DNS

Introduction The Domain Name System (DNS) is basically a large database which resides on various computers and it contains the names and IP addresses of various hosts on the internet and various domains. The Domain Name System is used to provide information to the Domain Name Service to use when queries are made.

DNS History ARPANET utilized a central file HOSTS.TXT Contains names to addresses mapping Maintained by SRI’s NIC (Stanford-Research-Institute: Network-Information-Center) Administrators email changes to NIC NIC updates HOSTS.TXT periodically Administrators FTP (download) HOSTS.TXT

DNS History Cont… As the system grew, HOSTS.TXT had problems with: Scalability (traffic and load) Name collisions Consistency In 1984, Paul Mockapetris released the first version (RFCs 882 and 883, superseded by 1034 and 1035 …)

What is DNS ? The “Domain Name System” What Internet users use to reference anything by name on the Internet The mechanism by which Internet software translates names to attributes such as addresses

A globally distributed, scalable, reliable database What is DNS ? A globally distributed, scalable, reliable database Comprised of three components A “name space” Servers making that name space available Resolvers (clients) which query the servers about the name space

Why we need DNS? DNS as a Database Global Distribution Loose Coherency Scalability Reliability Dynamicity

DNS as a Database Keys to the database are “domain names” www.foo.com, 18.in-addr.arpa, 6.4.e164.arpa Over 100,000,000 domain names stored Each domain name contains one or more attributes Known as “resource records” Each attribute individually retrievable

Global Distribution Data is maintained locally, but retrievable globally No single computer has all DNS data DNS lookups can be performed by any device Remote DNS data is locally cachable to improve performance

Loose Coherency The database is always internally consistent Each version of a subset of the database (a zone) has a serial number The serial number is incremented on each database change Changes to the master copy of the database are replicated according to timing set by the zone administrator Cached data expires according to timeout set by zone administrator

Scalability No limit to the size of the database One server has over 20,000,000 names Not a particularly good idea No limit to the number of queries 24,000 queries per second handled easily Queries distributed among masters, slaves, and caches

Reliability Data is replicated Clients can query Data from master is copied to multiple slaves Clients can query Master server Any of the copies at slave servers Clients will typically query local caches DNS protocols can use either UDP or TCP If UDP, DNS protocol handles retransmission, sequencing, etc.

Dynamicity Database can be updated dynamically Add/delete/modify of any record Modification of the master database triggers replication Only master can be dynamically updated Creates a single point of failure

DNS Components There are 3 components: Name Space: Specifications for a structured name space and data associated with the names Resolvers: Client programs that extract information from Name Servers. Name Servers: Server programs which hold information about the structure and the names.

Name Space Flat Name Space In a flat name space, a name is assigned to an address. A name in this space is a sequence of characters without structure. Hierarchical Name Space In a hierarchical name space, each name is made of several parts. The first part can define the organization, the second part can define the name, the third part can define departments, and so on.

Resolvers A Resolver maps a name to an address and vice versa. Query Response Name Server Resolver

Iterative Resolution 5 7 3 iterative response (referral) a.root server 5 a3.nstld.com 3 a.gtld- server 7 iterative response (referral) “I don't know. Try a.gtld-servers.net.” 6 iterative request “What is the IP address of www.google.com?” udel server 1 ns1.google.com 9 iterative response (referral) “I don't know. Try a.root-servers.net.” 4 iterative response (referral) “I don't know. Try ns1.google.com.” 8 iterative response “The IP address of www.google.com is 216.239.37.99.” 10 iterative response (referral) “I don't know. Try a3.nstld.com.” 2 client

Recursive Resolution root server 3 edu server 2 com server 4 8 7 recursive request “What is the IP address of www.google.com?” udel server 1 google server 5 9 recursive response “The IP address of www.google.com is 216.239.37.99.” 6 10 client

Name Server Architecture: Name Server Process Authoritative Data Zone data file From disk 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 Master server Zone transfer

Name Server (cont’d) Authoritative Data: Name Server Process (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

Name Server (cont’d) Using Other Name Servers: Name Server Process 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 Response Arbitrary name server Resolver Query Query

Name Server (cont’d) Cached Data : Name Server Process 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

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