Domain Name System (DNS)RFC 1034RFC 1035http://www.ietf.orgDNS: Domain Name SystemDNS name serversDNS: Root name serversThe DNS Name SpaceName ServersSimple DNS exampleDNS exampleDNS: Iterated queriesDNS: caching and updating recordsDomains, Zones, Authority, DelegationDeployment ExampleDNS Clients (resolver configuration)DNS ServersDNS recordsResource RecordsResource Records (2)DNS protocol, messagesDNS protocol, messagesnslookupDomain Name System (DNS)RFC 1034RFC 1035http://www.ietf.orgTCP/IP Protocol SuiteLink LayerEthernet/FastEthernet/802.11/PPPIPICMP IGMPNetwork LayerTransport LayerApplication LayerTCPUDPHTTP SMTP POPDNS SNMPARP ARPDHCPDNS: Domain Name SystemPeople: many identifiers: SSN, name, Passport #Internet hosts, routers: IP address (32 bit) - used for addressing datagrams “name”, e.g., gaia.cs.umass.edu - used by humansQ:map between IP addresses and name ?Domain Name System: distributed databaseimplemented in hierarchy of many name servers application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) note: core Internet function implemented as application-layer protocol complexity at network’s “edge”DNS name servers• no server has all name-to-IP address mappings• local name servers:• each ISP, company has local (default) name server• host DNS query first goes to local name server• authoritative name server:• for a host: stores that host’s IP address, name• can perform name/address translation for that host’s name Why not centralize DNS?• single point of failure• traffic volume• distant centralized database• maintenance• doesn’t scale!DNS: Root name servers• contacted by local name server that can not resolve name• root name server: contacts authoritative name server if name mapping not known gets mapping returns mapping to local name server• ~ 13 root name servers worldwideFurther information about the root name servers can be found at:http://netmon.grnet.gr/stathost/rootns/The DNS Name SpaceA portion of the Internet domain name space showingsome top Level Domains (TLDs).Name ServersPart of the DNS name space showing the division into zones.Simple DNS examplehost sun.lopsys.net wants IP address of mail.yahoo.com1. Contacts its local DNS server, dns.lopsys.net2. dns.lopsys.net contacts root name server, if necessary3. root name server contacts authoritative name server, dns.yahoo.com, if necessaryrequesting hostSun.lopsys.netroot name serverauthorititive name serverdns.yahoo.comlocal name serverdns.lopsys.net123456mail.yahoo.comDNS examplerequesting hostSun.lopsys.netCsy01.cs.wmich.eduroot name serverlocal name serverdns.lopsys.net23456authoritative name serverhal.cs.wmich.eduintermediate name serverdns.wmich.edu7Root name server: may not know authoratiativename server may know intermediate name server: who to contact to find authoritative name server18DNS: Iterated queriesrequesting hostSun.lopsys.netCsy01.cs.wmich.eduroot name serverlocal name serverdns.lopsys.net23456authoritative name serverdns.cs.wmich.eduintermediate name serverdns.wmich.edurecursive query: puts burden of name resolution on contacted name server heavy load?iterated query: contacted server replies with name of server to contact “I don’t know this name, but ask this server”iterated query718DNS: caching and updating records once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time (TTL usually 24 hours) update/notify mechanisms under design by IETF RFC 2136 http://www.ietf.org/html.charters/dnsind-charter.htmlDomains, Zones, Authority, Delegationwmichhreecsedurootwmich.edu domainwmich.edu zonecs.wmich.edu zone•Domain: is a node in the DNS tree, which includes all the nodes (domains) underneath it.•Zone: is a portion of the DNS tree that a particular DNS server is authoritative for.•A DNS Server may delegateauthority of its subdomainsto other organizations or departments.Deployment ExampleMail ServerDNS CacheDNS queries from mail server do not travel over any networkPrimaryDNS(External)SecondaryDNSDHCP 1DHCP 2PrimaryDNS(Internal)SecondaryDNSDHCP ProxyDMZInsideInternetISPDNS(as secondary)OutsideHOST(S)firewallDNS Clients (resolver configuration) A DNS client is called a resolver. A call to getByName()ishandled by a resolver(typically part of the client).UNIX: /etc/resolv.confnameserver 141.218.143.12nameserver 141.218.40.10nameserver 141.218.1.100domain cs.wmich.eduDNS Servers The name of the DNS server in UNIX is named The configuration file for named can be found usually in /etc/named.conf The zone files are usually kept in /var/named with all the the zone resource records (e.g., A, PTR, MX, NS, CNAME). BIND (Berkeley Internet Name Domain) is an common implementation of DNS server, source code and binaries are freely available http://www.isc.orgDNS recordsDNS: distributed db storing resource records (RR)RR format: (name, value, type, ttl)• Type=NS name is domain (e.g. foo.com) value is IP address of authoritative name server for this domain• Type=A name is hostname value is IP address• Type=CNAME name is an alias name for some “cannonical” (the real) name value is cannonicalname• Type=MX value is hostname of mailserver associated with nameResource RecordsThe principal DNS resource records types.Resource Records (2)A portion of a possible DNS database for cs.vu.nl.DNS protocol, messagesDNS protocol : query and reply messages, both with same message formatmsg header• identification: 16 bit # for query, reply to query uses same #• flags: query or reply recursion desired  recursion available reply is authoritativeDNS protocol, messagesName, type fieldsfor a queryRRs in responseto queryrecords forauthoritative serversadditional “helpful”info that may be usednslookup$ nslookup -d csy01.cs.wmich.edu------------Got answer:HEADER:opcode = QUERY, id = 6, rcode = NOERRORheader flags: response, auth. answer, want recursion, recursion avail.questions = 1, answers = 1, authority records = 4, additional = 4QUESTIONS:csy01.cs.wmich.edu, type = A, class = INANSWERS:-> csy01.cs.wmich.eduinternet address = 141.218.143.215ttl = 14400 (4 hours)AUTHORITY RECORDS:-> cs.wmich.edunameserver = gumby.cc.wmich.eduttl = 14400 (4 hours)-> cs.wmich.edunameserver = hal.cs.wmich.eduttl = 14400 (4 hours)ADDITIONAL RECORDS:->