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DNS

• NXDOMAIN vs SERVFAIL: what each DNS error means and how to fix it The two DNS response codes you will actually meet in production, what causes each, how to tell them apart in dig and a resolver comparison, and what to change. 5 min read · 2026-09-15
• How long does DNS propagation take? TTLs, caches and how to check DNS does not propagate, caches expire. What actually decides how quickly a record change is seen, how to plan a cutover, and how to check individual resolvers. 6 min read · 2026-09-15

Email

• The SPF 10 DNS lookup limit: why it breaks and how to fix it What counts towards the SPF lookup limit, why adding one more SaaS provider silently turns SPF off, how to read an SPF tree and the options for getting back under ten. 7 min read · 2026-09-15
• DMARC p=none vs quarantine vs reject: which policy to use and when to move What each DMARC policy tells receivers to do, why nearly everyone should start at none, how to read the reports and the checklist for moving to quarantine and then reject. 6 min read · 2026-09-15

TLS

• SSL certificate chains explained: leaf, intermediate, root and the missing intermediate problem Why a certificate that works in Chrome fails in curl, what a chain is, how browsers paper over a missing intermediate, and how to check what your server actually sends. 7 min read · 2026-09-15

HTTP

• 301 vs 302 vs 307 vs 308: choosing the right HTTP redirect The four redirect status codes, the difference between permanent and temporary, why 307 and 308 exist, what browsers cache, what search engines do with each, and which to use for HTTP to HTTPS. 5 min read · 2026-09-15
• HTTP security headers: what each one does and a sensible default set HSTS, Content-Security-Policy, X-Frame-Options, X-Content-Type-Options, Referrer-Policy, Permissions-Policy and the cross-origin headers, explained one at a time with a starting configuration. 9 min read · 2026-09-15

JWT

• JWT exp, iat and nbf claims: how token expiry actually works The three time claims in a JSON Web Token, the units they use, clock skew, why a token can be expired and still decode fine, and how to debug the token is expired error. 5 min read · 2026-09-15

HMAC

• How to verify webhook signatures with HMAC (Stripe, GitHub, Slack) Why webhooks are signed, how HMAC-SHA256 signatures are built, the raw-body mistake that causes most mismatches, timestamp replay protection and how to reproduce a signature by hand. 8 min read · 2026-09-15

Encoding

• Base64 vs Base64url: the difference, and why your JWT would not decode Two alphabets, one padding rule and the places each is used. What breaks when you mix them up and how to convert between them. 4 min read · 2026-09-15