DNS : Domain Name System

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1 1/30 DNS : Domain Name System Surasak Sanguanpong Last updated: May 24, 1999 Outline 2/30 DNS basic name space name resolution process protocol configurations

2 Why need DNS? 3/30 host table /etc/hosts compiled from HOST.TXT (maintain by SRI NIC) simple text file with has IP address to name mapping problems traffic and load name collision consistency A hierarchical name with distributed control is needed DNS basic 4/30 DNS is a distributed database TCP/IP applications use DNS to map hostname to IP address map IP address to hostname provide routing information mail > mail...ac.th handle aliases is actually is1...ac.th

3 Naming Scheme 5/30 cc...ac.th building tree from top to bottom th more specific ac name space is a tree of domain names are case-insensitive cc cc...ac.th Domain Name Space 6/30 root arpa com edu gov int mil net org au th in-addr usu ac cc cc.usu.edu tu cc cc...ac.th arpa domains generic domains country domains

4 DNS Management 7/30 managed by NIC root com th managed by Internic Thailand NIC manages root and top level domain name local admins manage 3rd level or more ac tu or manage by tu managed by Domain Name Concept 8/30 label domain name domain name every node has a label (except root) the list of labels, starting at that node, working up to the root, using a. to separate e.g..ac.th,..ac.th th ac absolute domain name absolute domain name domain name that ends with a period e.g. cc...ac.th. relative domain name relative domain name name to be completed e.g. cc cc

5 Domains 9/30 domain subtree of the domain name space th ac.th domain ac.ac.th domain tu cc cc...ac.th node Domains and Zones Zone is a subtree for which naming authority has been delegated.ac.th domain.ac.th zone ee.ac.th zone.ac.th domain ee 10/30 sci kps rdi lib sci kps rdi lib case 1 : single DNS administration case 2 : and have authority for their zones

6 Name Servers 11/30 Name server : Server that store information about the zone ns..ac.th ee ns...ac.th responsibility for..ac.th zone responsibility for.ac.th zone sci kps rdi lib ns...ac.th responsibility for..ac.th zone Type of Name Servers Primary Name server gets the data for zones from files on the host it runs on Secondary Name server gets its zone data from the primary for redundancy and workload distribution 12/30 nontri..ac.th: secondary ns..ac.th : primary ee ns...ac.th : primary ns2...ac.th : secondary sci kps rdi lib ns...ac.th: primary cc...ac.th: secondary

7 Zone Transfer 13/30 Secondary Name Server pulls zone data over from the primary called zone transfer. primary for.ac.th secondary for..ac.th ns..ac.th nontri..ac.th ns...ac.th primary for..ac.th secondary for.ac.th secondary for..ac.th ns2...ac.th secondary for..ac.th secondary for.ac.th secondary for..ac.th cc...ac.th secondary for..ac.th ns...ac.th primary for..ac.th secondary for.ac.th secondary for..ac.th Root Name Server 14/30 name server must contact other name servers for non local IP it has to know IP address of the top most server called root name server root name server - provide the names and address of the name server authoritative for top level domain name I have to connect root, when I don t have more info ns..ac.th root

8 Root Name Server (cont.) 15/30 13 root servers are currently available in Internet (Last updated Aug 97) A.ROOT-SERVERS.NET B.ROOT-SERVERS.NET C.ROOT-SERVERS.NET D.ROOT-SERVERS.NET E.ROOT-SERVERS.NET F.ROOT-SERVERS.NET G.ROOT-SERVERS.NET H.ROOT-SERVERS.NET I.ROOT-SERVERS.NET J.ROOT-SERVERS.NET K.ROOT-SERVERS.NET L.ROOT-SERVERS.NET M.ROOT-SERVERS.NET ftp://ftp.rs.internic.net/domain/named.root Name Resolution Process 16/30 name server address of ask th name servers root name server address of ask ac.th name servers th name server th address of ask.ac.th name server ac.th name server ac or address of ask ns..ac.th name server.ac.th name server tu address of address is ns..ac.th name server www address is address of resolver

9 Reverse Resolution root 17/30 arpa in-addr in-addr.arpa domain in reverse direction of IP address au ac th tu cc cc...ac.th in-addr.arpa Caching 18/30 all name servers employ a cache to reduce the DNS traffic standard UNIX keep cache in name server with time-out cache data is non-authoritative

10 DNS message format (I) 19/ identification:16 flags:16 number of questions :16 number of authority RRs :16 1 or more questions 1 or more answers 1 or more authority 1 or more additional information number of answer RRs:16 number of additional RRs:16 fixed header query reply fixed 12 byte header with 4 variable lth fields DNS message format is defined for both queries and answers DNS message format (II) 20/ identification:16 identification:16 flags:16 flags:16 set by the client and return by the server lets the client match responses to requests

11 DNS message format (III) 21/30 identification:16 identification:16 flags:16 flags:16 QR QR opcode opcode AA AA TC TC RD RD RA RA (zero) (zero) rcode rcode QR opcode AA TC RD RA rcode 0 query, 1 response 0 standard query, 1inverse query, 2server status request 0 authoritatived answer,1 non authoritatived answer 1 truncated. using UDP, reply was>512 bytes, return only 512 bytes 1 recursive desired, 0 iterative 1 recursion available (server support recursion) return code : 0no error, 3name error DNS message format (IV) 22/30 questions questions :32 :32 query query name name query query type type query query class class query class normally 1 means Internet Address query type indicates desired response A 1 IP address NS 2 name server CNAME 5 canonical name PTR 12 pointer record query name is the name being loop, sequence of label begins with 1-byte count 3www322ac2th0 HINFO 13 host info MX 15 mail exchange record

12 DNS message format (V) 23/30 reply 1 or more answers 1 or more authority 1 or more additional information these three fields share a common resource record (RR) domain domain name name type type class class time-to-live resource lth resource data data domain name : corresp. response name, (query name format) type : response RR type code (see query type) time-to-live : cache life time of RR (often day) resource lth : specify the size of resource data resource data : the answer, e.g. IP address or other type Operations 24/30 use port 53 typically UDP request and reply if answer is too big, use TCP ip ip hdr hdr UDP UDP hdr hdr DNS DNS hdr hdr query query answer#1 answer#2

13 Resolver file 25/30 resolver must have address for local name server /etc/resolv.conf on UNIX /etc/resolv.conf # domain domain..ac.th # list of name server nameserver nameserver nameserver Setting up DNS 26/30 BIND (Berkeley Internet Name Domain) package /usr/somewhere/in.named - BSD named DNS server /etc/named.boot - named configuration (tell named where to find database files)

14 Sample named.boot 27/30 ; Boot file for server ns...ac.th. directory /usr/local/named cache. root.cache primary localhost. primary/local primary in-addr.arpa primary/local.rev primary..ac.th primary/ secondary.ac.th secondary/ secondary cpc..ac.th secondary/cpc secondary..ac.th secondary/ secondary in-addr.arpa secondary/.rev primary in-addr.arpa primary/zone/zone32 primary in-addr.arpa primary/zone/zone33 : : : New named.conf format 28/30 BIND Version 8 defines a new format of boot file : named.conf options { directory "/usr/local/named"; }; zone "." { type hint; file "root.cache"; }; zone "localhost." { type master; file "primary/local"; }; zone " in-addr.arpa" { type master; file "primary/local.rev"; }; zone "..ac.th" { type master; file "primary/"; }; zone ".ac.th" { type slave; masters { ; }; : :

15 Sample database file 29/30 ; address file for server ns...ac.th. IN SOA ns...ac.th. dnsadmin.ns...ac.th. ( ; Last Updated May 13, ; Refresh every 3 hours 3600 ; Retry every 1 hour ; Expire after 30 days ; Minimum TTL of 1 day ) ; Name Servers IN NS ns...ac.th. IN NS ns...ac.th. IN NS ns..ac.th. (continue on next page) Sample database file (cont.) 30/30 ; Mail Hubs for the Domain IN MX 10 mailhost...ac.th. IN MX 15 cc...ac.th. ; AI Lab mars IN HINFO "MP 1101D/DECstation " IN MX 10 mailhost...ac.th. IN A maspar IN CNAME mars...ac.th. saturn IN HINFO "SPARCstation 2" "SunOS 4.1.3" IN MX 10 mailhost...ac.th. IN A ailab2 IN CNAME saturn...ac.th.

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