2. DNS - Domain Name Server

2. DNS - Domain Name Server
	Chapter 6. Network Operating Systems

Hostname Resolution
Hosts are generally known by "ordinary" names such as gauss or strange.
It is the application's duty to find the IP-address corresponding to this name. This process is called host name resolution.
DNS
DNS organizes host names in a hierarchy of domains.
Domain:
Domain is a collection of sites that are related in some sense:
- Because they form a proper network (e.g. all machines on a campus).
- Because they all belong to a certain organization (like the U.S. government).
FQDN:
A fully qualified domain name, or FQDN, uniquely identifies a host world-wide:
Figure 6.1. Section of The Name Space
Top-level domains:
These are a couple of top-level domains you may see frequently:
edu (Mostly US) educational institutions like universities, etc.
com Commercial organizations, companies.
org Non-commercial organizations.
net Gateways and other administrative host on a network.
mil US military institutions.
gov US government institutions.
Outside the US, each country generally uses a top-level domain of its own named after the two-letter country code defined in ISO-3166.
Note:
National domains do not imply that a host below that domain is actually located in that country; it only signals that the host has been registered with that country's NIC.
Advantages:
- It solves the problem of name uniqueness; with DNS, a host name has to be unique only within its domain to give it a name different from all other hosts world-wide.

Name Lookups with DNS
An DNS query:
1. When an application wants to look up information on erdos.maths.groucho.edu, it contacts a local DNS.
Types of DNSs:
Authoritative Servers:
Name servers that hold all information on hosts within a zone are called authoritative for this zone. There are at least two:
1. One primary server, that loads its zone information from data files.
2. Secondary servers, that transfer the zone data from the primary server at regular intervals.
Caching-only Servers:
A name server that is not authoritative for any domain. This type of server conducts DNS queries for applications and caches the information.

The DNS Database

Resource Record:

A single piece of information from the DNS database is called a resource record, or RR.

Each record has:

A class specifying the type of network it applies to:

Database file:

... 
            ; Name servers
                                  IN    NS       neils
                                  IN    NS       gauss.groucho.edu.
            gauss.groucho.edu.    IN    A        149.76.4.23

            ;
            ; Theoretical Physics (subnet 12)
            niels                 IN    A        149.76.12.1
                                  IN    A        149.76.1.12
            nameserver            IN    CNAME    niels
            otto                  IN    A        149.76.12.2
            quark                 IN    A        149.76.12.4
            strange               IN    A        149.76.12.6
            ...
            ; Collider Lab. (subnet 14)
            boson                 IN    A        149.76.14.1
            muon                  IN    A        149.76.14.7
            bogon                 IN    A        149.76.14.12
            ...


1. Introduction		3. NFS - Network File System