DNS, which stands for Domain Name System, is a foundational technology that makes the web usable for humans. When you type a domain name into your browser, DNS is the system that translates the domain name into an IP address that computers can use to route your request.
DNS works through a globally distributed database that stores mappings between domain names and IP addresses. This database is hierarchical and structured like an inverted tree. No single entity controls the entire DNS system - it is made up of many DNS servers run by organizations all over the world.
At the very top of the hierarchy is the DNS root zone, which is managed by the Internet Corporation for Assigned Names and Numbers (ICANN). The root zone delegates responsibility for Top Level Domains (TLDs) like .com, .net, and .org to different registry operators. Registrars work with end users to purchase domain names under various TLDs. The registries maintain authoritative name servers for their TLD zones, which in turn delegate responsibility for lower-level domains to DNS servers managed by the domain owner.
When you try to access a domain name, your computer first sends a DNS query to a local recursive DNS resolver, which may be provided by your ISP or local network. The resolver then follows the DNS hierarchy to retrieve the IP address for the domain name:
1. The resolver queries a DNS root nameserver to find the authoritative nameservers for the TLD in the domain name.
2. It then queries the TLD's authoritative nameservers to find the authoritative nameserver for the next part of the domain name.
3. This process continues until the resolver reaches the authoritative nameserver for the full domain name, which returns the IP address.
4. The IP address gets passed back through the chain of DNS servers to the original resolver, which caches it locally before returning it to the requesting computer.
This allows computers all over the world to find the IP address for any domain name by traversing the hierarchically distributed global DNS namespace.
Some key concepts around how DNS works:
- DNS uses both UDP and TCP protocols. UDP is used for standard queries since it's faster, while TCP provides reliability for zone transfers between servers.
- Caching is heavily used to improve DNS performance. Frequently requested domain/IP mappings are cached in local DNS resolvers to avoid unnecessary lookups.
- DNS records come in different types like A records (IP mappings), MX records (mail exchanges), CNAME records (aliases), and more. This provides flexibility in mapping domain names to different services.
- The DNS namespace is hierarchical, with different organizations managing zones and delegating responsibility based on domain levels. ICANN oversees the root zone.
- DNS security extensions like DNSSEC provide authentication by digitally signing records. This protects against attacks that forge or manipulate DNS data.
Some other technical aspects of DNS:
Load balancing and traffic management:
- DNS is often used to distribute load across multiple servers through round-robin IP mapping. This helps handle high traffic to popular domains.
Redundancy and failover:
- Having backup DNS servers provides built-in redundancy. If one server fails, others can take over.
- DNS time-to-live (TTL) values determine how long a result should be cached. Shorter TTLs allow faster failover by forcing clients to re-query DNS frequently.
- DNS allows changing IP addresses behind a domain name without impacting end users. This allows dynamic scaling and reconfiguring of hosting infrastructure.
- DNSSEC provides authenticated denial of existence using signed NSEC records that prove a record doesn't exist.
- DANE uses DNS TLSA records to store SSL certificate fingerprints, avoiding man-in-the-middle attacks.
- DNS over TLS and HTTPS protect against eavesdropping by encrypting DNS queries between resolvers and nameservers.
- Most operating systems include a local DNS resolver library that handles formulating queries and following the DNS hierarchy. Common implementations include BIND, Knot Resolver, dnsmasq, Unbound, etc.
- Web browsers and other apps normally interface with the OS resolver instead of directly handling DNS lookups themselves.
- Full-featured DNS servers like BIND provide naming authority by hosting DNS zones and responding to queries for domains they manage.
- Lightweight resolvers like dnsmasq and Unbound just recursively query other DNS servers on behalf of clients but don't host authoritative zones.
- Dynamic DNS allows clients to automatically update DNS records if their IP address changes frequently. This avoids manual updates.
- DNS zones can be managed programmatically using provisioning tools that integrate with DNS servers. This allows automating DNS record maintenance as part of application deployments.
- DDoS attacks often target DNS infrastructure since taking down DNS servers impacts access to websites for many users.
- DNS cache poisoning attacks try to inject malicious DNS records into resolvers by exploiting vulnerabilities in DNS implementations.
- Man-in-the-middle attacks can intercept DNS queries and send back forged responses. DNSSEC mitigates this by validating responses.
In summary, DNS is a massively distributed global hierarchy of servers that resolves human-readable domain names into computer-routable IP addresses. It allows decentralizing control across organizations while providing redundancy and performance. DNS enables the web as we know it today by seamlessly mapping memorable names to ephemeral IP addresses behind the scenes.