Informatics Institute of Technology 3SFE611 Network Design 1 DNS (Domain Name System) RFC1035 Why names? Computers use addresses. Humans cannot remember IP addresses ==> need namese.g. amon for Simplest solution: Each computer has a unique name and has built in table to address translation. Problem: not scalable. Solution: DNS (adopted 1983), supports distributed name database with fast response. Hierarchical names: amon.cpc.wmin.ac.uk

Informatics Institute of Technology 3SFE611 Network Design 2 Name Hierarchy Unique domain suffix is assigned by internet Authority. The domain administrator has complete control over the domain. No limit on number of subdomains or number levels. E.g. computer.site.division.company.com E.g. computer.site.subdivision.division.company.com Domains within an organisation do not have to be uniform in number of subdomains or levels. Name space is not related to physical interconnection. E.g. math.wmin.ac.uk and phy.wmin.ac.uk on same floor or different cities. Geographical hierarchy is also allowed e.g. rns.reston.va.us. A name could be a subdomain or an individual object.

Informatics Institute of Technology 3SFE611 Network Design 3 Name Hierarchy (cont) Unnamed root edugovcom usuk ses ohio ee cscis nasa vaac wmin cpc bae co... reston cnri

Informatics Institute of Technology 3SFE611 Network Design 4 Top Level Domains

Informatics Institute of Technology 3SFE611 Network Design 5 Server Hierarchy Servers are organised in a hierarchy. Each server has an authority over a part of the naming hierarchy. The server does not need to keep all names, it needs to know other servers who are responsible for other subdomains. A given level of the hierarchy can be partitioned into multiple servers. Authority has the name to address translation table. Responsible server has the name to address translation table or know the server who has. A single server can serve multiple domains. Root server knows about servers for top level domains. Each server knows about the root server.

Informatics Institute of Technology 3SFE611 Network Design 6 Server Hierarchy: Example Server ac.uk Server va.us Server nsf.gov Server osu.edu Server dec.com Root Server for uk Server for us Server for gov Server for edu Server for com

Informatics Institute of Technology 3SFE611 Network Design 7 Server Hierarchy: Better Root Server ac.uk Server va.us Server nsf.gov Server osu.edu Server dec.com

Informatics Institute of Technology 3SFE611 Network Design 8 Domain Name Server Name Server Cache Name Server Cache Name Resolver User Cache Name Server Cache Database Query Response

Informatics Institute of Technology 3SFE611 Network Design 9 Domain Name Server (cont) Name Server Cache Name Server Cache Name Resolver User Cache Name Server Cache Database Query Response Referral Response

Informatics Institute of Technology 3SFE611 Network Design 10 Name Resolution Each computer has a name resolver routine (e.g. gethostbyname). Each resolver knows the name of a local DNS server. Resolver sends a DNS request to the server, DNS server either gives the answer, forwards the request to another server or gives a referral (next server to whom request should be sent). Resolvers use UDP (single name) or TCP (group of names). Knowing the address of the root server is sufficient (recursive query of resolvers vs iterative query of servers) Recursive: Give me an answer. Iterative: Give me an answer or a referral.

Informatics Institute of Technology 3SFE611 Network Design 11 DNS Optimisation Caching: usually local set of domains a referenced repeatedly plus results of any recent requests are kept in a local cache. Each entry has a time to live (TTL) to prevent queries circulating. DNS servers ask the geographically closest server. Will use a different one if last one timed out. Servers respond to a full name only but humans may specify only a partial name or an alias such as “amon”. Resolvers may fill in the rest of the suffix: e.g. amon = amon.cpc.wmin.ac.uk Each resolver has a list of suffixes to try.