Space Name System DNS Reading: Section 9.1 .

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2. Overhaul on Assignment
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Space Name System (DNS) Reading: Section 9.1 COS 461: Computer Networks Spring 2006 (MW 1:30-2:50 in Friend 109) Jennifer Rexford Teaching Assistant: Mike Wawrzoniak

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Update on Assignment #1 Extending the due date Was expected Wednesday March 1 at 9pm Now due Monday March 6 at 9pm Performance Good to have an effective execution But not important to streamline each and every piece Help session on the task Last half hour of today\'s class Led by Mike Wawrzoniak E-mail inquiries to the course mailing list

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Goals of Today\'s Lecture Computer science ideas fundamental DNS Indirection: names set up of locations Hierarchy: in names, locations, and servers Caching: of mappings from names to/from locations Inner-workings of DNS resolvers and servers Iterative and recursive questions TTL-based storing Web and DNS Influence of DNS inquiries on Web execution Server determination and load adjusting

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Host Names versus IP addresses Host names Mnemonic name increased in value by people Variable length, alpha-numeric characters Provide nearly nothing (assuming any) data about area Examples: and IP addresses Numerical address increased in value by switches Fixed length, double number Hierarchical, identified with host area Examples: and

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Separating Naming and Addressing Names are less demanding to recollect versus Addresses can change underneath Move to E.g., renumbering while changing suppliers Name could guide to numerous IP addresses to various reproductions of the Web website Map to various addresses in better places Address of an adjacent duplicate of the Web webpage E.g., to decrease inactivity, or return diverse substance Multiple names for a similar address E.g., false names like and

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Strawman Solution: Local File Original name to address mapping Flat namespace/and so forth/has SRI kept fundamental duplicate Downloaded frequently Count of hosts was expanding: moving from a machine for each area to machine per client Many more downloads Many more updates

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Strawman Solution #2: Central Server Central server One place where all mappings are put away All questions go to the focal server Many functional issues Single purpose of disappointment High activity volume Distant concentrated database Single purpose of refresh Does not scale Need a conveyed, progressive gathering of servers

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Domain Name System (DNS) Properties of DNS Hierarchical name space partitioned into zones Distributed over an accumulation of DNS servers Hierarchy of DNS servers Root servers Top-level area (TLD) servers Authoritative DNS servers Performing the interpretations Local DNS servers Resolver programming

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13 root servers (see Labeled A through M DNS Root Servers A Verisign, Dulles, VA C Cogent, Herndon, VA (additionally Los Angeles) D U Maryland College Park, MD G US DoD Vienna, VA H ARL Aberdeen, MD J Verisign, ( 11 areas) K RIPE London (likewise Amsterdam, Frankfurt) I Autonomica, Stockholm (in addition to 3 different areas) E NASA Mt View, CA F Internet Software C. Palo Alto, CA (and 17 different areas) m WIDE Tokyo B USC-ISI Marina del Rey, CA L ICANN Los Angeles, CA

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TLD and Authoritative DNS Servers Top-level space (TLD) servers Generic spaces (e.g., com, organization, edu) Country spaces (e.g., uk, fr, ca, jp) Typically oversaw professionally Network Solutions keeps up servers for "com" Educause keeps up servers for "edu" Authoritative DNS servers Provide open records for hosts at an association For the association\'s servers (e.g., Web and mail) Can be kept up locally or by a specialist organization

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Distributed Hierarchical Database anonymous root zw arpa uk com edu organization air conditioning nonexclusive spaces nation spaces in-addr bar air conditioning west east 12 cam foo my 34 usr 56

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Using DNS Local DNS server ("default name server") Usually close to the end has who utilize it Local hosts designed with nearby server (e.g.,/and so forth/resolv.conf) or take in the server by means of DHCP Client application Extract server name (e.g., from the URL) Do gethostbyname() to trigger resolver code Server application Extract customer IP address from attachment Optional gethostbyaddr() to convert into name

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Host at needs IP address for neighborhood DNS server Example root DNS server 2 3 TLD DNS server 4 5 6 7 1 8 legitimate DNS server asking for host

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neighborhood DNS server Recursive versus Iterative Queries Recursive inquiry Ask server to get respond in due order regarding you E.g., ask for 1 and reaction 8 Iterative question Ask server who to ask next E.g., all other demand reaction sets root DNS server 2 3 TLD DNS server 4 5 6 7 1 8 definitive DNS server asking for host

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DNS Caching Performing every one of these inquiries require significant investment And this before the genuine correspondence happens E.g., 1-second inactivity before beginning Web download Caching can considerably lessen overhead The top-level servers infrequently change Popular destinations (e.g., went to frequently Local DNS server regularly has the data reserved How DNS reserving functions DNS servers reserve reactions to inquiries Responses incorporate an "opportunity to live" (TTL) field Server erases the stored section after TTL lapses

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Negative Caching Remember things that don\'t work Misspellings like www.cnn.comm and These can set aside a long opportunity to fizzle the first run through Good to recall that they don\'t work … so the disappointment takes less time whenever around

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DNS: dispersed db putting away asset records (RR) Type=NS name is area (e.g. esteem is hostname of definitive name server for this area RR design: (name, esteem, sort, ttl) DNS Resource Records Type=A name is hostname esteem is IP address Type=CNAME name is moniker name for some "accepted" (the genuine) name is truly esteem is sanctioned name Type=MX esteem is name of mailserver related with name

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DNS convention : inquiry and answer messages, both with same message arrange DNS Protocol Message header Identification: 16 bit # for question, answer to question utilizes same # Flags: Query or answer Recursion wanted Recursion accessible Reply is legitimate

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Reliability DNS servers are imitated Name benefit accessible if no less than one reproduction is up Queries can be load adjusted between copies UDP utilized for inquiries Need dependability: must actualize this on top of UDP Try substitute servers on timeout Exponential backoff while retrying same server Same identifier though inquiries Don\'t couldn\'t care less which server reacts

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Inserting Resource Records into DNS Example: just made startup "FooBar" Register at Network Solutions Provide recorder with names and IP locations of your legitimate name server (essential and optional) Registrar embeds two RRs into the com TLD server: (,, NS) (,, A) Put in legitimate server Type A record for Type MX record for

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Playing With Dig on UNIX Dig program Allows questioning of DNS framework Use banners to discover name server (NS) Disable recursion so that works slowly and carefully unix> burrow +norecurse NS ;; AUTHORITY SECTION: edu. 2D IN NS L3.NSTLD.COM. edu. 2D IN NS D3.NSTLD.COM. edu. 2D IN NS A3.NSTLD.COM. edu. 2D IN NS E3.NSTLD.COM. edu. 2D IN NS C3.NSTLD.COM. edu. 2D IN NS G3.NSTLD.COM. edu. 2D IN NS M3.NSTLD.COM. edu. 2D IN NS H3.NSTLD.COM.

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DNS and the Web

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DNS Query in Web Download User sorts or taps on a URL E.g., Browser removes the website name E.g., Browser calls gethostbyname() to learn IP deliver Triggers resolver code to inquiry the neighborhood DNS server Eventually, the resolver gets an answer Resolver gives back the IP deliver to the program Then, the program contacts the Web server Creates and associates attachment, and sends HTTP ask for

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Multiple DNS Queries Often a Web page has implanted articles E.g., HTML document with installed pictures Each inserted protest has its own particular URL … and conceivably lives on an alternate Web server E.g., Browser downloads installed questions Usually done consequently, unless designed generally Requires taking in the address for

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When are DNS Queries Unnecessary? Program is designed to utilize an intermediary E.g., program sends all HTTP asks for through an intermediary Then, the intermediary deals with issuing the DNS ask for Requested Web asset is privately reserved E.g., store has No compelling reason to get the asset, so no compelling reason to question Browser as of late questioned for this host name E.g., client as of late went by, the program as of now called gethostbyname() … and might be locally reserving the subsequent IP address

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Web Server Replicas Popular Web destinations can be effortlessly over-burden Web website frequently keeps running on various server machines Internet

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Directing Web Clients to Replicas Simple approach: distinctive names,, But, this obliges clients to choose particular copies More rich approach: diverse IP addresses Single name (e.g.,, numerous locations E.g.,,,, … Authoritative DNS server returns many locations And the nearby DNS server chooses one address Authoritative server may var

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