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@ -135,14 +135,9 @@ Two ways of handling certificate rollover are known to work well, in combination
- One with the SHA2-256 fingerprint of the mail server's current public key (3 1 1)
- And a second with the SHA2-256 fingerprint of the public key of an issuing CA that directly or indirectly signed the server certificate (2 1 1). This need not be (and typically is not) a root CA.
## Current + next details
With the "current + next" approach, because both fingerprint are **key** fingerprints, the second can be known in advance of obtaining the corresponding certificate. In particular, if keys are rotated often enough (every 30 to 90 days or so), the next key can be pre-generated as soon-as the previous key and certificate are deployed. This allows plenty of time to publish the corresponding **next** "3 1 1" TLSA record to replace the legacy record for the decommissioned key.
With TLSA records that will match the next key long in place, when it is time to deploy that key with a new certificate some 30 to 90 days later, a new certificate is obtained for *that* key and deployed, and the process begins again with another "next" key generated right away.
Deployment of a new certificate and key must be predicated (automated check) on the corresponding TLSA "3 1 1" record being in place for some time, not only on the primary DNS nameserver, but also on all secondary nameservers. Explicit queries against all the servers are to check for this are highly recommended.
## Points of attention when rolling over
## Points of attention when rolling over using "current + next"
* With the "current + next" approach, the second key pair (of which the public key can be used for the **next** TLSA "3 1 1" fingerprint) can be known in advance of obtaining the corresponding certificate for *that* key. In particular, if keys are rotated often enough (every 30 to 90 days or so), the next key can be pre-generated as soon as the previous key and certificate are deployed. This allows plenty of time to publish the corresponding **next** "3 1 1" TLSA record to replace the legacy record for the decommissioned key. The process begins again with another "next" key generated right away.
* Deployment of a new certificate and key must be predicated (automated check) on the corresponding TLSA "3 1 1" record being in place for some time, not only on the primary DNS nameserver, but also on all secondary nameservers. Explicit queries against all the servers are to check for this are highly recommended.
* Some servers have keys and certificates for multiple public key algorithms (e.g. both RSA and ECDSA). In that case, not all clients will negotiate the same algorithm and see the same key. This means that a single "3 1 1" record cannot match the server's currently deployed certificate chains. Consequently, for such servers the "3 1 1" current + "3 1 1" next TLSA records need to be provisioned separately for each algorithm. Failure to do that can result in hard to debug connectivity problems with some sending systems and not others.
* Use of the same key (and perhaps wildcard certificate) across all of a domain's SMTP servers (all MX hosts) is **not** recommended. Such keys and certificates tend to be rotated across all the servers at the same time, and any deployment mistakes then lead to an outage of inbound email. Large sites with proper monitoring and carefully designed and automated rollover processes can make wildcard certificates work, but if in doubt, don't overestimate your team's ability to execute this flawlessly.
* When monitoring your systems, test every IPv4 and IPv6 address of each MX host, not all clients will be able connect to every address, and none should encounter incorrect TLSA records, neglected certificates, or even non-working STARTTLS. Also test each public key algorithm, or stick to just one. All DANE-capable SMTP servers support both RSA and ECDSA P-256, so you can pick just RSA (2048-bit key) or ECDSA (P-256).