DNS The Domain Name System 14

DNS : The Domain Name System 14

Introduction - Problem n Computers are used to work with numbers n Humans are used to work with names n ==> IP addresses are NUMBERS : -/ ? 101101101110110110 14

Introduction n Hosts. txt n Distributed Database n Mapping between IP-address and Hostnames n Mail routing n Client-Server n Resolver - Name servers 14

Naming convention n Hierarchical www. groept. be wwwsnmp. cs. utwente. nl n [0. . 9, a. . z, A. . Z, -] Not case sensitive Geography . domain No Relation ? nl utwente IP Subnet cs hostname wwwsnmp 14

DNS Tree Root TLD arpa com in-addr monsanto 193 ea 58 edu gov int . mil net org be nl de groept generic domains country domains 9 1 1. 9. 58. 193. in-addr. arpa. FQDN 14

Basics n Delegation by zone n Primary Name Server n Root Name Server 14

Forward Lookup www. groept. be 1. Internet Connection initiated from webbrowser or other service to request the name. RESOLVER DNS Server 14

Forward Lookup www. groept. be 1. Connection initiated from webbrowser or other service to request the ip address. RESOLVER 2. DNS replies with ip address Internet DNS Server 14

Forward Lookup www. groept. be 1. Connection initiated from webbrowser or other service to request the ip address. RESOLVER 2. DNS replies with ip address 3. Web browser requests page using the ip address Internet DNS Server 14

Forward Lookup www. groept. be 1. Connection initiated from webbrowser or other service to request the ip address. RESOLVER 2. DNS replies with ip address 3. Web browser requests page using the ip address 4. Web server replies with webpage Internet DNS Server 14

Lookup to www. cisco. com Root name server 1. . com. Resolver issues a recursive lookup 2. Local nameserver issues an itterative lookup to root name server for. com Internet. cisco. com. itterative www. cisco. com groept. be. recursive 14

Lookup to www. cisco. com Root name server 1. . com. Resolver issues a recursive lookup 2. Local nameserver issues an itterative lookup to root name server for. com Internet. cisco. com. itterative www. cisco. com groept. be. recursive 14

Lookup to www. cisco. com Root name server 1. . com. Resolver issues a recursive lookup 2. Local nameserver issues an itterative lookup to root name server for. com Internet. cisco. com. itterative www. cisco. com groept. be. recursive 14

Lookup to www. cisco. com Root name server 1. . com. Resolver issues a recursive lookup 2. Local nameserver issues an itterative lookup to root name server for. com Internet. cisco. com. itterative www. cisco. com groept. be. 3. Local nameserver answers with the IP address recursive 14

Lookup to www. cisco. com Root name server 1. . com. Resolver issues a recursive lookup 2. Local nameserver issues an itterative lookup to root name server for. com Internet. cisco. com. itterative www. cisco. com groept. be. 3. 4. recursive Local nameserver answers with the IP address Browser request the page using the IP address 14

Sequential Lookup to www. cisco. com Root name server 1. . com. Internet. cisco. com. www. cisco. com Second machine issues the same recursive lookup groept. be. recursive 14

Sequential Lookup to www. cisco. com Root name server 1. Second machine issues the same recursive lookup 2. . com. Local nameserver provides a cached answer Internet. cisco. com. www. cisco. com groept. be. recursive 14

Sequential Lookup to www. cisco. com Root name server 1. Second machine issues the same recursive lookup 2. . com. Local nameserver provides a cached answer Internet. cisco. com. www. cisco. com groept. be. 3. Browser request the page using the IP address recursive 14

Name server types n Root name server n Primary name server n Secondary name server n Caching-only name server n Slave servers – forwarding server 14

NSLOOKUP – Your tool ! n Forward Lookup n Reverse Lookup nslookup gateway. pharmacia. com nslookup 193. 235. 243. 3 Name: gateway. pharmacia. com Address: 193. 235. 243. 3 14

New developments n DNS dynamic updates using Secure DNS n Prompt notification of zone changes n Incremental zone transfers 14

Resource records n SOA-record n NS-record n A-record n CNAME-record n HINFO-record n PTR-record n MX-record 14

SOA-record # nslookup Default Server: bebrump 001. bebru. eu. pnu. com Address: 10. 240. 72. 65 > set type=SOA > monsanto. com Server: bebrump 001. bebru. eu. pnu. com Address: 10. 240. 72. 65 Non-authoritative answer: monsanto. com origin = srvsn 0. monsanto. com mail addr = hostmaster. srvsn 0. monsanto. com serial = 2001103098 refresh = 3600 (1 H) retry = 1800 (30 M) expire = 432000 (5 D) minimum ttl = 86400 (1 D) Authoritative answers can be found from: srvsn 0. monsanto. com internet address = 164. 144. 230. 67 14

NS-Record > set type=NS > groept. be Server: bebrump 001. bebru. eu. pnu. com Address: 10. 240. 72. 65 Non-authoritative answer: groept. be nameserver = mail. groept. be nameserver = ns. eu. net groept. be nameserver = dns. eunet. be Authoritative answers can be found from: mail. groept. be internet address = 193. 75. 206. 83 ns. eu. net internet address = 192. 16. 202. 11 dns. eunet. be internet address = 193. 74. 208. 137 > 14

A-record www. groept. be IN A 193. 75. 206. 93 > set type=A > www. groept. be Server: bebrump 001. bebru. eu. pnu. com Address: 10. 240. 72. 65 Name: www. groept. be Address: 193. 75. 206. 93 > 14

CNAME-record IN chekov. Belgium. EU. net relay. eunet. be A IN IN A 193. 74. 208. 163 CNAME chekov. Belgium. EU. net > relay. eunet. be Server: bebrump 001. bebru. eu. pnu. com Address: 10. 240. 72. 65 Name: relay. eunet. be Addresses: 193. 74. 208. 163, 193. 74. 22. 138, 193. 74. 208. 147 14

HINFO-record chekov. Belgium. EU. net IN HINFO “SUN E 450” “Solaris” 14

PTR-record 137. 208. 74. 193. in-addr. arpa IN PTR jupiter. Belgium. EU. net > set type=PTR > 193. 74. 208. 137 Server: bebrump 001. bebru. eu. pnu. com Address: 10. 240. 72. 65 137. 208. 74. 193. in-addr. arpa name = jupiter. Belgium. EU. net 208. 74. 193. in-addr. arpa nameserver = dns. eunet. be 208. 74. 193. in-addr. arpa nameserver = ns. EU. net dns. eunet. be internet address = 193. 74. 208. 137 > 14

MX-record pharmacia. com IN MX 100 IN MX 200 gateway. pharmacia. com ns 3. pharmacia. com gateway 3. pharmacia. com > set type=MX > pharmacia. com Server: bebrump 001. bebru. eu. pnu. com Address: 10. 240. 72. 65 Non-authoritative answer: pharmacia. com preference = 100, mail exchanger = gateway. pharmacia. com preference = 200, mail exchanger = gateway 3. pharmacia. com preference = 100, mail exchanger = ns 3. pharmacia. com Authoritative answers can be found from: gateway. pharmacia. com internet address = 193. 235. 243. 3 gateway 3. pharmacia. com internet address = 193. 235. 243. 8 > 14

DNS Message Format 0 15 16 identification 31 flags QR opcode AA TC RD RA zero rcode number of questions number of answer RRs number of authority RRs 12 bytes number of additional RRs questions answers (variable number of resource records) authority (variable number of resource records) additional information (variable number of resource records) 14

Question Portion of DNS Query Message 0 15 16 31 query name query type query class 14