Network & DNS
Technical guides about network & dns, written for practical follow-up.
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13 articlesTwo IP checks disagree. Which connection did each one measure?
Compare IPv4, dual-stack requests and VPN changes using a small connection record instead of an unexplained screenshot.
A website works but its email fails: follow the DNS evidence
Separate website records, mail routing and resolver caches when a domain behaves differently across services.
Will these IPv4 hosts fit? Check the boundary before the address count
Use subnet boundaries and reserved endpoints to decide whether a proposed network range can hold the intended devices.
One IPv6 address, two spellings: compare the bytes and the prefix
Recognise equivalent IPv6 notation and distinguish address scope from evidence of an actual internet connection.
A DNSSEC change broke resolution. Check the delegation first
Investigate DS, DNSKEY and resolver validation behaviour when a signed domain suddenly becomes unreachable.
Before adding a domain to a rule, inspect the exact hostname
Review Unicode labels, Punycode and URL boundaries so allowlists target the intended hostname.
A certificate renewal meets a CAA policy: prepare the issuer change
Check inherited CAA records, wildcard rules and intended certificate issuers before changing certificate providers.
Lowering a DNS TTL today does not clear yesterday's cache
Plan a cutover using the previous TTL, negative answers and the service overlap needed during propagation.
Follow one delivered email through SPF, DKIM and DMARC
Use message headers and DNS evidence to explain which sending identities passed authentication and aligned with From.
An SPF pass still leaves DMARC failing. Trace the sender identity
Separate SPF authorisation from From alignment, and examine DNS lookup limits when a record grows.
Before p=reject, find the legitimate email your policy would block
Use alignment evidence and aggregate reports to stage a DMARC policy change around real sending services.
A DKIM key exists in DNS. Does the message use that selector?
Connect a delivered signature to its selector, signing domain and published key before planning a rotation.
After a VPN change, compare HTTP and WebRTC as separate routes
Build a repeatable before-and-after connection record that preserves protocols, address families and missing observations.
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13 guides