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improved tips & tricks section

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DANE-for-SMTP-how-to.md
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@ -186,15 +186,17 @@ Two ways of handling certificate rollover are known to work well, in combination
# Tips, tricks and notices for implementation
This section describes several pionts for attention when implementing DANE for SMTP.
* DANE is meant to be used for the MX domain. So if you are using another domain's mail server, make sure to ask the administrator of that domain to support DANE by setting up a TLSA record.
* Make sure that DNSSEC is implemented properly. A lot of DANE breakage stems from receiving/recipient domains with broken DNSSEC implementation.
* DANE is meant to be used for the MX domain. So if you are using another domain's mail servers, make sure to ask the administrator of that domain (your mail provider) to support DANE by setting up a TLSA record.
* When implementing DANE we advise to first publish DANE records on your MX domains, and then enable DANE verification on your sending mail servers.
* DANE is backwards compatible. So if your mail server supports DANE and a connecting mail server does not support it yet, usually STARTTLS or plain text is used.
* DANE relies on the security that is provided by DNSSEC. Make your primary domain and MX domain support DNSSEC before implementing DANE. It is important that DNSSEC is implemented properly. A lot of DANE breakage stems from receiving/recipient domains with broken DNSSEC implementations.
* Purchasing of expensive certificates for mail servers has no to little added value for the confidentiality since mail servers 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 possible to use self-signed certificates.
* [Section 3.2 of RFC 7672](https://tools.ietf.org/html/rfc7672#section-3.2) states that SMTP clients must **not** perform certificate name checks when using an end-entity certificate (usage type 3), but must perform certificate name checks when using an intermediate or root certificate (usage type 2).
* [Section 3.1 of RFC 7672](https://tools.ietf.org/html/rfc7672#section-3.1) states that the expiration date of the end-entity certificate MUST be ignored.
* 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.
* 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. Alghough it is wise te refresh your key material once in a while, note 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.
* Mail servers by default 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](https://en.wikipedia.org/wiki/PKIoverheid)) this makes it easier to switch supplier / certificate without impacting DANE. (Remember [DigiNotar](https://en.wikipedia.org/wiki/DigiNotar)).
@ -202,6 +204,8 @@ This section describes several pionts for attention when implementing DANE for S
* 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](https://tools.ietf.org/html/rfc2181#section-10.3) a CNAME in MX records is not allowed, while [RFC 7671](https://tools.ietf.org/html/rfc7671#section-7) and [RFC 5321](https://tools.ietf.org/html/rfc5321#section-5.1) imply that the usage of a CNAME in MX records is allowed.
* Using Server Name Indication (SNI) in an e-mail environment (for matching the certificate offered by a recieving e-mail server) is only usefull when DANE and DNSSEC are used. DNSSEC to perform a reliable MX lookup and DANE to verify the authenticity of the certificate. Sending e-mail servers (the TLS client) usually don't use SNI, because some receiving e-mail servers (the TLS server) cannot handle this; in some cases the setting up of a TLS connection fails. For more information see [RFC 7672 section 8.1](https://tools.ietf.org/html/rfc7672#section-8.1) and [this blogpost by Filippo Valsorda](https://blog.filippo.io/the-sad-state-of-smtp-encryption/).
* Make sure you keep an eye on the logs of your sending mail server to see what domains fail DANE verification.
* Some software allows for a test mode. This means that DANE verification is done and logged but there’s no consequence for delivery if DANE verification fails.
# Inbound e-mail traffic (publishing DANE DNS records)
This part of the how-to describes the steps that should be taken with regard to your inbound e-mail traffic, which primairily involves publishing DANE DNS records. This enables other parties to use DANE for validating the certificates offered by your e-mail servers.