Name Resolution and DNS

Domain names and IP addresses r People prefer to use easy-to-remember names instead of IP addresses r Domain names are alphanumeric names for IP addresses e.g., syslab.csd.uwo.ca, ietf.org r The domain name system (DNS) is an Internet-wide distributed database that translates between domain names and IP addresses r How important is DNS? Imagine what happens when the local DNS server is down.

Before there was DNS …. …. there was the HOSTS.TXT file r Before DNS (until 1985), the name-to-IP address was done by downloading a single file (hosts.txt) from a central server with FTP. m Names in hosts.txt are not structured. m The hosts.txt file still works on most operating systems. It can be used to define local names.

DNS: Domain Name System r Distributed database implemented in hierarchy of many name servers r Application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) m note: core Internet function, implemented as application-layer protocol m complexity at network’s “edge”

Hierarchy of Name Servers r The resolution of the hierarchical name space is done by a hierarchy of name servers r Each server is responsible (authoritative) for a contiguous portion of the DNS namespace, called a zone. r Zone is a part of the subtree r DNS server answers queries about hosts in its zone

Hierarchical Names r Internet hosts and other resources need globally unique names r Difficult to keep unstructured names unique m would require a single list of all names in use r Hierarchical names are much easier to make unique

Why Not Centralize DNS? r Single point of failure r Traffic volume r Distant centralized database r Maintenance doesn’t scale!

Design principle of DNS r The naming system on which DNS is based is a hierarchical and logical tree structure called the domain namespace. r An organization obtains authority for parts of the name space, and can add additional layers of the hierarchy r Names of hosts can be assigned without regard of location on a link layer network, IP network or autonomous system r Let’s discuss the different levels

Domain Name Hierarchy. edu Root domain comgovmilnetorg rofrat... jp icirncasepubuttvsat roearn ns std cslmndsp ulise paul accogvor... uni-linztuwien eunet cc univie matexpitc phytia alpha chris Top-Level-Domains Second Level Domains

DNS Name hierarchy r DNS hierarchy can be represented by a tree r Root and top-level domains are administered by an Internet central name registration authority (ICANN) r Below top-level domain, administration of name space is delegated to organizations r Each organization can delegate further

DNS: Root Name Servers r Contacted by local name server that can not resolve name r Root name server: m Contacts authoritative name server if name mapping not known m Gets mapping m Returns mapping to local name server m Does not have name 13 root name servers worldwide b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 17 other locations) i Autonomica, Stockholm (plus 3 other locations) k RIPE London (also Amsterdam, Frankfurt) m WIDE Tokyo a Verisign, Dulles, VA c Cogent, Herndon, VA (also Los Angeles) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 11 locations)

Top-level domains r Types of top-level domains: m Organizational: 3-character code indicates the function of the organization Used primarily within the US Examples: gov, mil, edu, org, com, net m 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) comCommercial organizations eduEducational institutions govGovernment institutions intInternational organizations milU.S. military institutions netNetworking organizations orgNon-profit organizations

TLD and Authoritative Servers r Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp. r Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail). m Can be maintained by organization or service provider

Local Name Server r Does not strictly belong to hierarchy r Each ISP (residential ISP, company, university) has one. m Also called “default name server” r When a host makes a DNS query, query is sent to its local DNS server m Acts as a proxy, forwards query into hierarchy.

Root DNS Servers com DNS servers org DNS serversedu DNS servers poly.edu DNS servers umass.edu DNS servers yahoo.com DNS servers amazon.com DNS servers pbs.org DNS servers Distributed, Hierarchical Database Client wants IP for 1 st approximation: r Client queries a root server to find com DNS server r Client queries com DNS server to get amazon.com DNS server r Client queries amazon.com DNS server to get IP address for

requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server Example r Host at cis.poly.edu wants IP address for gaia.cs.umass.edu. r First, check locally. r Then, check root. Root notes.edu suffix and suggests TLD servers for.edu. r Check TLD server. This provides the authoritative server. r Check authoritative server, and get the IP address! r This uses both iterative and recursive queries. (Usually the way its done.)

requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server 3 Recursive queries recursive query: r puts burden of name resolution on contacted name server r heavy load? iterated query: r contacted server replies with name of server to contact r “I don’t know this name, but ask this server”

DNS: Caching and Updating Records r Once (any) name server learns mapping, it caches mapping m Cache entries timeout (disappear) after some time m TLD servers typically cached in local name servers Thus root name servers not often visited r Update/notify mechanisms under design by IETF m RFC 2136 m

Summary r We have examined how DNS works