Transcript DNS
Name Resolution and DNS Domain names and IP addresses People prefer to use easy-to-remember names instead of IP addresses Domain names are alphanumeric names for IP addresses e.g., syslab.csd.uwo.ca, www.google.com, ietf.org The domain name system (DNS) is an Internet-wide distributed database that translates between domain names and IP addresses How important is DNS? Imagine what happens when the local DNS server is down. Before there was DNS …. …. there was the HOSTS.TXT file Before DNS (until 1985), the name-to-IP address was done by downloading a single file (hosts.txt) from a central server with FTP. Names in hosts.txt are not structured. The hosts.txt file still works on most operating systems. It can be used to define local names. DNS: Domain Name System Distributed database implemented in hierarchy of many name servers Application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) note: core Internet function, implemented as application-layer protocol complexity at network’s “edge” Hierarchy of Name Servers The resolution of the hierarchical name space is done by a hierarchy of name servers Each server is responsible (authoritative) for a contiguous portion of the DNS namespace, called a zone. Zone is a part of the subtree root server org server uci.edu server edu server gov server .virginia.edu server cs.virginia.edu server com server Hierarchical Names Internet hosts and other resources need globally unique names Difficult to keep unstructured names unique would require a single list of all names in use Hierarchical names are much easier to make unique Why Not Centralize DNS? Single point of failure Traffic volume Distant centralized database Maintenance doesn’t scale! Design principle of DNS The naming system on which DNS is based is a hierarchical and logical tree structure called the domain namespace. An organization obtains authority for parts of the name space, and can add additional layers of the hierarchy Names of hosts can be assigned without regard of location on a link layer network, IP network or autonomous system Let’s discuss the different levels Domain Name Hierarchy Root domain . edu com gov mil net org Top-Level-Domains ro fr . . . . . . at jp Second Level Domains ici rnc ase pub utt vsat eunet univie roearn ns std cs ulise lmn ac uni-linz co gv or ... tuwien ....... itc . . . . . . dsp paul cc mat exp phytia alpha chris DNS Name hierarchy DNS hierarchy can be represented by a tree Root and top-level domains are administered by an Internet central name registration authority (ICANN) Below top-level domain, administration of name space is delegated to organizations Each organization can delegate further DNS: Root Name Servers Contacted by local name server that can not resolve name Root name server: Contacts authoritative name server if name mapping not known Gets mapping Returns mapping to local name server Does not haveDulles, name a Verisign, VA c Cogent, Herndon, VA (also Los Angeles) d U Maryland College Park, MD k RIPE London (also Amsterdam, Frankfurt) g US DoD Vienna, VA i Autonomica, Stockholm (plus 3 h ARL Aberdeen, MD other locations) j Verisign, ( 11 locations) m WIDE Tokyo e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 17 other locations) 13 root name servers worldwide b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA Top-level domains Types of top-level domains: Organizational: 3-character code indicates the function of the organization • Used primarily within the US • Examples: gov, mil, edu, org, com, net Geographical: 2-character country or region code • Examples: us, va, jp, de There are more than 200 top-level domains. Organizational top-level domains (TLD) com Commercial organizations edu Educational institutions gov Government institutions int International organizations mil U.S. military institutions net Networking organizations org Non-profit organizations TLD and Authoritative Servers Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp. Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail). Can be maintained by organization or service provider Local Name Server Does not strictly belong to hierarchy Each ISP (residential ISP, company, university) has one. Also called “default name server” When a host makes a DNS query, query is sent to its local DNS server Acts as a proxy, forwards query into hierarchy. Distributed, Hierarchical Database Root DNS Servers com DNS servers yahoo.com amazon.com DNS servers DNS servers org DNS servers pbs.org DNS servers edu DNS servers poly.edu umass.edu DNS serversDNS servers Client wants IP for www.amazon.com; 1st approximation: Client queries a root server to find com DNS server Client queries com DNS server to get amazon.com DNS server Client queries amazon.com DNS server to get IP address for www.amazon.com Example Host at cis.poly.edu wants root DNS server 2 IP address for gaia.cs.umass.edu. First, check locally. Then, check root. Root local DNS server notes .edu suffix and dns.poly.edu suggests TLD servers for .edu. 1 8 Check TLD server. This provides the authoritative server. Check authoritative requesting host server, and get the IP cis.poly.edu address! This uses both iterative and recursive queries. (Usually the way its done.) 3 TLD DNS server 4 5 7 6 authoritative DNS server dns.cs.umass.edu gaia.cs.umass.edu Recursive queries root DNS server recursive query: 2 puts burden of name resolution on contacted name server heavy load? iterated query: contacted server 3 7 6 TLD DNS server local DNS server dns.poly.edu 1 5 4 8 replies with name of server to contact “I don’t know this requesting host name, but ask this cis.poly.edu server” authoritative DNS server dns.cs.umass.edu gaia.cs.umass.edu DNS: Caching and Updating Records Once (any) name server learns mapping, it caches mapping Cache entries timeout (disappear) after some time TLD servers typically cached in local name servers • Thus root name servers not often visited Update/notify mechanisms under design by IETF RFC 2136 http://www.ietf.org/html.charters/dnsind-charter.html Summary We have examined how DNS works