DNS (Domain Name System)

DNS resolves DNS names to IP addresses and vice versa. A DNS name has two parts: the host name, identifying a single host, and the domain name, identifying a group of hosts in a shared namespace. The two are separated by a dot.

Hierarchy and FQDN

DNS is a hierarchical system, structured from the root downwards:

• Root – The top of the hierarchy (written as .)
• Top-Level Domain (TLD) – e.g. com, net, de, org
• Second-Level Domain – e.g. heise in heise.de
• Subdomain / Host – further levels below the second-level domain, e.g. www

When all parts up to the root are specified, the result is the FQDN (Fully Qualified Domain Name), which must be unique in the network, e.g. www.heise.de.

Zones

Each DNS server is responsible for a delimited part of the namespace, called a zone (e.g. heise.de). The server managing a zone's file holds the authority for that zone.

• Primary zone – Read and write access; the authoritative copy of the zone.
• Secondary zone – A read-only copy of a primary zone (for redundancy or load distribution). It can answer queries but cannot update the zone file.

Zone data is exchanged between servers via zone transfer (two DNS servers without a domain controller) or zone replication (Active Directory-integrated zones on domain controllers).

By direction:

• Forward lookup zone – Resolves domain names to IP addresses.
• Reverse lookup zone – Resolves IP addresses to domain names.

Record Types

Record	Purpose
A	Domain name to IPv4 address
AAAA	Domain name to IPv6 address
CNAME	Alias pointing to another host record
SRV	Resolves a service to an IP address
PTR	Reverse lookup: IP address to domain name

Recursive vs. Iterative Queries

• Recursive query – The client sends this to its name server and expects a final answer (the IP address). If the server holds the zone, it returns an authoritative answer.
• Iterative query – If the server cannot answer itself, it queries other DNS servers along the hierarchy. Each may only point to the next responsible server rather than the final answer, until the authoritative server is reached.
• Caching – Every involved server stores results in its DNS cache. A cached answer is returned as a non-authoritative answer.

Example Resolution (www.example.com)

1. The client sends a recursive query to its configured DNS server.
2. That server is not authoritative and has no cached entry, so it sends an iterative query to a root name server.
3. The root server replies with the address of the com. TLD name server.
4. The DNS server queries the com. name server.
5. The com. server replies with the address of the example.com. name server.
6. The DNS server queries the example.com. name server.
7. That server, authoritative for the zone, replies with the IP address of the FQDN.
8. The DNS server returns the IP address to the client (and caches it).

HOSTS File

For very small networks, a static HOSTS file (C:\Windows\System32\Drivers\etc\HOSTS) can map host names to IP addresses instead of DNS. Because Active Directory requires DNS, this alternative is rarely used today.

Useful Commands (Client)

Command	Purpose
ipconfig /displaydns	Show the local DNS cache
ipconfig /flushdns	Clear the local DNS cache

Naming Rules for Windows Domains

• For internal networks, use a subdomain of an official internet domain (e.g. media.ct.de instead of media.ct.local).
• Keep names short (domains max. 64 characters).