Module 2.2: Domain Name System

1. Module 2.2: Domain Name System • IP assigns 32-bit addresses to hosts (interfaces) • Binary addresses easy for computers to manage • All applications use IP addresses through the TCP/IP protocol software • Difficult for humans to remember: % telnet 134.82.11.70 • The Domain Name System (DNS) provides translation between symbolic names and IP addresses • DNS runs over UDP and uses port 53 of messages less than 512 bytes; otherwise, it uses TCP port 53 K. Salah

2. Structure of DNS names • Each name consists of a sequence of alphanumeric components separated by periods • Examples: www.eg.bucknell.edu www.netbook.cs.purdue.edu charcoal.eg.bucknell.edu • Names are hierarchical, with most-significant component on the right • Left-most component is computer name K. Salah

3. DNS naming structure • Top level domains (right-most components; also known as TLDs) defined by global authority com Commercial organization edu Educational institution gov Government organization mil Military organization • Organizations apply for names in a top-level domain: kfupm.edu macdonalds.com • Organizations determine own internal structure ccse.kfupm.edu cs.purdue.edu K. Salah

4. Geographic structure • Top-level domains are US-centric • Geographic TLDs used for organizations in other countries: • Countries define their own internal hierarchy: ac.uk and .edu.au are used for academic organizations in the United Kingdom and Australia. In SA, it is edu.sa. K. Salah

5. Domain names within an organization • Organizations can create any internal DNS hierarchy • Uniqueness of TLD and organization name guarantee uniqueness of any internal name (much like file names in your directories) • All but the left-most component of a domain name is called the domain for that name: • Authority for creating new subdomains is delegated to each domain • Administrator of kfupm.edu has authority to create eg.kfupm.edu and need not contact any central naming authority • DNS domains are logical concepts and need not correspond to physical location of organizations • DNS domain for an organization can span multiple networks K. Salah

6. Domain name space • Names are defined in an inverted-tree structure with the root at the top. • Can have 128 levels: level 0 (root) to level 127. • Label: • Each node in the tree has a level • Maximum of 63 characters. • Root label is a null string (empty string). • Children of a node have different labels. K. Salah

7. Domain names and labels • Full domain name is a sequence of labels separated by dots. • Domain names are always read from the node up to the root. Last label is the label of root (null). So, full domain name always ends in a null label [means dot]. K. Salah

8. FQDN and PQDN • Fully Qualified Domain Name (FQDN) or Absolute Domain Name • Label is terminated by a null string. • Contains the full name of a host. • Partially Qualified Domain Name (PQDN) or Relative Domain Name • Not terminated by a null string. • Used when the name to be resolved belongs to the same site as the client. • Resolver supplies the missing part called as suffix. • Why Absolute and Relative? Convenience. K. Salah

9. Domains • Domain: • Subtree of the domain name space. • Name of the domain is the domain name of the node at the top of the subtree. • A domain can be divided into subdomains. K. Salah

10. DNS and client-server computing • DNS names are managed by a hierarchy of DNS servers • Root server at top of tree knows about next level servers. • Next level servers, in turn, know about lower level servers • Some Jargon • Each DNS server is the authoritative server for the names it manages • What a server is responsible for or has authority over is called a zone. A domain can span multiple servers. • Primary server is also called authoritative server • Second server has a copy K. Salah

11. a NSI Herndon, VA c PSInet Herndon, VA d U Maryland College Park, MD g DISA Vienna, VA h ARL Aberdeen, MD j NSI (TBD) Herndon, VA k RIPE London i NORDUnet Stockholm m WIDE Tokyo e NASA Mt View, CA f Internet Software C. Palo Alto, CA b USC-ISI Marina del Rey, CA l ICANN Marina del Rey, CA Root Name Servers Root Server 13 root name servers worldwide, according to www.dnso.org 2005 K. Salah

12. Choosing DNS server architecture • Small organizations can use a single server • Easy to administer • Inexpensive • Large organizations often use multiple servers • Reliability through redundancy • Improved response time through load-sharing • Delegation of naming authority • Locality of reference applies - users will most often look up names of computers within same organization • All DNS servers are linked together to form a unified system. Each server knows how to reach a root server and how to reach servers that are authorities for names further down the hierarchy. K. Salah

13. host surf.eurecom.fr wants IP address of gaia.cs.umass.edu 1. contacts its local DNS server, dns.eurecom.fr 2.dns.eurecom.fr contacts root name server, if necessary 3. root name server eventually contacts authoritative name server, dns.cs.umass.edu, if necessary This is called “Recursive Resolution” local name server dns.eurecom.fr intermediate name server dns.umass.edu Name Resolution root name server 6 2 3 7 5 4 1 8 authoritative name server dns.cs.umass.edu requesting host surf.eurecom.fr gaia.cs.umass.edu K. Salah

14. recursive 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 the following server(s)” Gives more control to client local name server dns.eurecom.fr Types of Queries root name server iterated query 2 3 4 7 5 6 1 8 authoritative name server dns.cs.umass.edu requesting host surf.eurecom.fr gaia.cs.umass.edu K. Salah

15. DNS caching • DNS resolution can be very inefficient • Every host referenced by name triggers a DNS request • Every DNS request for the address of a host in a different organization goes through the root server • Servers and hosts use caching to reduce the number of DNS requests • Cache is a list of recently resolved names and IP addresses • Authoritative server include time-to-live with each reply • DDNS (Dynamic Domain Name System) • When a new node is added/deleted authoritative DNS server is updated accordingly. • When a new domain is added, update message is sent to upper servers • Servers query other servers periodically for latest changes • update/notify mechanisms under design by IETF • RFC 2136 • http://www.ietf.org/html.charters/dnsind-charter.html K. Salah

16. DNS: distributed db storing resource records (RR) Type=NS name is domain (e.g. foo.com) value is IP address of authoritative name server for this domain RR format: (name, TTL, class, type, value) DNS records • Type=CNAME • name is alias name for some “cannonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com • value is cannonical name • Type=A • name is hostname • value is IP address • Type=MX • value is name of mailserver associated with name Class is typically “IN” information, and TTL is in seconds K. Salah

17. DNS protocol :queryand reply messages, both with same message format DNS protocol, messages msg header • identification: 16 bit # for query, reply to query uses same # • flags: • query or reply • recursion desired • recursion available • reply is authoritative K. Salah

18. DNS protocol, messages Name, type fields for a query RRs in reponse to query records for authoritative servers additional “helpful” info that may be used K. Salah

19. Example using “dig” The number next to MX is the preference value in case you get multiple replies. K. Salah

20. A different business model than web caching The content providers(CNN, Yahoo, Lycos, Google, etc) are the CDN customers. Content replication CDN company (e.g. Akamai.com) installs hundreds of CDN servers throughout Internet in lower-tier ISPs, close to users CDN replicates its customers’ content in CDN servers. When provider updates content, CDN updates servers But how it works? Content distribution networks (CDNs) origin server in North America CDN distribution node CDN server in S. America CDN server in Asia CDN server in Europe K. Salah

21. Origin server www.foo.com Distributes only GIF files to CDN In each HTML page, it replaces: http://www.foo.com/sports.ruth.gifwithhttp://www.cdn.com/www.foo.com/sports/ruth.gif HTTP request for www.foo.com/sports/sports.html Origin server 1 DNS query for www.cdn.com 2 CDNs authoritative DNS server 3 HTTP request for www.cdn.com/www.foo.com/sports/ruth.gif Nearby CDN server CDN example CDN company • cdn.com • distributes gif files • uses its authoritative DNS server to return the IP of the nearby CDN server Note: No changes is required to DNS, HTTP, or browser to implement this. K. Salah

22. routing requests CDN creates a “map”, indicating distances from leaf ISPs and CDN nodes when query arrives at authoritative DNS server: server determines ISP from which query originates uses “map” to determine best CDN server shortest distance Least loaded not just Web pages streaming stored audio/video streaming real-time audio/video More about CDNs K. Salah