mail_dns_check 0.0.2

Check a domain's email DNS records: SPF, DMARC, DKIM, and MX, with stable issue codes for missing, duplicate, and risky settings. Pure Dart.

mail_dns_check

Check whether a domain's email DNS is set up correctly: SPF, DMARC, DKIM, and MX records, with a stable issue code for every problem. Queries go through public DNS-over-HTTPS, so there is no API key and no paid service in between.

final checker = MailDnsChecker();
final report = await checker.check('github.com', dkimSelectors: ['google']);

print(report.dmarc?.policy); // quarantine
print(report.spfLookups);    // DNS lookups SPF evaluation needs
for (final issue in report.issues) {
  print('${issue.severity.name} ${issue.code}: ${issue.message}');
}

checker.close();

Platform support

Pure Dart on top of package:http: Dart VM, Flutter on Android, iOS, Windows, macOS, Linux, and the web. The default resolver, Cloudflare DNS, and Google Public DNS (https://dns.google/resolve) both send Access-Control-Allow-Origin: *, so checks also run in the browser.

Features

• MX: records sorted by priority, missing MX, and null MX (RFC 7505).
• SPF: parsed terms, duplicate records, unknown mechanisms, +all, a missing all, deprecated ptr, includes without SPF, and the RFC 7208 limits of 10 DNS lookups and two void lookups, counted through includes and redirects. A redirect next to all is ignored, as the RFC requires.
• DMARC: parsed tags, duplicate records, a missing or invalid policy, p=none, partial pct, and missing aggregate reports.
• DKIM: keys for the selectors you pass, followed through CNAMEs, and revoked keys.
• DNS failures become lookup_failed issues instead of exceptions or false "missing record" reports.
• Parsers (SpfRecord.parse, DmarcRecord.parse, DkimRecord.parse, MxRecord.tryParse) work on plain strings without any network.

Installation

dependencies:
  mail_dns_check: ^0.0.2

Issue codes

Code	Severity	Meaning
mx_missing	warning	No MX record
mx_null	info	Null MX: the domain accepts no mail
spf_missing	warning	No SPF record
spf_multiple	error	More than one SPF record
spf_invalid	error	Unknown SPF mechanism
spf_plus_all	error	+all allows every sender
spf_no_all	warning	No all mechanism or redirect
spf_ptr	warning	Deprecated ptr mechanism
spf_include_missing	error	An include or redirect target has no SPF record
spf_too_many_lookups	error	More than 10 DNS lookups
spf_too_many_void_lookups	error	More than two include or redirect targets with no records
dmarc_missing	warning	No DMARC record
dmarc_multiple	error	More than one DMARC record
dmarc_invalid	error	Missing or unknown p= policy
dmarc_policy_none	info	p=none only monitors
dmarc_pct_partial	info	pct below 100
dmarc_no_rua	info	No aggregate report address
dkim_missing	warning	No key for a selector
dkim_revoked	warning	Empty p= key
lookup_failed	error	A DNS query failed or timed out

report.isHealthy is true when every issue is info.

Usage

Use another resolver, a shorter timeout, or your own http.Client:

final checker = MailDnsChecker(
  endpoint: Uri.parse('https://dns.google/resolve'),
  timeout: const Duration(seconds: 5),
);

An injected client is not closed by close().

Limitations

• DKIM selectors are not discoverable through DNS. Pass the ones your mail provider uses, such as google for Google Workspace or selector1 and selector2 for Microsoft 365.
• A subdomain without its own DMARC record is reported as dmarc_missing, even when the organizational domain's record covers it.
• SPF macros such as %{i} are not expanded, and the targets of a:, mx:, and exists: are not resolved: they count toward the limit of 10 lookups but not toward the void lookup limit.
• Every queried name is sent to the DNS-over-HTTPS resolver.

Flutter example

FutureBuilder<MailDnsReport>(
  future: checker.check('seungpyo.online'),
  builder: (context, snapshot) {
    final report = snapshot.data;
    if (report == null) return const LinearProgressIndicator();
    return Column(
      children: [
        for (final issue in report.issues)
          ListTile(title: Text(issue.code), subtitle: Text(issue.message)),
      ],
    );
  },
)