SPF flattening

The 10-Lookup Constraint

Every mechanism that triggers a DNS resolution counts towards the 10-lookup limit:

include: (resolves domain and counts nested lookups recursively)
a and mx (triggers DNS queries for records)
ptr (deprecated and expensive)
redirect (resolves target SPF record)

Mechanisms like ip4, ip6, and all do not trigger additional lookups.

Common Failure Scenario

Enterprise domains frequently stack multiple SaaS sending vendors over time:

v=spf1 include:_spf.google.com include:sendgrid.net include:mailgun.org include:salesforce.com ~all

When each vendor includes multiple nested domains, the lookup count easily hits 12–16 lookups, producing silent delivery failures and quarantine drops.

What is SPF Flattening?

SPF flattening replaces dynamic include: mechanisms with direct, resolved ip4: and ip6: CIDR blocks:

Recursively traverse all include: and redirect statements in your current record.
Resolve all underlying IP ranges.
Deduplicate and consolidate overlapping CIDRs.
Publish the flattened IP list directly into DNS TXT records.

# Before (Nested Lookups: 12)
v=spf1 include:_spf.google.com include:sendgrid.net ~all

# After Flattening (Lookups: 0-1)
v=spf1 ip4:35.190.247.0/24 ip4:167.89.0.0/17 ip4:198.2.128.0/18 ~all

Split-Record Strategy for 255-Byte Limits

DNS TXT records have a 255-byte limit per string (and UDP packets historically prefer under 512 bytes). When an organization has dozens of sending IPs, split the record across dedicated sub-records:

v=spf1 include:_spf1.example.com include:_spf2.example.com -all

v=spf1 ip4:35.190.247.0/24 ip4:35.191.0.0/16 ~all

v=spf1 ip4:167.89.0.0/17 ip4:198.2.128.0/18 ~all

Automated Refresh & Drift Prevention

Because SaaS providers can rotate their IP ranges, manual flattening risks stale records. Implement an automated workflow:

Daily Cron/Scheduled Job: Resolve upstream SaaS provider SPF records.
Diff Detection: Compare new CIDR sets against published DNS records.
Automated Pull Request / DNS API Update: Apply DNS TXT updates via Cloudflare or Route 53 API when changes are detected.

# Verify SPF record lookups and syntax locally
dig +short TXT example.com

Verification Checklist

Evaluated total DNS lookup count using a lookup checker (under 10).
Validated RFC 7208 syntax without duplicate mechanisms.
Enforced ~all (softfail) during transition, moving to -all (fail) with DMARC alignment.
Automated daily verification against upstream SaaS vendor IP changes.