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Glossary

Technical definitions of DNS, TLS, and networking concepts. Each entry explains what the technology does, how it works, and where things commonly go wrong.

DNS

Maps an alias domain name to a canonical (true) domain name.

Adds cryptographic signatures to DNS records to prevent response forgery.

Specifies which mail servers accept incoming email for a domain.

Holds arbitrary text used for SPF, DKIM, DMARC, and domain verification.

Marks zone authority and sets serials, transfer timers, and negative-caching TTL.

TTL (Time to Live)

How long resolvers may cache a DNS record before fetching a fresh copy.

Reverse DNS (PTR)

Resolves an IP address back to a hostname using PTR records in in-addr.arpa.

DNS response code indicating a queried name does not exist in the namespace.

Restricts which certificate authorities may issue TLS certificates for a domain.

Email

Authorizes which mail servers can send email on behalf of your domain.

Adds a cryptographic signature to outgoing email for sender verification.

Ties SPF and DKIM together with a policy for handling authentication failures.

Enforces TLS on inbound SMTP, closing the STARTTLS downgrade attack window.

Displays a verified brand logo in supporting email clients for authenticated senders.

TLS

Certificate Chain

The ordered list of certificates from server leaf to trusted root CA.

Certificate Transparency

Public append-only logs of all issued TLS certificates for auditability.

SAN (Subject Alternative Name)

The authoritative list of hostnames a cert covers; browsers ignore Common Name.

Wildcard Certificate

Covers any single-label subdomain with *.example.com — not apex or multi-level.

Real-time certificate revocation check; stapling fixes privacy and latency issues.

Named bundle of key exchange, authentication, encryption, and integrity algorithms.

Web Security

Forces browsers to use HTTPS for all future requests to a domain.

Content-Security-Policy

HTTP header that controls which resources a page is allowed to load.

X-Frame-Options

Prevents clickjacking by controlling whether browsers can embed a page in a frame.

Referrer-Policy

Controls how much URL information the browser shares via the Referer header.

Permissions-Policy

Gates powerful browser features like camera, microphone, and geolocation per origin.

X-Content-Type-Options

The nosniff directive stops browsers from MIME-sniffing responses away from their declared type.

RFC 9116 file at /.well-known/security.txt for machine-readable vulnerability disclosure contacts.

Networking

The routing protocol that determines how traffic flows between autonomous systems on the internet.

ASN (Autonomous System Number)

Unique number identifying an autonomous system — a network under a single routing policy.

One IP prefix, many locations — BGP routes each client to the topologically nearest site.

Signed ROAs bind prefixes to authorized origin ASNs, enabling BGP route origin validation.