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DNS : The Domain Name System

DNS : The Domain Name System. Introduction - Problem. Computers are used to work with numbers Humans are used to work with names ==> IP addresses are NUMBERS :-/. ?. 1011011011101110110110. Introduction. Hosts.txt Distributed Database Mapping between IP-address and Hostnames

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DNS : The Domain Name System

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  1. DNS : The Domain Name System

  2. Introduction - Problem • Computers are used to work with numbers • Humans are used to work with names • ==> IP addresses are NUMBERS :-/ ? 1011011011101110110110

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

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

  5. arpa com edu gov int mil net org be nl de in-addr monsanto groept 193 ea 58 9 1 DNS Tree Root . TLD generic domains country domains 1.9.58.193.in-addr.arpa. FQDN

  6. Basics • Delegation by zone • Primary Name Server • Root Name Server

  7. Internet Forward Lookup www.groept.be Connection initiated from web-browser or other service to request the name. RESOLVER DNS Server

  8. Internet Forward Lookup www.groept.be Connection initiated from web-browser or other service to request the ip address. RESOLVER DNS replies with ip address DNS Server

  9. Internet Forward Lookup www.groept.be Connection initiated from web-browser or other service to request the ip address. RESOLVER DNS replies with ip address Web browser requests page using the ip address DNS Server

  10. Internet Forward Lookup www.groept.be Connection initiated from web-browser or other service to request the ip address. RESOLVER DNS replies with ip address Web browser requests page using the ip address Web server replies with webpage DNS Server

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

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

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

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

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

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

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

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

  19. Name server types • Root name server • Primary name server • Secondary name server • Caching-only name server • Slave servers – forwarding server

  20. NSLOOKUP – Your tool ! • Forward Lookup • nslookup gateway.pharmacia.com •  Name: gateway.pharmacia.com •  Address: 193.235.243.3 • Reverse Lookup • nslookup 193.235.243.3 •  Name: gateway.pharmacia.com •  Address: 193.235.243.3

  21. New developments • DNS dynamic updates using Secure DNS • Prompt notification of zone changes • Incremental zone transfers

  22. Resource records • SOA-record • NS-record • A-record • CNAME-record • HINFO-record • PTR-record • MX-record

  23. SOA-record # nslookup Default Server: bebrump001.bebru.eu.pnu.com Address: 10.240.72.65 > set type=SOA > monsanto.com Server: bebrump001.bebru.eu.pnu.com Address: 10.240.72.65 Non-authoritative answer: monsanto.com origin = srvsn0.monsanto.com mail addr = hostmaster.srvsn0.monsanto.com serial = 2001103098 refresh = 3600 (1H) retry = 1800 (30M) expire = 432000 (5D) minimum ttl = 86400 (1D) Authoritative answers can be found from: srvsn0.monsanto.com internet address = 164.144.230.67

  24. NS-Record > set type=NS > groept.be Server: bebrump001.bebru.eu.pnu.com Address: 10.240.72.65 Non-authoritative answer: groept.be nameserver = mail.groept.be 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 >

  25. www.groept.be IN A 193.75.206.93 A-record > set type=A > www.groept.be Server: bebrump001.bebru.eu.pnu.com Address: 10.240.72.65 Name: www.groept.be Address: 193.75.206.93 >

  26. <bijnaam> IN A <echte naam> chekov.Belgium.EU.net IN A 193.74.208.163 relay.eunet.be IN CNAME chekov.Belgium.EU.net CNAME-record > relay.eunet.be Server: bebrump001.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

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

  28. 137.208.74.193.in-addr.arpa IN PTR jupiter.Belgium.EU.net PTR-record > set type=PTR > 193.74.208.137 Server: bebrump001.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 >

  29. MX-record pharmacia.com IN MX 100 gateway.pharmacia.com pharmacia.com IN MX 100 ns3.pharmacia.com pharmacia.com IN MX 200 gateway3.pharmacia.com > set type=MX > pharmacia.com Server: bebrump001.bebru.eu.pnu.com Address: 10.240.72.65 Non-authoritative answer: pharmacia.com preference = 100, mail exchanger = gateway.pharmacia.com pharmacia.com preference = 200, mail exchanger = gateway3.pharmacia.com pharmacia.com preference = 100, mail exchanger = ns3.pharmacia.com Authoritative answers can be found from: gateway.pharmacia.com internet address = 193.235.243.3 gateway3.pharmacia.com internet address = 193.235.243.8 >

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

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

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