docs(rate-limit): correct IPv6 subnet width — /56 not /64#170
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… /56 not /64 The docstring and inline comment claimed express-rate-limit's ipKeyGenerator collapses IPv6 source IPs to a /64 prefix, but the helper's default is actually /56 (per its `ipv6Subnet = 56` default parameter). The code calls `ipKeyGenerator(ip)` without overriding, so /56 is what we get — meaning a single IPv6 client could in theory rotate through 256 distinct /64 prefixes inside their /56 allocation and all still bucket to the same rate-limit budget. That's the correct behavior; the docs just lied about the width. Also fixed a smaller inaccuracy: the doc claimed the helper takes `(req, res)` but the real signature is `(ip, ipv6Subnet = 56)`. Added two unit tests pinning the actual /56 behavior — addresses inside one /56 collapse to one key, addresses in different /56s diverge. This makes the boundary observable so a future helper upgrade that quietly changes the default surfaces as a test failure instead of silent rate-limit drift. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Closes #169.
Summary
Docstring + inline comment in
app/middleware/rate-limit-key.jsclaimed express-rate-limit's
ipKeyGeneratorcollapses IPv6 to a/64prefix. The helper's actual signature isipKeyGenerator(ip, ipv6Subnet = 56)and we don't override thedefault, so the real bucket width is
/56.The behavior is correct —
/56is the right granularity. The docsjust lied about the width, and the helper-signature mention was
off too (
(req, res)vs the real(ip)).Test plan
npm run lintcleannpm test— 615 passed → 617 passed (+2 IPv6 tests pinning the/56boundary)/56collapse to one key, addresses in different/56s divergeProudly Made in Nebraska. Go Big Red! 🌽 https://xkcd.com/2347/