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Proof of nonexistence for authorization record

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DMARC-how-to.md
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This how-to is created by the Dutch Internet Standards Platform (the organization behind [internet.nl](https://internet.nl)) and is meant to provide practical information and guidance on implementing DMARC.
# What is DMARC?
DMARC is short for **D**omain based **M**essage **A**uthentication, **R**eporting and **C**onformance and is described in [RFC 7489](https://tools.ietf.org/html/rfc7489). With DMARC the owner of a domain can, by means of a DNS record, publish a policy that states how to handle e-mail (deliver, quarantine, reject) which is not properly authenticated using SPF and/or DKIM.
DMARC is short for **D**omain based **M**essage **A**uthentication, **R**eporting and **C**onformance and is described in [RFC 7489](https://tools.ietf.org/html/rfc7489). With DMARC the owner of a domain can, by means of a DNS record, publish a policy that states how to handle e-mail (deliver, quarantine, reject) which is not properly authenticated using SPF and/or DKIM. Because DMARC depends on the security of DNS, the use of DNSSEC is highly recommended.
At the same time DMARC also provides the means for receiving reports which allows a domain's administrator to detect whether their domainname is used for phishing or spam.
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* There is a workaround: Forward the appointment as an "iCalendar file" or as an attachment.
* When processing incoming mail we advise to favor a DMARC policy over an SPF policy. Do not configure SPF rejection to go into effect early in handling, but take full advantage of the enhancements DMARC is offering. A message might still pass based on DKIM.
* At the same time, be aware that some operaters still allow a hard fail (-all) to go into effect early in handling and skip DMARC operations.
* When using a different domain for the rua and/or ruf address, make sure that the DMARC authorization records (example.nl._report._dmarc.differentdomain.nl) are properly DNSSEC signed. A regularly occuring mistake is the presence of "proof of non-existence" (NSEC3) for the ancestor domain (_report._dmarc.differentdomain.nl). If this happens then resolvers that use qname minimization (like the resolver used by [Internet.nl](https://internet.nl)) think that example.nl._report._dmarc.differentdomain.nl does not exists since _report._dmarc.differentdomain.nl does not exists. Therefore the resolver can't get the DMARC authorization record which makes DMARC fail.
* Check your DNSSEC implementation on [DNSViz](https://dnsviz.net/). Enter "example.nl._report._dmarc.differentdomain.nl".
* You can also manually check for this error. `dig example.nl._report._dmarc.differentdomain.nl +dnssec` results in a NOERROR response, while `dig _report._dmarc.differentdomain.nl +dnssec` results in a NXDOMAIN response.
# Overview of DMARC configuration tags
The DMARC policy is published by means of a DNS TXT record. A DMARC record can contain several configuration tags. The table below will list all configuration tags and explain their purpose.