Domain Name System

The Domain Name System ABDUL GAFOOR KV Big leap software solution Pvt Ltd

History of DNS Invented in 1983 and first used in ARPANET Jon Postel, Paul Mockapetris HOST.TXT Tree of domain name have two or more part Eg: www.bigleaponline.com

Host names IP Addresses are great for computers IP address includes information used for routing. IP addresses are tough for humans to remember. IP addresses are impossible to guess. ever guessed at the name of a WWW site?

The Domain Name System The domain name system is usually used to translate a host name into an IP address and vice versa. Domain names comprise a hierarchy so that names are unique, yet easy to remember.

DNS Hierarchy edu com org jp rpi albany

Host name structure Each host name is made up of a sequence of labels separated by periods. Each label can be up to 63 characters The total name can be at most 255 characters. Examples: whitehouse.gov barney.the.purple.dinosaur.com monica.cs.rpi.edu

Domain Name The domain name for a host is the sequence of labels that lead from the host (leaf node in the naming tree) to the top of the worldwide naming tree. A domain is a subtree of the worldwide naming tree.

Top level domains edu, gov, com, net, org, mil , … Countries each have a top level domain (2 letter domain name). New top level domains include: .aero .biz .coop .info .name .pro

DNS Organization Distributed Database The organization that owns a domain name is responsible for running a DNS server that can provide the mapping between hostnames within the domain to IP addresses. So - some machine run by RPI is responsible for everything within the rpi.edu domain.

DNS Distributed Database There is one primary server for a domain, and typically a number of secondary servers containing replicated databases. rpi.edu DNS DB rpi.edu DNS DB rpi.edu DNS DB Authoritative rpi.edu DNS DB Replicas rpi.edu DNS server

DNS Clients A DNS client is called a resolver .

DNS Servers Servers handle requests for their domain directly. Servers handle requests for other domains by contacting remote DNS server(s). Servers cache external mappings.

Server - Server Communication If a server is asked to provide the mapping for a host outside it’s domain (and the mapping is not in the server cache): The server finds a nameserver for the target domain. The server asks the nameserver to provide the host name to IP translation. To find the right nameserver, use DNS!

DNS Data DNS databases contain more than just hostname-to-address records: Name server records NS Hostname aliases CNAME Mail Exchangers MX Host Information HINFO

The Root DNS Server The root server needs to know the address of 1st (and many 2nd) level domain nameservers. rpi edu com org jp albany

Server Operation If a server has no clue about where to find the address for a hostname, ask the root server. The root server will tell you what nameserver to contact. A request may get forwarded a few times.

DNS Message Format HEADER QUERIES Response RESOURCE RECORDS Response AUTHORITY RECORDS Response ADDITIONAL INFORMATION

Recursion A request can indicate that recursion is desired - this tells the server to find out the answer (possibly by contacting other servers). If recursion is not requested - the response may be a list of other name servers to contact.

Question Format Name: domain name (or IP address) Query type (A, NS, MX, …) Query class (1 for IP)

UDP & TCP Both UDP and TCP are used: TCP for transfers of entire database to secondary servers (replication). UDP for lookups If more than 512 bytes in response - requestor resubmits request using TCP.

Domain Name System

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Domain Name System