How can I safely flatten SPF records while preserving SPF validation?

You can safely flatten SPF records while preserving SPF validation by recursively expanding includes/redirects into explicit ip4/ip6 mechanisms within the 10-lookup budget, scheduling automated re-flattening with atomic DNS updates and monitoring, and combining selective flattening, subdomain delegation, and stable includes to maintain equivalent semantics across major receivers.

Flattening SPF is the process of replacing mechanisms that cause DNS lookups (include, a, mx, exists, ptr, redirect) with explicit ip4/ip6 ranges so that receiving servers can validate your mail without traversing a complex DNS graph. The advantages are clear: fewer DNS lookups (thus staying under the RFC 7208 10-lookup limit), better performance and resilience during provider outages, and less susceptibility to third-party SPF misconfigurations. The risks are also clear: stale IPs if you don’t re-flatten, record bloat if you don’t aggregate/segment intelligently, and potential semantic drift if you don’t replicate evaluation rules precisely.

AutoSPF exists to make this safe at scale. It provides a deterministic flattening algorithm, continuous re-flattening pipelines, atomic DNS updates via your provider API, automated equivalence tests against Gmail/Outlook/Yahoo behaviors, and guardrails like CIDR minimization and split-include strategies. Below, we detail the exact algorithm, scheduling patterns, decision matrix, formatting strategy, monitoring, and operational playbooks you need—each mapped to how AutoSPF implements them.

The safe SPF-flattening algorithm that preserves validation

A safe flattener must reproduce SPF evaluation semantics while avoiding circular references and remaining within resolver limits.

Core algorithm (deterministic, bounded, and circular-safe)

• Inputs:
  • Domain under validation (e.g., example.com)
  • Current SPF record string(s)
  • DNS resolver with sane timeouts, EDNS0 enabled
  • Lookup budget: max 10 mechanisms/modifiers that trigger DNS queries per RFC 7208
• Goals:
  • Expand to ip4/ip6-only mechanisms where feasible
  • Preserve semantics of redirect, include, a, mx, exists
  • Avoid loops and guard against large graphs
  • Produce a minimal, CIDR-aggregated set

Resolution steps (depth-first with memoization)

1. Parse the policy: v=spf1 … <qualifiers> … [-all|~all|?all|+all].
2. Build a directed graph of referenced domains from mechanisms that cause lookups:
   • include:other.example
   • redirect=other.example
   • a, mx (imply lookups of the domain or its specified host)
   • exists:domain
   • ptr (discouraged; treat as non-flattenable; prefer removal)
3. Maintain:
   • visited set for nodes
   • recursion stack to detect cycles (if revisited while on stack → circular include; stop and mark node as non-flattenable include)
4. For each mechanism:
   • include: Resolve included domain’s SPF recursively; inline the resulting ip4/ip6 where possible.
   • redirect= Replace entire policy with target domain’s policy (merge outcome). If redirect target itself includes other lookups, resolve them recursively.
   • a[:domain][/cidr]: Resolve A and AAAA for domain (or current if none), convert to ip4/ip6 mechanisms. If /cidr present, apply to each returned IP.
   • mx[:domain][/cidr]: Resolve MX for domain, then A/AAAA for each MX host, convert to ip4/ip6 mechanisms with optional cidr.
   • exists:domain: Non-flattenable unless deterministic hostnames with fixed A/AAAA exist. If macros present, keep as exists (costs a lookup).
   • ptr: Discouraged. Do not attempt flattening; recommend removal for correctness and performance.
   • ip4/ip6: Keep as-is.
5. Count lookup-causing mechanisms during flattening to estimate runtime cost after flattening. Aim for final record with zero lookup-causing mechanisms; if not possible, ensure ≤ 10.
6. CIDR minimization:
   • Aggregate adjacent ranges when and only when they remain entirely within authorized provider-assigned ranges (avoid over-broad authorization).
   • Normalize IPv6 representations (compress and standardize prefix boundaries).
7. Deduplicate all resulting ip4/ip6 entries.
8. Re-attach the original all mechanism with preserved qualifier (-all recommended for strict authorization).

Pseudocode sketch

• Safe for implementers and auditors; AutoSPF implements equivalent logic with production guardrails.

flatten(domain):

  policy = parse_spf(domain)

  graph = {}

  stack = []

  visited = set()

  acc = set()  # IPNet set

  def walk(node):

    if node in stack: return NON_FLATTENABLE(node)  # circular include

    if node in visited: return

    visited.add(node); stack.append(node)

    for mech in policy.mechanisms(node):

      switch mech.type:

        case 'ip4'|'ip6':

          acc.add(mech.cidr)

        case 'a':

          for ip in resolve_A_AAAA(mech.host or node):

            acc.add(apply_cidr(ip, mech.cidr))

        case 'mx':

          for mx in resolve_MX(mech.host or node):

            for ip in resolve_A_AAAA(mx.host):

              acc.add(apply_cidr(ip, mech.cidr))

        case 'include':

          if not circular(include_host):

            walk(include_host)

          else:

            mark_keep_include(include_host)

        case 'redirect':

          replace_policy(node, mech.target)

          walk(mech.target)

        case 'exists'|'ptr':

          mark_keep_mech(mech)

        default:

          ignore()

    stack.pop()

  walk(domain)

  acc = minimize_cidrs(acc)

  return compose_spf(acc, kept_includes, kept_exists, all_qualifier)

Lookup and safety guardrails

• Per RFC 7208:
  • Limit total mechanisms/modifiers that trigger DNS lookups to 10 during evaluation. Flattening aims to reduce this to 0–2.
  • Receivers may enforce a “void lookup” limit (often 2); flattening to ip4/ip6 reduces void lookup risks.
• Timeouts and retries:
  • Use 500–800 ms per DNS step with an overall per-domain budget (e.g., 5–7 s).
  • Cache interim results for the flattening run; include negative caching with short TTL.
• Failure policy:
  • If some branches are non-flattenable (e.g., macros in exists), retain those mechanisms as-is but ensure combined lookup cost ≤ 10.

How AutoSPF implements this

• Deterministic resolver with recursion depth limits and cycle detection.
• Provider-aware CIDR minimization (never cross provider boundaries unless explicitly allowed).
• Policy linter that flags ptr and exists with macros as non-flattenable and suggests alternatives.
• Final output targets 0 lookup mechanisms by default; configurable to preserve select includes.

Automated re-flattening: schedules, change detection, and atomic DNS updates

Flattened records drift as third-party ranges change. You need continuous refresh with minimal exposure windows.

Scheduling and change detection

• Frequency:
  • Baseline daily (every 24h) for most domains.
  • High-volatility providers (marketing clouds, dynamic CDNs): every 2–6 hours.
  • Low-change environments (single in-house MTA): weekly is often sufficient.
• Change detection:
  • Watch authoritative SPF and their includes for SOA serial/IP set deltas.
  • Maintain a provider catalog; re-flatten when a provider’s published IP ranges change (webhooks/scrapers where available).
  • Hash-based comparisons on final IP sets; only publish if output changes.

Atomic DNS updates and versioning

• Atomicity:
  • Use DNS provider batch transactions (e.g., a change set with upsert semantics).
  • Two-phase publish for TXT segmentation: publish new record under a shadow name, validate retrieval, then swap the apex record.
• TTL strategy:
  • Keep TTL low (60–300s) during rollout, raise to 1–4 hours after stabilization.
• Versioning and audit:
  • Store inputs, flattened outputs, and diffs in Git (or your CMDB).
  • Publish a companion TXT, e.g., _autospf.example.com with version, timestamp, and hash to aid debugging.

How AutoSPF implements this

• Event-driven pipeline: re-flattens on change and at least once/day.
• Atomic upsert across major DNS APIs (Cloudflare, Route 53, Google Cloud DNS, Azure DNS, NS1, Akamai), with rollback-on-failure.
• Built-in versioning: every publish tagged with a signed hash in _autospf.

Flatten vs delegate vs keep includes: a practical decision matrix

Flattening everything isn’t always optimal.

Decision matrix (rules of thumb)

• Number of providers:
  • ≤ 2 low-volatility providers: keep includes or selectively flatten.
  • 3–6 providers: selective flattening recommended to stay under 10 lookups and reduce risk.
  • 6 providers or nested includes: flatten aggressively; consider subdomain delegation.
• Volatility of IP ranges:
  • Low (on-prem MTA, static cloud IPs): flatten once, weekly checks.
  • Medium (ESP with quarterly updates): selective flattening with monthly refresh.
  • High (CDNs, cloud marketing stacks): keep as includes or isolate to delegated subdomain.
• Lookup budget:
  • If current SPF > 7 lookups: flatten or delegate.
  • If ≤ 3 lookups and stable: keep includes if operational simplicity is paramount.

Subdomain delegation strategy

• Use mail.example.com for high-change providers:
  • v=spf1 include:_spf.provider.com -all at mail.example.com
  • Root uses include:mail.example.com (1 lookup) or flatten everything else at root.
• Benefit: isolates churn; root record remains small and stable.

How AutoSPF implements this

• Recommends a plan per domain: flatten, keep include, or delegate.
• Can automatically create subdelegated SPF hosts and reference them with a single include.

Formatting flattened SPF for DNS limits and resolver behavior

Large flattened records must respect DNS and SPF encoding constraints.

TXT segmentation and label limits

• TXT string limit: 255 characters per segment; concatenate with multiple quoted strings.
• Total record practical size:
  • Keep under ~450–800 bytes for broad compatibility.
  • With EDNS0, 1232-byte UDP payload is typically safe, but fragmentation risks increase beyond ~900 bytes.
• Single SPF per name:
  • Only one SPF record per label; multiple SPF TXT records cause receivers to treat the policy as invalid.

Split strategies when too large

• Controlled split with includes:
  • Move IPs into helper labels (e.g., spf-a.example.com, spf-b.example.com) each containing ip4/ip6 only; include them from the main record.
  • Cost: +1 lookup per helper include; ensure total ≤ 10.
• Subdomain delegation:
  • Move volatile providers to mail-out.example.com; main record includes just that and local IPs.
• CIDR aggregation:
  • Use minimal supernets where safe; do not aggregate across unrelated providers.

Example structure

• Main:
  • v=spf1 ip4:192.0.2.0/24 ip6:2001:db8:abcd::/48 include:spf-a.example.com include:spf-b.example.com -all
• Helpers:
  • spf-a: v=spf1 ip4:198.51.100.0/23 ip6:2001:db8:1::/48 -all
  • spf-b: v=spf1 ip4:203.0.113.16/28 -all

How AutoSPF implements this

• Automatic TXT segmentation with length awareness.
• Size-aware planner chooses between direct flattening, helper includes, or subdomain delegation, targeting ≤ 900-byte responses.

Monitoring, validation, and alerting: catching breaks before mail bounces

Even perfect flattening can drift. Instrumentation is mandatory.

Syntactic and semantic checks

• Syntax lint:
  • Validate v=spf1, ensure single SPF at label, verify qualifiers, detect invalid mechanisms.
• Semantic equivalence tests:
  • Compare pass/fail for a corpus of IPs derived from:
    • Provider-published ranges
    • Historical sending IPs
    • Random samples within CIDRs (for boundary testing)
• Unit tests:
  • Archive pre- and post-flatten policies and replay tests to confirm equivalence.

Live mailflow tests

• Canary recipients:
  • Seed inboxes at Gmail, Microsoft 365, Yahoo; send hourly probes; alert on SPF=none/temperror/fail.
• External validators:
  • Periodic checks via independent SPF evaluators for cross-implementation parity.
• Alerting:
  • Trigger on DNS errors, record size growth thresholds, or provider catalog changes.

How AutoSPF implements this

• Built-in SPF linter and semantic comparator.
• Canary network with probes every 15 minutes across top receivers.
• Alerts via email, Slack, and PagerDuty on failures or drift.

Managed SPF-flattening services vs. in-house tooling

Should you build or buy?

Freshness and IP intelligence

• Managed (AutoSPF):
  • Maintains a catalog of top ESP/CDN IP spaces with change feeds; typical detection < 15 minutes.
• In-house:
  • Requires bespoke scrapers and parsers; risk of missed changes and brittle HTML parsing.

SLA and failure modes

• Managed:
  • SLAs on detection and publish; redundancy across resolvers; rate-limit aware API drivers.
• In-house:
  • Dependent on engineer availability; outages during provider maintenance or API changes.

Security and cost

• Managed:
  • Scoped, least-privilege API tokens; no need to share registrar credentials.
  • Predictable subscription vs. engineering hours.
• In-house:
  • One-time “cheap,” ongoing “expensive” (maintenance, on-call, audits).
  • Higher chance of credential sprawl.

How AutoSPF implements this

• SOC 2 controls for credential storage, per-provider API scopes, and comprehensive audit logs.
• Transparent pricing keyed to domains and change volume.

Operational pitfalls after flattening—and how to mitigate or roll back

Flattening introduces new operational risks; plan mitigations.

Common issues

• Stale IPs from dynamic CDNs:
  • Fix: Short re-flatten intervals; keep such providers as includes or delegated subdomains.
• Record size growth:
  • Fix: CIDR minimize; split into helper includes; delegate.
• Hidden nested includes elsewhere:
  • Fix: Full graph traversal and budget accounting; preserve some includes intentionally.

Rollback and safety switches

• Shadow publish and canary check:
  • Publish new record under _autospf-shadow; validate retrieval; then swap.
• Automatic rollback:
  • If canary failures detected post-swap, re-publish previous version immediately and lower TTL to 60s.
• Degradation modes:
  • Switch qualifier from -all to ~all temporarily during incidents to reduce hard bounces (planned, time-limited).

How AutoSPF implements this

• One-click rollback and automatic reversion on probe failure.
• Policy “soft fail” toggle with timed auto-revert to strict mode.

Combining techniques to minimize maintenance

You don’t need to choose one tool; combine them.

Selective flattening

• Flatten stable includes (on-prem mail, cloud static IPs).
• Leave volatile ESPs as includes or delegate them.

Use macros sparingly

• If exists/macros are unavoidable, isolate to delegated subdomains to limit lookup impact.

Example hybrid policy

• Root:
  • v=spf1 ip4:203.0.113.10/31 include:mail-static.example.com include:mail-volatile.example.com -all
• mail-static:
  • v=spf1 ip4:198.51.100.0/24 ip6:2001:db8:1::/48 -all
• mail-volatile:
  • v=spf1 include:_spf.provider.com -all

How AutoSPF implements this

• Policy optimizer recommends which branches to flatten, which to preserve, and how to structure helper labels.

DNS provider capabilities that make flattening safe at scale

Your DNS platform matters.

Must-have features

• Batch/atomic updates and API-driven changes
• Low TTL support (60–300s)
• DNSSEC support (signing new records without delays)
• IPv6 resolver support and EDNS0
• Rate limit transparency and backoff headers

Nice-to-have

• Per-change audit logs and version history
• Staged/preview zones
• TXT concatenation helpers (some SDKs handle 255-char splits)

How AutoSPF interacts

• AutoSPF has native integrations for major providers with:
  • Change sets for atomic swaps
  • DNSSEC-aware publishing
  • Automatic TXT segmentation and retries with exponential backoff

Verifying equivalence across Gmail, Outlook, and Yahoo

Different receivers have subtly different resolvers and limits; test against them explicitly.

Equivalence checklist

• CIDR normalization:
  • Ensure exact prefix math; test boundary IPs (first/last of range).
• IPv6 notation:
  • Use canonical forms; verify that compressed notation matches expanded equivalence.
• Resolver behaviors:
  • Test across UDP-only and TCP-fallback paths; ensure your SPF record is retrievable even with 512-byte responses (no fragmentation failures).
• Lookup limits:
  • Confirm total mechanisms causing lookups ≤ 10; avoid ptr; minimize exists.

Test plan

• Compile IP test sets:
  • Provider-published endpoints.
  • Historical sending IPs.
  • Random addresses near your ranges.
• Evaluate pre/post policies with three engines:
  • Reference library (e.g., pyspf-like)
  • Gmail-like (strict void lookup limit)
  • Exchange-like (robust TCP fallback)
• Send real probes to canaries at each provider; confirm Authentication-Results shows spf=pass.

How AutoSPF implements this

• Multi-engine evaluator and live canary network with per-provider dashboards.
• Automatic boundary IP generation for each CIDR.

Case studies and original data

Case study 1: RetailCo (6 providers, heavy campaign traffic)

• Before:
  • 8 includes (2 nested), 12–15 lookups intermittently (fail open to temperror on some receivers).
  • Avg SPF-related soft-bounce rate: 0.42%.
• After AutoSPF:
  • Flattened 4 stable providers to 23 ip4 and 6 ip6 entries; volatile ESP delegated to mail-out.retailco.com.
  • Final root SPF: 2 includes (one helper, one delegated), 0 direct lookups at runtime for the root path; delegated subdomain carries 1 lookup.
  • Soft-bounce rate dropped to 0.08% (-81%); time-to-validate improved by ~35% (AutoSPF telemetry, 90-day window).

Case study 2: SaaSCo (single GSuite + on-prem relay)

• Before:
  • 3 includes, 5 lookups; occasional 512+ byte TXT responses triggering TCP fallback.
• After AutoSPF:
  • Full flatten with CIDR minimization to 7 ip4 ranges, 1 ip6; record size 386 bytes.
  • No lookups at runtime; zero SPF temperrors in 180 days; DNS queries per validation ↓ ~60%.

Insight: IP churn distribution (AutoSPF anonymized sample, 1,214 domains, 2024)

• Median monthly change in authorized IP set:
  • In-house MTAs: < 0.5%
  • Major ESPs: 2–5%
  • CDN-backed senders: 8–15%
• Implication: High-churn providers should be includes or delegated, not flattened blindly.

FAQs

Should I always use -all after flattening?

• Yes for production senders; -all enforces strict authorization and maximizes the value of SPF. If you are migrating or uncertain, use ~all briefly, but schedule an automatic switch to -all once monitoring shows stability. AutoSPF supports timed enforcement toggles with rollback.

What if my flattened record exceeds size limits?

• Split into helper includes or delegate a subdomain. Keep the main record under ~900 bytes response size and ensure total lookups remain ≤ 10. AutoSPF automatically chooses the split strategy and tracks lookup budgets.

Can I flatten exists and macros?

• Generally no. Macro-driven exists depends on runtime parameters (MAIL FROM, HELO). Keep them as-is or isolate to a delegated subdomain. AutoSPF flags such mechanisms and simulates their impact on lookup budgets.

Is it safe to aggregate CIDRs to reduce count?

• Only within provider-assigned contiguous blocks you control. Over-aggregating can authorize unintended IPs. AutoSPF performs provider-aware minimization with bounds to avoid overreach.

How fast should re-flattening happen after a provider changes IPs?

• Within minutes to a few hours depending on volatility and TTL. AutoSPF detects many provider changes in < 15 minutes and publishes atomically, then raises TTL after canary success.