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DNS – Domain Name System

DNS - Domain Name System

As most of you know DNS stands for Domain Name System and translates the names of websites, computers,.. to the correct IP-addres.

The practical implementation contists out of three parts namely the stub resolver, caching/recursing resolver (recursor) and the authoritative nameserver.

The stub resolver builds the DNS-packet that needs to be send to the recursor and the recursor queries the authorative nameservers.

A URL consits out of different labels and these labels are delimited by dots for such as darkdevelopments.org.

The right most label is the top-level domain or root (.org). The hierarchy of domains descends from right to left, each label to the left specifies a subdomain (darkdevelopments).


Data in the DNS is saved as a Resource Record, this consits out of a name, type, TTL and data. The data can be an IP-address or another name.
The following types can be specified within DNS:

- SOA : Start-Of-Authority, holds the settings for the domain like TTL, primairy DNS.
- A : Returns a IPv4 address to map hostnames to an IP address.
- AAAA : Resturn an IPv6 address to map hostnames to an IP address.
- CNAME : Alias of one name to another.
- MX : Maps a domain name to a list of message transfer agents.
- PTR : Used for reverse lookup.
- SPF : SPF allows administrators to specify which hosts are allowed to send mail from a given domain by creating a specific SPF record (or TXT record) in the Domain Name System (DNS).
- SRV : Generalized service location record, used for newer protocols instead of creating protocol-specific records such as MX.

Forward / Reverse Lookup Zones

A zone is a group of addresses where the DNS-server is responsible for wihtin a certain namespace. In most cases this means the domain for example Active Directory.

If different subdomains arent in the same zone then those subzones are deligated. This means that the parent domain has created a pointer to the DNS-server that is responsible for the subzone.

Forward zones convert the hostname to the appropriate IP by using A, CNAME, MX, NS... Reverse lookup zones are used to translate the IP to the appropriate hostname by using the PTR-records.

Advanced DNS

It is possible to setup a secundairy DNS-server that contains a readonly copy of the primary DNS-server. The servers will replicate the DNS-records at certain times (Refresh interval, Retry Interval, Expire Interval). TTL contains the information how long a DNS-records needs to be cached.

Cheap Load Balancing

By using the different A-records with the same name but different IP DNS will use round robin.

DNS Server Roles
Primary - Is the only server where the content of the zone-file can be altered. This means adding, deleting, modifing records within a zone. Only one server can be the primary DNS-server in exception of Active Directory.
Secundairy - All servers that contain a read-only copy of the zone-file of the primary server, can be multiple servers.
Cache-Only - Does not contain zone information but cached queries for clients.
Forwarder - Transfers queries to another DNS-server (recursive).

DNS & Active Directory
When installing an Active Directory environment with DCPROMO, DNS will be automaticly installed. Because otherwise the domainname could not be resolved to the domain controller.

More detailed information about DNS can be found at en.wikipedia.org/wiki/Domain_Name_System

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