Add SPF macro guide
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code {
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font-family: "JetBrains Mono", monospace;
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padding: 0 0.25rem;
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}
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<p>
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<p>
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For advanced usage, domain fields may contain macros. These start with a percentage sign and will expand
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For advanced usage, domain fields may contain macros. These start with a percentage sign and will expand
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to a dynamic value. For example, <b>%{d}</b> expands to the current domain and <b>%{i}</b> to the
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to a dynamic value. For example, <b>%{d}</b> expands to the current domain and <b>%{i}</b> to the
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current IP address. See
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current IP address. See the <a href="/spf-macro-guide">Macro Guide</a> for a list of all macros.
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<a href="https://www.rfc-editor.org/rfc/rfc7208#section-7.2" target="_blank">the SPF specification</a>
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for a list of macros you can use.
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</p>
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</p>
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<center>
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<center>
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152
spf-macro-guide/index.html
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spf-macro-guide/index.html
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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<title>SPF Macro Guide - Explanation of all SPF macros with examples</title>
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<link rel="stylesheet" href="/assets/styles/main.css">
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<script type="module" src="/assets/scripts/ui/validator.js"></script>
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</head>
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<body>
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<h1>SPF Macro Guide</h1>
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<main>
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<h2>Overview of SPF macros</h2>
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<p>
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Using SPF, you can specify which IP addresses are authorized to send emails from a mail server.
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Many of the directives you can use allow you to specify a domain name, but here SPF comes with an
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extra feature: Macros.
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</p>
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<p>
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Macros allow you to insert dynamic values into the values of SPF directives, which can be used for
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e.g. per-user authentication and more. This guide will go through all macros, along with some examples.
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</p>
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<p>
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The mechanisms and modifiers that allow macros are: <b>include</b>, <b>a</b>, <b>mx</b>, <b>ptr</b>,
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<b>exists</b>, <b>redirect</b> and <b>exp</b>.
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</p>
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<h3>List of macros</h3>
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<ul>
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<li>
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<p><code>%{s}</code> - Sender email address</p>
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<p>Expands to the email address which the current email is being sent from, e.g. <code>john@example.com</code></p>
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</li>
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<li>
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<p><code>%{o}</code> - Sender domain</p>
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<p>Expands to only the domain part of the sender email address, e.g. <code>example.com</code></p>
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</li>
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<li>
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<p><code>%{l}</code> - Sender username</p>
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<p>Expands to only the local part of the sender email address, e.g. <code>john</code></p>
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</li>
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<li>
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<p><code>%{d}</code> - Current domain</p>
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<p>
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This starts out identical to the sender domain, but when hitting an <b>include</b> mechanism or
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a <b>redirect</b> modifier, this value will change to the domain specified in that term
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during the processing of it.
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</p>
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</li>
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<li>
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<p><code>%{i}</code> - IP address</p>
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<p>
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Expands to the IP address of the email client that is sending the mail.
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This can both be an IPv4 and IPv6 address.
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</p>
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</li>
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<li>
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<p><code>%{v}</code> - IP version</p>
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</li>
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<li>
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<p>Expands to the string <b>"in-addr"</b> if the sender address is IPv4, or <b>"ip6"</b> if it is IPv6.</p>
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</li>
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<li>
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<p><code>%{p}</code> - Validated domain name</p>
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<p>
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Does a reverse DNS lookup of the sender IP address, and validates that the resulting domain
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is a subdomain of the current domain. Expands to the validated domain, or the string "unknown".
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<b>NOTE:</b> It is not recommended to use this macro. From the specification:
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</p>
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<blockquote>
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This mechanism is slow, it is not as reliable as other
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mechanisms in cases of DNS errors, and it places a large burden on
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the .arpa name servers. If used, proper PTR records have to be in
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place for the domain's hosts and the "ptr" mechanism SHOULD be one of
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the last mechanisms checked. After many years of SPF deployment
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experience, it has been concluded that it is unnecessary and more
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reliable alternatives should be used instead.
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</blockquote>
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</li>
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<li>
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<p><code>%{h}</code> - HELO/EHLO domain</p>
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<p>Expands to the domain given on the SMTP HELO/EHLO commands.</p>
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</li>
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</ul>
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<h3>Transformers</h3>
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<p>SPF macros can be transformed in a few different ways, by adding another character after the macro letter.</p>
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<ul>
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<li>
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<p>Reverse transformer (r)</p>
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<p>
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Adding "r" after a macro will reverse the domain name or IP address.
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E.g. if <code>%{d}</code> expands to <b>example.com</b>, <code>%{dr}</code> will be
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<b>com.example</b>.
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Likewise, if <code>%{i}</code> expands to <b>192.0.2.1</b>, <code>%{ir}</code> will become
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<b>1.2.0.192</b>.
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</p>
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</li>
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<li>
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<p>Digit transformer (1-9)</p>
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<p>
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Adding a number after a macro, will take that amount of right-hand parts of the domain name /
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IP address. This may be combined with reversing. E.g. for the domain <b>mail.example.com</b>,
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<code>%{d2}</code> will expand to <b>example.com</b>.
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</p>
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</li>
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</ul>
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<h3>Explanations</h3>
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<p>
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SPF allows you to set custom error messages in case of failed validations using the <b>exp</b> modifier.
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The message is retrieved from the TXT records of the domain name defined by the modifier.
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This error message also supports macros, and has extended support for a few more than the ones above:
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</p>
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<ul>
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<li>
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<p><code>{%c}</code> - SMTP client IP (easily readable format)</p>
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</li>
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<li>
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<p><code>{%r}</code> - Domain name of host performing the check</p>
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</li>
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<li>
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<p><code>{%t}</code> - Current timestamp</p>
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</li>
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</ul>
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<center>
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<h3>SPF tools:</h3>
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<a href="/spf-validator">SPF Validator Tool</a> •
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<a href="/spf-creator">SPF Creator Tool</a>
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</center>
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</main>
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</body>
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</html>
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@ -64,7 +64,7 @@
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<center>
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<center>
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<h3>More tools:</h3>
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<h3>More tools:</h3>
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<a href="/dmarc-validator">DMARC Validator Tool</a>
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<a href="/spf-creator">SPF Creator Tool</a>
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</center>
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</center>
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</main>
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</main>
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</body>
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</body>
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