Domain Names System The Domain Name System (DNS) is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities. Most importantly, it translates domain names meaningful to humans into the numerical identifiers associated with networking equipment for the purpose of locating and addressing these devices worldwide.

A domain name is the sequence of labels from a node to the root, separated by dots (“.”s), read left to right The name space has a maximum depth of 127 levels Domain names are limited to 255 characters in length A node’s domain name identifies its position in the name space

Subdomains One domain is a subdomain of another if its apex node is a descendant of the other’s apex node More simply, one domain is a subdomain of another if its domain name ends in the other’s domain name So sales.nominum.com is a subdomain of nominum.com com nominum.com is a subdomain of com

Domain name space The domain name space consists of a tree of domain names. Each node or leaf in the tree has zero or more resource records, which hold information associated with the domain name. The tree sub-divides into zones beginning at the root zone. A DNS zone may consist of only one domain, or may consist of many domains and sub-domains, depending on the administrative authority delegated to the manager. Administrative responsibility over any zone may be divided by creating additional zones. Authority is said to be delegated for a portion of the old space, usually in the form of sub-domains, to another nameserver and administrative entity. The old zone ceases to be authoritative for the new zone

Domain Name Space Domain Top-level Domain Delegation Forward / Reverse com, edu, gov, mil, net, org, int Delegation Forward / Reverse winnie.corp.hp.com 15.16.192.152

Name servers The Domain Name System is maintained by a distributed database system, which uses the client-server model. The nodes of this database are the name servers. Each domain has at least one authoritative DNS server that publishes information about that domain and the name servers of any domains subordinate to it. The top of the hierarchy is served by the root nameservers, the servers to query when looking up (resolving) a TLD

Name Servers Name servers store information about the name space in units called “zones” The name servers that load a complete zone are said to “have authority for” or “be authoritative for” the zone Usually, more than one name server are authoritative for the same zone This ensures redundancy and spreads the load Also, a single name server may be authoritative for many zones

Types of Name Servers Two main types of servers Authoritative – maintains the data Master – where the data is edited Slave – where data is replicated to Caching – stores data obtained from an authoritative server The most common name server implementation (BIND) combines these two into a single process Other types exist… No special hardware necessary

Name Servers and Zones Name Servers Zones nominum.com isc.org 128.8.10.5 serves data for both nominum.com and isc.org zones Name Servers Zones 128.8.10.5 nominum.com 202.12.28.129 serves data for nominum.com zone only 202.12.28.129 isc.org 204.152.187.11 serves data for isc.org zone only 204.152.187.11

Name Resolution Name resolution is the process by which resolvers and name servers cooperate to find data in the name space To find information anywhere in the name space, a name server only needs the names and IP addresses of the name servers for the root zone (the “root name servers”) The root name servers know about the top-level zones and can tell name servers whom to contact for all TLDs

Name Resolution A DNS query has three parameters: A domain name (e.g., www.nominum.com), Remember, every node has a domain name! A class (e.g., IN), and A type (e.g., A) A name server receiving a query from a resolver looks for the answer in its authoritative data and its cache If the answer isn’t in the cache and the server isn’t authoritative for the answer, the answer must be looked up

Name Resolution Resolver Root Name Server Recursive Iterative Caching Time to Live (TTL)

Authoritative name server An authoritative name server is a name server that gives answers that have been configured by an original source, for example, the domain administrator or by dynamic DNS methods, in contrast to answers that were obtained via a regular DNS query to another name server. An authoritative-only name server only returns answers to queries about domain names that have been specifically configured by the administrator. An authoritative name server can either be a master server or a slave server. A master server is a server that stores the original (master) copies of all zone records. A slave server uses an automatic updating mechanism of the DNS protocol in communication with its master to maintain an identical copy of the master records. Every DNS zone must be assigned a set of authoritative name servers that are installed in NS records in the parent zone.

Authoritative name server Give authoritative answers for one or more zones. The master server normally loads the data from a zone file A slave server normally replicates the data from the master via a zone transfer slave master slave

on behalf of resolvers) Authoritative Data Authoritative Data (primary master and slave zones) Agent (looks up queries on behalf of resolvers) Cache Data (responses from other name servers) Name Server Process Response Resolver Query

Delegation Administrators can create subdomains to group hosts According to geography, organizational affiliation or any other criterion An administrator of a domain can delegate responsibility for managing a subdomain to someone else But this isn’t required The parent domain retains links to the delegated subdomain The parent domain “remembers” who it delegated the subdomain to

Delegation Creates Zones Each time an administrator delegates a subdomain, a new unit of administration is created The subdomain and its parent domain can now be administered independently These units are called zones The boundary between zones is a point of delegation in the name space Delegation is good: it is the key to scalability

Zones & Delegations Zones are “administrative spaces” Zone administrators are responsible for portion of a domain’s name space Authority is delegated from a parent and to a child net zone net com ripe www edu isi tislabs • disi ws1 ws2 ftp sun moon google net domain ripe.net zone disi.ripe.net zone

DNS resource records A Resource Record (RR) is the basic data element in the domain name system. Each record has a type (A, MX, etc.), an expiration time limit, a class, and some type-specific data. Resource records of the same type define a resource record set (RRset). The order of resource records in a set, returned by a resolver to an application, is undefined, but often servers implement round-robin ordering to achieve load balancing. DNSSEC, however, works on complete resource record sets in a canonical order

Resource Records Resource records consist of it’s name, it’s TTL, it’s class, it’s type and it’s RDATA TTL is a timing parameter IN class is widest used There are multiple types of RR records Everything behind the type identifier is called rdata www.ripe.net. 3600 IN A 10.10.10.2 ttl Label type rdata class

Types Resource Record Comments SOA NS A PTR CNAME TXT RP

Resource Record: SOA Every Zone contains a Start Of Authority(SOA)resource record at the beginning of Zone The SOA and NS records are used to provide information about the DNS itself. SOA timers are used for maintaining consistency between primary and secondary servers The NS indicates where information about a given zone can be found: The SOA record provides information about the start of authority, i.e. the top of the zone, also called the APEX. ripe.net. 7200 IN NS ns.ripe.net. ripe.net. 7200 IN NS ns.eu.net.

SOA Name Server Host, Admin. E-mail Serial (YYMMDDNN) Refresh Time (24 hrs.) Retry Time (2 hrs.) Expire Time (30 days) Min. TTL Time (4 days) ee IN SOA sun4.ee.ncku.edu.tw. root.sun4.ee.ncku.edu.tw. ( 1998106002 3600 900 3600000 3600 )

Resource Record: SOA Master server Contact address Version number net. 3600 IN SOA A.GTLD-SERVERS.net. nstld.verisign-grs.com. ( 2002021301 ; serial 30M ; refresh 15M ; retry 1W ; expiry 1D ) ; neg. answ. ttl Version number Timing parameter

DNS Protocol The DNS protocols contain various kinds of DNS messages that are progressions according to the data in their message fields. DNS protocols consist of message type which consists of   queries; updates, and responses, DNS (Domain Name System) query message format has fixed length which is 12 bytes. DNS Header has fixed length and all other such as:- Question Entries, Answer Resource Records, authority Resource Records, Additional Resource Records have variable length. There are many other type of DNS (Domain Name System) messages or protocol such as DNS query message header, DNS query question entries, DNS resource records, Name query message, Name query response, Reverse name query message, DNS update message format, DNS (Domain Name System) update message flags, Dynamic update response message.

DNS Protocol Overview Part of the confusion associated with the DNS protocol is that it lacks a special name. Thus DNS can refer either to the entire system, or to the protocol that makes it work. This page documents the protocol, which operates in one of two basic modes - lookups or zone transfers. DNS Lookups Normal resource records lookups are done with UDP. An "intelligent retransmission" is to be used, though one is not specified in the protocol, resulting in a mix of poor strategies with good ones. The protocol itself is stateless; all the information needed is contained in a single message, fully documented in RFC 1035 §4.1, and having the following format: +---------------------+ | Header | | Question | the question for the name server | Answer | resource records answering the question | Authority | resource records pointing toward an authority | Additional | resource records holding additional information

DHCP Servers DHCP stands for Dynamic Host Configuration Protocol. DHCP Servers is used to assign an IP to the computers on a network. DHCP was first introduced in RFC 1531 in October 1993. The connected computers of a network must be configured before the starting of communication with all other computers on the network. DHP servers facilitate the clients with automatic option and save user time because DHCP Servers configured all computers used in a network automatically. DHCP servers also offer a central database for continuing track of computers that have been attached to the network and also avoid more than one computer from by accident being configured with the similar IP address. IPv4 and IPv6 are DHCP versions which are available in market having same performance and name. Dynamic Host Configuration Protocol makes uses of two ports consigned by LANA for BOOTP.  DHCP Servers use 67/udp for the sending of data to server and 68/udp for client. Whole operation performed in four stages such as IP lease, request, lease offer, and IP discovery.