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DNS Introduction www.what-is-my-ip-address.com 2005/03/11 (C) Herbert Haas Except for Great Britain. According to ISO 3166 and Internet tradition, Great Britain's top-level domain name should be gb. Instead, most organizations in Great

  1. DNS Introduction www.what-is-my-ip-address.com 2005/03/11 (C) Herbert Haas

  2. “Except for Great Britain. According to ISO 3166 and Internet tradition, Great Britain's top-level domain name should be gb. Instead, most organizations in Great Britain and Northern Ireland (i.e., the United Kingdom) use the top-level domain name uk. They drive on the wrong side of the road, too.” DNS and BIND book Footnote to the ISO 3166 two-letter country code TLDs

  3. DNS Tree Growth 162,128,493 by 2002/7 2005/03/11 (C) Herbert Haas 3

  4. Top Host Names – Worldwide Top Host Names July 2002 Top Host Names Jan 1992 956841 www 3883 venus 384 venus 204 mac4 172 mac9 336393 mail 3867 dev 356 pluto 201 hobbes 172 mac11 56958 cpe 3795 zeus 323 mars 201 hermes 170 mac8 36107 router 3765 jupiter 288 jupiter 198 thor 169 phoenix 35004 ftp 3720 mars 286 saturn 198 sirius 169 mac12 33720 ns2 3656 l0 285 pc1 196 gw 169 hal 33128 gw 3647 t3 282 zeus 195 calvin 168 snoopy 27548 ns1 3567 www3 262 iris 194 mac5 168 mac13 23019 pc1 3511 260 mercury 191 mac10 167 mac15 21775 pc2 loopback0 259 mac1 190 fred 167 mac14 16432 smtp 3470 pop 258 orion 189 titan 167 grumpy 15265 pc3 3452 mercury 254 mac2 189 pc3 163 gandalf 15177 pc4 3438 intranet 240 newton 186 opus 162 pc4 14979 broadcast 3404 demo 234 neptune 186 mac6 160 uranus 14891 pc5 3397 alpha 233 pc2 185 charon 159 mac16 14877 gateway 3388 pc13 224 gauss 185 apollo 158 sleepy 14138 server 3330 pluto 222 eagle 179 mac7 158 io ...big gap... 3308 exchange 213 mac3 179 athena 157 earth 3884 cisco 3253 linux 209 merlin 177 alpha 156 europa 207 cisco 172 mozart 155 rigel 2005/03/11 (C) Herbert Haas 4

  5. History  Even in the early Arpanet hosts have been identified by names  For People, not machines!  Name/Address bindings in HOSTS.TXT files /etc/hosts 127.0.0.1 eric localhost Eric 10.0.1.1 eric.spark eric 10.0.1.1. 10.0.1.2 kenny.spark kenny "SPark" 10.0.1.3 stan.spark stan (Kenny and Stan have similar hostfiles) Kenny Stan 10.0.1.2 10.0.1.3 2005/03/11 (C) Herbert Haas 5

  6. Hostfile Problems  Centrally maintained by Network Information Center (NIC)  Copied by all hosts  Scalability problem  Consistency problem  Maintenance problem 2005/03/11 (C) Herbert Haas 6

  7. 1984: DNS  Paul Mockapetris (IAB) created DNS  Distributed database  World-wide and redundant  Maintained by Name Servers  Simulates hierarchical tree of mnemonic names  Each domain name is a node in a database  Goal: Simple "Hostname resolution"  But also stores other information 2005/03/11 (C) Herbert Haas 7

  8. Logical Tree of Names Root  IP net-IDs are "flat" Domain .  Arbitrary assignment TLDs without semantical or logical considerations … COM ORG AT EDU BIZ …  Hard to remember  DNS maps addresses to 2 n d Level Domain names DEBIAN AC  DNS allows hierarchical 3 n d Level Domain tree of names WWW TUWIEN  No name collisions WWW.DEBIAN.ORG. anymore! 192.25.206.10  Max 127 levels WWW GD  Concatenation results in WWW.TUWIEN.AC.AT. GD.TUWIEN.AC.AT. Fully Qualified Domain 128.130.102.130 192.35.244.50 Name (FQDN) 2005/03/11 (C) Herbert Haas 8

  9. Name Servers  The DNS tree is realized by Name Servers  The Domain Name Tree does NOT reflect the physical network structure!  Each NS cares for a subset of the DNS tree: zones  Flexible mappings  1:n (Routers or servers with several network interfaces)  n:1 (Multiple services behind a single IP address) 2005/03/11 (C) Herbert Haas 9

  10. Terminology  A "Domain" is a subtree of the domain name . space  A "Domain Name" is the GOV COM name of a node in the tree  Concatenated labels from the root to the Domain Name (node) FBI current domain SECRET.FBI.GOV.  Listed from right to left  Separated by dots SECRET  Max 255 characters Domain  A "Label" is a GOV X-FILES MIB component of the domain name  Max 63 characters Domain FBI.GOV 2005/03/11 (C) Herbert Haas 10

  11. The Root Domain  The root of the DNS tree is represented as a dot "."  A true FQDN includes the dot  Otherwise "relative" domain name  Most people/applications don't care  However, DNS does care!  The root is implemented by several root- servers (currently 13)  Below the root, a domain may be called top-level, second-level, third-level etc... 2005/03/11 (C) Herbert Haas 11

  12. Top Level Domains  Seven "generic domains" (gTLDs)  COM, EDU, GOV, INT, ORG, MIL, NET  Initially inside USA, now globally used  244 Two-letter country codes  E.g. AT, DE, UK, ES, RU, CH, IT, AQ, …  Initially outside USA only, now also "US"  Country code does not necessarily reflect real location!  Seven new TLDs  BIZ, INFO, NAME, MUSEUM, COOP, AERO, PRO 2005/03/11 (C) Herbert Haas 12

  13. Delegation and Zones  To ease administration, . Delegation Zone "." the authority over subdomains is delegated ORG Zone ORG to other nameservers Delegation  A zone is a point of Delegation delegation or "Start of BAR Authority" (SOA)  Zones relate to the way the database is CROSS FOO partitioned and distributed Zone CROSS.BAR.ORG Zone FOO.BAR.ORG 2005/03/11 (C) Herbert Haas 13

  14. Hostname Resolution  Recursive queries = the job is forwarded  The response must be exact (or error message)  Most burden on next name server  Iterative queries = All NS are queried top-down  The response contains best answer already known  Requested name server makes no further queries Root + gTLDs (e.g. EDU) ? . List of mit name servers u d e . t i m . w w w www.mit.edu. ? www.mit.edu. ? MIT server 18.181.0.31 18.181.0.31 Recursive Iterative 2005/03/11 (C) Herbert Haas 14

  15. A Detailed Real-World Example zone "." gd.tuwien.ac.at . ns2.univie.ac.at List of at name servers ns1.univie.ac.at a.root-servers.net ns.uu.net … gd.tuwien.ac.at AQ AT List of tuwien.ac.at tunamed.tuwien.ac.at name servers tunamec.tuwien.ac.at ns2.univie.ac.at zone "ac.at" gd.tuwien.ac.at gd.tuwien.ac.at 192.35.244.50 AC CO Address = 192.35.244.50 tunamed.tuwien.ac.at TUWIEN UNIVIE Let me FTP something zone "tuwien.ac.at" gd.tuwien.ac.at 192.35.244.50 GD ZID INFO 2005/03/11 (C) Herbert Haas 15

  16. Note  Each questioned name server replies with more detailed information…or the desired information itself!  A reference to another NS gives precious information about new zone authority – cached! 2005/03/11 (C) Herbert Haas 16

  17. Caching Root NS  First, the local NS resolves the name kenny.southpark.edu  Hereby it learns also the addresses of the southpark.edu NS  All this information is cached! southpark.edu NS Local NS Root NS  When resolving the name seamen.superbestfriends.southpa rk.edu the local NS notices that this name is member of southpark.edu  Address of southpark.edu NS is southpark.edu NS cached Local NS  No need to start at root NS! superbestfriends.southpark.edu NS 2005/03/11 (C) Herbert Haas 17

  18. Reverse Lookups  Very often reverse lookups are necessary  "Have address but want name"  For logging purposes or service restriction  Therefore the in-addr.arpa domain was created  Given an IP-address the associated hostname can be found  Otherwise an exhaustive search in the domain space would be necessary to find any desired hostname 2005/03/11 (C) Herbert Haas 18

  19. In-Addr.Arpa What's the Domain Name of 192.25.206.10 ? .  Each byte of an IP address is treated as ORG ARPA … … … label and attached under the in-addr.arpa TLD  Expressed as character DEBIAN … IN-ADDR … string for its decimal value ("0" - "255") WWW … 191 192 193 194 …  Labels are concatenated WWW.DEBIAN.ORG. in reverse order … 24 25 26 …  "10.206.25.192.in-addr.arpa" 205 206 207 … … Pointer (PTR) … 9 10 11 … 2005/03/11 (C) Herbert Haas 19

  20. BIND  Berkeley Internet Name Domain (BIND)  Implemented by Paul Vixie as an Internet name server for BSD-derived systems  Most widely used name server on the Internet  Version numbers: 4 (old but still used), 8, 9  BIND consists of  A name server program "named"  A resolver library for client applications  BIND deals with zones! 2005/03/11 (C) Herbert Haas 20

  21. Resolver and Name Server CLIENT FOREIGN user queries queries User Foreign Progra Resolver NS m user responses responses cache additions references  All DNS messages use Shared port 53 Database  Zone transfers use TCP  Simple queries use UDP SERVER Shared Databas e FOREIGN refreshes references responses Foreign NS Maste (name Resolve r Files d) r queries maintenance queries Foreign maintenance NS responses 2005/03/11 (C) Herbert Haas 21

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