Name Resolution and DNS Domain names and IP

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, www. google. com, 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

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 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 Root domain. Top-Level-Domains edu com gov mil net org 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 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 m Contacts authoritative name server if name mapping not known Gets mapping Returns mapping to local name server Does nota Verisign, have. Dulles, name VA c Cogent, Herndon, VA (also Los Angeles) d U Maryland College Park, MD k RIPE London (also Amsterdam, Frankfurt) g US Do. D 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 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: code 2 -character country or region • 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 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.

Distributed, Hierarchical Database Root DNS Servers com DNS servers yahoo. com amazon. com DNS servers org DNS servers pbs. org DNS servers edu DNS servers poly. edu umass. edu DNS servers Client wants IP for www. amazon. com; 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 www. amazon. com

Example r Host at cis. poly. edu wants r r 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 cis. poly. edu server, and get the IP address! This uses both iterative and recursive queries. (Usually the way its done. ) 3 4 TLD DNS server 5 7 6 authoritative DNS server dns. cs. umass. edu gaia. cs. umass. edu

Recursive queries root DNS server recursive query: 2 r puts burden of name resolution on contacted name server r heavy load? iterated query: r contacted server 3 7 local DNS server dns. poly. edu 1 6 TLD DNS server 5 4 8 replies with name of server to contact r “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 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 http: //www. ietf. org/html. charters/dnsind-charter. html

Summary r We have examined how DNS works