SPF Record Syntax: Mechanisms, Qualifiers, Modifiers, and Macros

3 hours ago

SPF records are single TXT records starting with v=spf1; any syntax error invalidates the entire record.
Eight mechanisms exist: all, include, a, mx, ptr, ip4, ip6, exists; each either matches the client IP or not.
Qualifiers (+,-,~,?) set the result when a mechanism matches; default is + (Pass).
Modifiers like redirect= and exp= provide extra information; redirect= hands evaluation to another domain after all mechanisms fail.
Macros (%{...}) expand dynamic values like sender or client IP, enabling per-IP authorization via exists.
Evaluation is first-match-wins; order matters and mechanisms after all are ignored.
DNS lookup limit is 10 (include, a, mx, ptr, exists, redirect count); ip4, ip6, all, exp are exempt.
Common mistakes: using ipv4 instead of ip4, spaces after colons, multiple SPF records, terms after all.
Long records (over 255 characters) must be split into multiple quoted strings in one TXT record, concatenated without spaces.
Practical recommendations: use -all for strict policies, ~all for discovery, and avoid ptr and macro letters that break caching.

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Introduces the Fintech Engineering Handbook, a guide on patterns for building money-handling software.
Targets fintech newcomers, practitioners, and outsiders to explain trustworthiness in money systems.
Core principles: No invented data, no lost data, no trust.

Moving beyond turn-based interaction: session/update notifications can occur anytime, prompt response acknowledges message not end of turn, agent idle state signals readiness.
Messages, tool calls, and terminal output use stable IDs for uniform patching (omit, null, replace, append); tool calls can be streamed without buffering.
Diff overhaul: replaced oldText/newText with structured file changes (add, delete, modify, move, copy, binary, non-text) plus optional git_patch.

Functional programming in hica emphasizes composing functions rather than changing state, with everything as an expression.
Immutability is default using 'let'; mutation uses 'var' locally, promoting pure functions without side effects.
Functions are first-class values, supporting higher-order functions, closures, and combinators like map, filter, and fold.

Mastodon serves different content (HTML, ActivityPub JSON, API JSON) for the same URL based on the Accept header, requiring careful cache key normalization.
The cache key must include a normalized variant of the Accept header to prevent serving the wrong content type to clients.
Key assumptions: AUTHORIZED_FETCH disabled, no signed URLs, federation uses HTTP Signatures (not Authorization header).

No standardized strings – ISO only defines atoms or lists of character codes, leading to incompatibility across implementations.
No functions – code is expressed only in predicates that return true/false; simple operations like adding 1 to a length require multiple steps.
Limited collection types – only linked lists and compound terms; no built-in maps or structs, and custom solutions lack cross-compatibility.

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