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OpenBSD httpd configuration repetition reduced using m4 macro processor
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Illustration only Photo by Khashayar Kouchpeydeh on Unsplash
An engineer demonstrates using m4 to eliminate repeated code blocks in OpenBSD httpd configurations
For engineers managing OpenBSD web servers, m4 offers a way to reduce configuration boilerplate without relying on language-specific macros. This approach trades some safety for flexibility, as m4 operates purely on text substitution rather than syntax-aware transformations. The technique may be useful where built-in templating is unavailable or insufficient.
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m4 is a standalone text macro processor available on OpenBSD and most Unix-like systems
It enables parameterized text substitution to avoid repeating identical configuration blocks
Unlike AST-based macros, m4 operates on raw text and requires careful handling to avoid expansion errors
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The event describes using m4 to address repetitive configuration in OpenBSD's httpd. The engineer faced multiple identical code blocks for blocking malicious routes, which m4 can consolidate through macro definitions. This demonstrates m4's utility as a general-purpose text preprocessor, particularly in environments lacking built-in configuration templating.
m4 operates differently from language-specific macros like those in Lisp or Rust. It performs pure text substitution without understanding the underlying syntax, which means it can generate invalid output if macros are poorly constructed. The example shows how m4's positional arguments ($1 through $9) allow parameterized reuse of configuration patterns.
The tradeoff becomes clear when comparing m4 to AST-based alternatives. While m4 offers more flexibility across different file types, it lacks the safety guarantees of syntax-aware macro systems. Engineers must manually ensure expansions produce valid configuration, particularly when dealing with nested expressions or complex substitutions.
This approach may be particularly valuable for OpenBSD users, where system utilities often prioritize simplicity over built-in templating features. The technique requires no additional dependencies beyond what's already present in the base system. However, the lack of syntax awareness means engineers must thoroughly test macro expansions to avoid subtle configuration errors.
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