tool.medowar.de/dkim-check.php

<?php
declare(strict_types=1);

/** Selectors tried when the user leaves the selector field blank. */
const COMMON_SELECTORS = [
    'default', 'selector1', 'selector2', 'google', 'k1', 'k2', 's1', 's2',
    'dkim', 'mail', 'smtp', 'mx', 'pm', 'zoho', 'mandrill', 'sendgrid',
    'amazonses', 'mailjet', 'mailgun',
];

/**
 * Looks up a DKIM selector TXT record, parses its tags and validates the
 * public key (RSA or Ed25519).
 */
class DkimChecker
{
    public function check(string $domain, string $selector): array
    {
        $fqdn = $selector . '._domainkey.' . $domain;
        $report = [
            'fqdn'     => $fqdn,
            'selector' => $selector,
            'record'   => null,
            'tags'     => [],
            'key'      => null,
            'errors'   => [],
            'warnings' => [],
            'valid'    => false,
        ];

        $record = self::getTxtRecord($fqdn);
        if ($record === null) {
            $report['errors'][] = 'No TXT record found at ' . $fqdn;
            return $report;
        }
        $report['record'] = $record;

        $tags = self::parseTags($record);
        $report['tags'] = $tags;

        $version = $tags['v'] ?? null;
        if ($version !== null && strtoupper($version) !== 'DKIM1') {
            $report['errors'][] = 'Unexpected v= tag: "' . $version . '" (expected "DKIM1").';
        }

        $keyType = strtolower($tags['k'] ?? 'rsa');
        if (!in_array($keyType, ['rsa', 'ed25519'], true)) {
            $report['warnings'][] = 'Unknown key type k=' . $keyType . ' — treating as opaque.';
        }

        if (!isset($tags['p'])) {
            $report['errors'][] = 'Missing p= tag — record is not a valid DKIM key record.';
        } elseif ($tags['p'] === '') {
            $report['errors'][] = 'Empty p= tag — this key has been revoked.';
        } else {
            $report['key'] = self::analyzeKey($tags['p'], $keyType);
            if (!$report['key']['valid']) {
                $report['errors'][] = 'Public key could not be parsed: ' . $report['key']['error'];
            } elseif ($keyType === 'rsa' && $report['key']['bits'] !== null && $report['key']['bits'] < 1024) {
                $report['warnings'][] = 'RSA key is only ' . $report['key']['bits'] . ' bits — below the recommended 1024-bit minimum.';
            } elseif ($keyType === 'rsa' && $report['key']['bits'] !== null && $report['key']['bits'] < 2048) {
                $report['warnings'][] = 'RSA key is ' . $report['key']['bits'] . ' bits — 2048 bits is recommended for new keys.';
            }
        }

        if (($tags['t'] ?? '') !== '' && str_contains($tags['t'], 'y')) {
            $report['warnings'][] = 'Testing mode is on (t=y) — receivers may not enforce this key\'s signatures.';
        }

        if (isset($tags['h'])) {
            $algos = array_map('trim', explode(':', $tags['h']));
            if (!array_intersect($algos, ['sha256'])) {
                $report['warnings'][] = 'h= tag does not list sha256 — allowed hash algorithms: ' . $tags['h'];
            }
        }

        $report['valid'] = empty($report['errors']);
        return $report;
    }

    /** @return array<string,string> */
    private static function parseTags(string $record): array
    {
        $tags = [];
        foreach (explode(';', $record) as $part) {
            $part = trim($part);
            if ($part === '' || !str_contains($part, '=')) {
                continue;
            }
            [$key, $value] = explode('=', $part, 2);
            $tags[strtolower(trim($key))] = trim($value);
        }
        return $tags;
    }

    private static function analyzeKey(string $p, string $keyType): array
    {
        $clean = preg_replace('/\s+/', '', $p) ?? '';
        $der = base64_decode($clean, true);
        if ($der === false) {
            return ['valid' => false, 'error' => 'p= is not valid base64.', 'bits' => null, 'type' => $keyType];
        }

        if ($keyType === 'ed25519') {
            if (strlen($der) !== 32) {
                return [
                    'valid' => false,
                    'error' => sprintf('Ed25519 keys must be 32 raw bytes, got %d.', strlen($der)),
                    'bits' => null,
                    'type' => $keyType,
                ];
            }
            return ['valid' => true, 'error' => null, 'bits' => 256, 'type' => $keyType];
        }

        // RSA (or unknown, treated as RSA-encoded SubjectPublicKeyInfo).
        $pem = "-----BEGIN PUBLIC KEY-----\n" . chunk_split(base64_encode($der), 64, "\n") . "-----END PUBLIC KEY-----\n";
        $pubKey = @openssl_pkey_get_public($pem);
        if ($pubKey === false) {
            return ['valid' => false, 'error' => 'OpenSSL could not parse the public key.', 'bits' => null, 'type' => $keyType];
        }
        $details = openssl_pkey_get_details($pubKey);
        return [
            'valid' => true,
            'error' => null,
            'bits'  => $details['bits'] ?? null,
            'type'  => $keyType,
        ];
    }

    private static function getTxtRecord(string $fqdn): ?string
    {
        foreach ((@dns_get_record($fqdn, DNS_TXT) ?: []) as $r) {
            $txt = $r['txt'] ?? (isset($r['entries']) ? implode('', $r['entries']) : '');
            if ($txt !== '') {
                return $txt;
            }
        }
        return null;
    }
}

$domain   = trim((string) ($_GET['domain'] ?? ''));
$selector = trim((string) ($_GET['selector'] ?? ''));
$report   = null;
$scanResults = [];
$inputError = null;

if ($domain !== '') {
    // Accept a bare hostname; strip scheme/path if a URL was pasted.
    $domain = preg_replace('#^\w+://#', '', $domain);
    $domain = explode('/', $domain)[0];
    $domain = strtolower(trim($domain));

    if (!preg_match('/^(?=.{1,253}$)([a-z0-9](-?[a-z0-9])*\.)+[a-z]{2,}$/', $domain)) {
        $inputError = 'Please enter a valid domain name (e.g. example.com).';
    } elseif ($selector !== '' && !preg_match('/^[a-zA-Z0-9]([a-zA-Z0-9._-]{0,127})?$/', $selector)) {
        $inputError = 'Please enter a valid selector (letters, digits, dot, dash, underscore).';
    } else {
        $checker = new DkimChecker();
        if ($selector !== '') {
            $report = $checker->check($domain, $selector);
        } else {
            foreach (COMMON_SELECTORS as $candidate) {
                $result = $checker->check($domain, $candidate);
                if ($result['record'] !== null) {
                    $scanResults[] = $result;
                }
            }
        }
    }
}

/** Renders a single DKIM report (verdict, checks, tag table). */
function renderReport(array $report): void
{
    $h = 'htmlspecialchars';
    $key = $report['key'];
    ?>
    <?php if ($report['valid']): ?>
        <div class="verdict ok">✅ <?= $h($report['fqdn']) ?> — valid DKIM key record.</div>
    <?php else: ?>
        <div class="verdict fail">❌ <?= $h($report['fqdn']) ?> — invalid or unusable DKIM record.</div>
    <?php endif; ?>

    <?php foreach ($report['errors'] as $e): ?>
        <div class="error">⚠️ <?= $h($e) ?></div>
    <?php endforeach; ?>
    <?php foreach ($report['warnings'] as $w): ?>
        <div class="warning">⚠️ <?= $h($w) ?></div>
    <?php endforeach; ?>

    <?php if ($report['record'] !== null): ?>
        <div class="record"><?= $h($report['record']) ?></div>

        <div class="checks">
            <?php
            $renderCheck = function (bool $pass, string $label, string $detail, bool $criticalIfFail = true) use ($h) {
                $cls = $pass ? 'pass' : ($criticalIfFail ? 'crit' : 'warn');
                $ic  = $pass ? '✔️' : ($criticalIfFail ? '❌' : '⚠️');
                printf(
                    '<div class="check %s"><span class="ic">%s</span><div><strong>%s</strong><br>%s</div></div>',
                    $cls, $ic, $h($label), $h($detail)
                );
            };
            $renderCheck(($report['tags']['p'] ?? '') !== '', 'Public key present', ($report['tags']['p'] ?? '') !== '' ? 'p= tag is populated' : 'Key missing or revoked');
            if ($key) {
                $renderCheck($key['valid'], 'Key parses', $key['valid'] ? strtoupper($key['type']) . ($key['bits'] ? ', ' . $key['bits'] . ' bits' : '') : ($key['error'] ?? 'Parse error'));
            }
            $renderCheck(!str_contains($report['tags']['t'] ?? '', 'y'), 'Not in testing mode', str_contains($report['tags']['t'] ?? '', 'y') ? 't=y is set' : 'No testing flag', false);
            ?>
        </div>

        <table>
            <thead><tr><th>Tag</th><th>Meaning</th><th>Value</th></tr></thead>
            <tbody>
                <?php
                $meanings = [
                    'v' => 'Version', 'k' => 'Key type', 'p' => 'Public key (base64)',
                    'h' => 'Hash algorithms', 't' => 'Flags', 's' => 'Service type',
                    'n' => 'Notes', 'g' => 'Granularity',
                ];
                foreach ($report['tags'] as $tag => $value):
                ?>
                    <tr>
                        <td class="mono"><?= $h($tag) ?></td>
                        <td><?= $h($meanings[$tag] ?? '—') ?></td>
                        <td class="mono" style="word-break: break-all;">
                            <?= $tag === 'p' ? $h(substr($value, 0, 60) . (strlen($value) > 60 ? '…' : '')) : $h($value) ?>
                        </td>
                    </tr>
                <?php endforeach; ?>
            </tbody>
        </table>
    <?php endif; ?>
    <?php
}
?>
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>DKIM Record Checker</title>
    <style>
        body {
            font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
            max-width: 1000px;
            margin: 40px auto;
            padding: 0 20px;
            background: #f5f5f5;
            color: #333;
        }
        h1 { color: #333; }
        .info { background: #e3f2fd; padding: 15px; border-radius: 5px; margin-bottom: 20px; }
        form.lookup { margin-bottom: 20px; display: flex; gap: 10px; flex-wrap: wrap; }
        input[type=text] {
            flex: 1; min-width: 180px; padding: 10px; font-size: 15px;
            border: 1px solid #ccc; border-radius: 5px;
        }
        input[name=selector] { flex: 0 0 200px; min-width: 140px; }
        .btn {
            display: inline-block; padding: 10px 20px; background: #2196F3; color: white;
            text-decoration: none; border-radius: 5px; border: none; cursor: pointer; font-size: 15px;
        }
        .btn:hover { background: #1976D2; }
        .error { background: #ffebee; color: #c62828; padding: 10px; border-radius: 5px; margin: 10px 0; }
        .warning {
            background: #fff8e1; color: #e65100; padding: 10px; border-radius: 5px;
            margin: 10px 0; border-left: 5px solid #ff9800;
        }
        .verdict {
            display: flex; align-items: center; gap: 12px; padding: 16px 20px; border-radius: 6px;
            font-size: 17px; font-weight: 600; margin: 20px 0 10px;
        }
        .verdict.ok   { background: #e8f5e9; color: #2e7d32; border-left: 6px solid #43a047; }
        .verdict.fail { background: #ffebee; color: #c62828; border-left: 6px solid #e53935; }
        .record {
            background: #263238; color: #aed581; padding: 8px 12px; border-radius: 4px;
            font-family: monospace; font-size: 13px; word-break: break-all; margin: 10px 0;
        }
        .checks { display: grid; grid-template-columns: repeat(auto-fill, minmax(200px, 1fr)); gap: 10px; margin: 12px 0; }
        .check {
            display: flex; align-items: center; gap: 8px; padding: 12px 14px; border-radius: 5px;
            background: white; box-shadow: 0 1px 3px rgba(0,0,0,0.1); font-size: 14px;
        }
        .check .ic { font-size: 18px; }
        .check.pass { border-left: 4px solid #43a047; }
        .check.warn { border-left: 4px solid #fb8c00; }
        .check.crit { border-left: 4px solid #e53935; }
        table { width: 100%; border-collapse: collapse; background: white; box-shadow: 0 1px 3px rgba(0,0,0,0.1); margin-top: 10px; }
        th, td { padding: 8px 12px; text-align: left; border-bottom: 1px solid #eee; font-size: 13px; }
        th { background: #fafafa; color: #555; font-weight: 600; }
        td.mono, .mono { font-family: monospace; }
        h2 { color: #333; margin-top: 30px; }
        details.scan { margin: 14px 0; }
        summary { cursor: pointer; font-weight: 600; color: #1976D2; }
        .empty { text-align: center; color: #999; padding: 40px; }
    </style>
</head>
<body>
    <h1>✉️ DKIM Record Checker</h1>

    <div class="info">
        Looks up the <strong>DKIM</strong> TXT record for a domain + selector
        (<code>selector._domainkey.domain</code>), parses its tags and validates the
        public key. Leave the selector blank to scan a list of common selectors.
    </div>

    <form class="lookup" method="GET">
        <input type="text" name="domain" placeholder="example.com" value="<?= htmlspecialchars($domain) ?>" autofocus>
        <input type="text" name="selector" placeholder="selector (optional)" value="<?= htmlspecialchars($selector) ?>">
        <button type="submit" class="btn">🔍 Check DKIM</button>
    </form>

    <?php if ($inputError): ?>
        <div class="error"><?= htmlspecialchars($inputError) ?></div>
    <?php endif; ?>

    <?php if ($report !== null): ?>
        <?php renderReport($report); ?>
    <?php elseif ($domain !== '' && $selector === '' && !$inputError): ?>
        <h2>Common selector scan</h2>
        <?php if (empty($scanResults)): ?>
            <div class="empty">No DKIM record found at any of the <?= count(COMMON_SELECTORS) ?> common selectors tried.<br>
            If you know the selector, enter it above for a direct lookup.</div>
        <?php else: ?>
            <?php foreach ($scanResults as $result): ?>
                <details class="scan" open>
                    <summary><?= htmlspecialchars($result['selector']) ?></summary>
                    <?php renderReport($result); ?>
                </details>
            <?php endforeach; ?>
        <?php endif; ?>
    <?php endif; ?>
</body>
</html>