DNS

DNS (Domain Name System) is a hierarchical and distributed name service that translates human-readable domain names into numeric IP addresses. The client sends a query to a DNS resolver that traverses the name hierarchy — from root name servers via top-level domain servers (TLD) to authoritative name servers — to find the corresponding IP address.

DNS operates primarily over UDP port 53 for regular lookups and TCP port 53 for zone transfers. The system uses multiple record types: A records point to IPv4 addresses, AAAA to IPv6, MX specifies mail servers, CNAME creates aliases, TXT stores arbitrary text (used for SPF and DKIM), and NS defines authoritative name servers.

Each record has a TTL value (Time To Live) that determines how long it can be cached. DNSSEC adds cryptographic signing to verify that responses have not been tampered with.

The namespace itself is organised as a tree: the root zone sits at the top, followed by top-level domains (TLDs such as .com, .no, .org), then second-level domains like example.com, and under those subdomains and individual hosts. Responsibility for each zone is delegated through NS records so no single party owns the whole namespace. A lookup typically starts at a recursive resolver that queries a root server, then the TLD server, and finally the authoritative name server for the zone.

Beyond the most common record types, CNAME is used for aliases, PTR for reverse lookups, and SPF, DKIM and DMARC are all stored as TXT records used in mail authentication. Modern transports such as DNS over TLS (DoT) and DNS over HTTPS (DoH) encrypt traffic between client and resolver. Low TTLs allow faster changes at the cost of more query traffic; high TTLs improve cache efficiency but slow propagation when records change. Because web, email, APIs and cloud service discovery all depend on DNS, an outage at the name layer typically feels like "the whole internet is down" even when underlying services are healthy.