Domain Name Service (DNS) Training Division, NIC New Delhi

Domain Name Service (DNS) Training Division, NIC New Delhi

Domain Name Service (DNS) I. History of DNS II. DNS structure and its components III. Functioning of DNS IV. Replicating DNS V. Dynamic update of DNS VI. Maintaining and Monitoring DNS Servers VII.DNS command line utilities

I.History of DNS Resources on the Internet was originally supported by HOSTS file. Names and the corresponding IP addresses were entered by the network administrators into this file. HOSTS.TXT was maintained by the Network Information Centre (NIC) and contained the Host name to address mappings.

Updating the HOSTS file became difficult with the explosive growth of Internet. The file grew bigger and could not be partitioned as it used a flat namespace. The task became management intensive as networks grew. This called for a more sophistciated and well defined naming service structure that was hierarchically structured.

Thus DNS was introduced in the year 1984 for translating the resource names into IP addresses. The host names reside in a database and can be distributed among multiple servers. The hierarchical namespace also provided rules for dividing the namespace into subsets of names. Information about the host names and IP addresses could be partitioned and distributed.

II. DNS Structure and its components a. DNS domain namespace Structured hierarchy of domains to organize names. b. Resource records Maps DNS domain names to a specific type of resource information when the name is registered or resolved in the namespace. c. DNS Zones Zone is a range of responsibility within the domain name space that spans a subtree or a portion of a sub-tree d. DNS Name servers Stores and answer name queries for resource records. e. DNS clients or resolvers Query servers to look up and resolve names to a type of resource record specified in the query.

a. DNS Domain Namespace Domain name space is a hierarchical tree structure containing the names in a DNS database. The database consists of Hosts name and Domain name. A Domain Can be considered as a subtree of the domain name space. Domain names are of a specific pattern that is concatenation of node names eg.training.nic.in.

The Internet can be thought of as a single DNS namespace. The root or the top-most level of the Internet domain namespace is managed by the Internet name registration authority. The root has no name but is represented by a period or. Below the root DNS domain are the top level domains. The Top level domains are child of the root.

Three Types of Top Level Domains Organisational Are represented by a 3-character code that gives a clear indication of the primary activity of the domain.they are mainly for organisations within the United States. Geographical Are represented by a 2-character code that represents the country,region code.these codes are established by the International Standards organisation (ISO). Reverse Domains This is named as in-addr.arpa which is used for IP addressto-name mappings.

Name Domain levels Child of Domain Top-level domain Root in First-Level Domain Second-Level Domain Third-level Domain Top-level domain First-level domain Second-level domain nic.in delhi.nic.in gamma.training.nic.in

.com.arpa.in.gov.edu nic mah asm kar Domain Name System (Inverted Tree Structure)

Top-level domain Domain levels Description Domain name example.com Commercial organisation yahoo.com.edu Education institutions buffalo.edu.gov Government organisations nasa.gov

b.resource Records DNS database consists of Resource Records(RR). Each resource record is a member of a class.(internet Class is the most popular) The Class is further broken down into Types. The type corresponds to the type of data stored in the record. eg: server1.com IN A 124.x.y.z (IN stands for INTERNET and A stands for address information.)

c. DNS Zones DNS database is comprised of multiple zones. Zones allowed the management of the domain space to be delegated.

Zone in com org Zone nic training.nic.in domain nic.in domain Zone training Fig 1:Zones within the Domain Namespace

Zone in com org nic.in domain Zone nic asm Zone training.nic.in domain Zone training Fig 2:

Different types of Zones and Zone files There are two types of Zones A query uses forward zone when resolution starts with a domain name and result in an IP address. A query uses reverse zone when the resolution starts with an IP address and results in a domain name.

Creating Forward Lookup Zone Forward Lookup IP address for trglab.nic.in IP address = 192.168.0.16 DNS Server

Zone Types,Zone Names and Zone File Standard Zones Change Zone Transfer Primary Zone Secondary Zone Active Directory Integrated Zones Zone Transfer Change Change Change

Creating Reverse Lookup Zones Reverse Lookup Name for 192.168.0.16? Name = trglab.nic.in DNS Server

Configuring Standard Zones A Primary Zone DNS Server A Zone Information B Secondary Zone (Master DNS Server = DNS Server A) Secondary Zone (Master DNS Server = DNS Server A) C DNS Server B DNS Server C

d.dns Name Servers DNS zone database is stored in and accessed through a name server. Name servers can store data for one zone or multiple zones. A name server is said to have authority for the domain namespace that the zone encompasses. There must be at least one name server for a zone

III.Functioning of DNS DNS uses a client/server architecture. Domain name clients are called name resolvers. DNS client requests information from a DNS Server s database. The request include type of information and a key(either a domain name or IP address).

Resolution A DNS Server receives two types of requests Recursive Iterative DNS clients always makes recursive requests whereas DNS server uses both types of requests.

The following is a list of responses : Positive authoritative Positive Referral Negative

4 DNS Server Primary zone Root. 6 DNS Server Primary zone in in 3 5 7 8 DNS Server Primary zone nic.in nic DNS Server 1 2 Cache <empty> 9 10 DNS Server Primary zone training.nic.in training 1 11 DNS Client Fig1:Iterative Name Resolution starting from the root name server. training.nic.in domain

4 DNS Server Primary zone Root. 6 DNS Server Primary zone in in 3 5 7 8 DNS Server Primary zone nic.in nic DNS Server 1 2 Cache <empty> 9 10 DNS Server Primary zone training.nic.in training 1 11 DNS Client training.nic.in domain Fig 2

IV.Replication of DNS Replicating a zone file to multiple name servers is called zone transfer. Zone transfer is accomplished by copying the zone file information from master server to secondary server. There are two types of zone file replication Full zone transfer (AXFR), replicates the entire zone file. Incremental zone transfer (IXFR), replicates only the changed records of the zone.

Zone Transfer Process DNS Server (Master) DNS Server Primary Zone Database File trglab Secondary Zone Database File expt1 expt2 Zone 1

Configuring Zone transfers

V.Dynamic Update of DNS Enables DNS client computers to register and dynamically update their resource records with a DNS server whenever changes occur. Allow Windows 2000 clients to update their own forward lookup records, and they allow DHCP and WINS servers to update the forward and reverse lookup records of all clients.

VI.Maintaining and monitoring DNS Using Cache-only servers to reduce network traffic Using the DNS Snap-in Using the nslookup utility.

VII. DNS Command Line Utilities DNSCMD IPCONFIG NET NETSH NSLOOKUP