#1715 DNS cache: document dns_cache_negative_ttl

[juliebin]

Documents the new dns_cache_negative_ttl option from SparkPost/Momentum#1752:

• a new option page;
• rows in the config index and the options summary (5.3.1 and later).

It also corrects the max_dns_ttl page. That page said the DNS cache was not affected by max_dns_ttl and min_dns_ttl, but the cache has applied both to positive answers since 2007, and max_dns_ttl now also caps the lifetime of negative lookup results.

The 5.3.1 changelog line will follow once the 5.3.1 changelog page is on main.

🤖 Generated with Claude Code

---

Note

Low Risk
Documentation-only changes to Momentum 4 config reference pages; no runtime or application code.

Overview
Documents dns_cache_negative_ttl (Momentum 5.3.1+, global scope, default 300s) for how long failed DNS lookups stay in the response cache (NXDOMAIN, SERVFAIL, timeouts), including interaction with max_dns_ttl and dns_cache_purge_interval, plus an RFC 2308 note on SERVFAIL caching.

Adds a dedicated reference page and list entries in the configuration index and options summary. Updates max_dns_ttl: drops the incorrect claim that min/max DNS TTL do not affect the cache, and notes that max_dns_ttl also caps negative cache lifetime.

^{Reviewed by Cursor Bugbot for commit 8e2cd68. Bugbot is set up for automated code reviews on this repo. Configure here.}

[netlify]

✅ Deploy Preview for support-docs ready!

Name	Link
🔨 Latest commit	8e2cd68
🔍 Latest deploy log	https://app.netlify.com/projects/support-docs/deploys/6ac82a3f28944b0008cae579
😎 Deploy Preview	https://deploy-preview-874--support-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.