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Field notes from the network

DNS quirks, email-auth pitfalls, security-header deep dives, post-mortems and how-tos — written by the people running the probes.

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Operations · 5 min

How to monitor SSL certificate expiry without surprises

Five practical ways to never wake up to an expired cert again — including the host.tools cert-watch API.

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E
Email
From p=none to p=reject — a 30-day DMARC roadmap

Step-by-step rollout that won't blackhole your transactional mail. Real-world DMARC reports, parsed.
D
DNS
The "DNS propagation" myth — it's actually caching

Why "propagation" isn't a thing, what TTL really controls, and how to ship a low-risk record change.
S
Security
How a single cookie leak fingerprinted half the internet

A post-mortem of a real CDN-side cookie issue — and the audit you can run today to make sure you're safe.
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Bulk lookups · monitors · webhooks · 10,000 API calls/day

Upgrade to Pro for $19/mo. Cancel anytime. Works with the same API you already use.
I
IP
ASNs, peering and you — a webmaster's primer

Why your IP belongs to an autonomous system, how peering shapes latency, and what a hosting provider's ASN says about reputation.
H
HTTP
Going from F to A+ on security headers in one afternoon

CSP, HSTS, COOP, COEP, Permissions-Policy. Copy-paste-ready configs for nginx, Apache, Cloudflare Workers and Vercel.

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