Dns Email Auth

DNS Email Auth Check

Grade a domain's SPF, DKIM, and DMARC setup with plain dig. No signup required.

Prerequisites

• dig (bind-utils / dnsutils — preinstalled on macOS and most Linux)

Trigger

• "Check SPF/DKIM/DMARC for example.com"
• "Is my domain protected against spoofing?"
• "Why does my mail land in spam?"
• "Audit email auth on example.com"

Workflow

Set DOMAIN to the bare domain. All queries are read-only DNS lookups.

1. SPF (RFC 7208):

   dig +short TXT "$DOMAIN"
   

   Find the record starting v=spf1 and evaluate:

   • Missing → CRITICAL: anyone can send as this domain.
   • More than one v=spf1 record → CRITICAL: permerror, receivers ignore SPF entirely (RFC 7208 §3.2).
   • Terminal qualifier: -all (hard fail) = best; ~all (softfail) = acceptable, weaker; ?all = WARN (neutral, useless); +all = CRITICAL (explicitly authorizes the entire internet).
   • Lookup limit: count mechanisms that cost a DNS lookup — include:, a, mx, ptr, exists:, and the redirect= modifier — then resolve each include: target with dig +short TXT <target> and count recursively. More than 10 total = permerror (RFC 7208 §4.6.4) and SPF silently stops working.
   • Flag ptr anywhere: deprecated and unreliable per RFC 7208 §5.5.

2. DKIM (RFC 6376). Selectors are not enumerable via DNS, so probe the common ones:

   for s in default google k1 s1 s2 selector1 selector2; do
     echo "== $s"; dig +short TXT "$s._domainkey.$DOMAIN"
   done
   

   • A record containing v=DKIM1 and a non-empty p= = key found. Note which selector.
   • p= empty = revoked key (RFC 6376 §3.6.1) — WARN.
   • Estimate key size from the base64 p= length: ~216 chars ≈ RSA-1024 (WARN — RFC 8301 recommends 2048, ~392 chars), ~392+ ≈ RSA-2048 (OK).
   • Honesty: no hit among these selectors does NOT prove DKIM is absent — the domain may use a custom selector. Report "not found under common selectors" and ask the user for their provider's selector; never report "DKIM missing" as fact.

3. DMARC (RFC 7489):

   dig +short TXT "_dmarc.$DOMAIN"
   

   • Missing v=DMARC1 record → CRITICAL: SPF/DKIM results are never enforced.
   • Policy strength: p=none (monitor only) < p=quarantine < p=reject (full enforcement).
   • rua= absent → WARN: no aggregate reports, drift goes unnoticed.
   • pct= below 100 → WARN: policy applies to a sample only.
   • sp= weaker than p= → WARN: subdomains can be spoofed.

4. Grade:

Report

## Email Auth Report: [domain]

**Grade: [A-F]**

| Layer | Record | Verdict |
|-------|--------|---------|
| SPF   | v=spf1 ... [qualifier], [N]/10 lookups | [OK/WARN/CRITICAL] |
| DKIM  | [selector or "not found under common selectors"], [key size] | [OK/WARN/UNKNOWN] |
| DMARC | p=[policy], rua=[yes/no], pct=[N] | [OK/WARN/CRITICAL] |

### Findings
1. [CRITICAL/WARN] [finding] — [RFC section] — [spoofing/deliverability impact]

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