Atomicity Strong need consistency 6

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1 CS640: Naming Computer AdityaAkella Lecture and the 17DNS Networks DNS Today Design The Road Ahead 2 Need Once How identified, naming to identify Naming Challenge: area? Naming DNS: to do name we hierarchy How efficiently location resource? do we scale (IP locate resources address) must these resources? be to located the wide 3 1

2 Lookup Traffic Distant Single Doesn t Obvious a point Central scale! centralized volume of failure update DNS? Solutions database(1) 4 Why Original Count Flatnamespace Lookup SRI Downloaded not Obvious kept use Name mapping main /etc/hosts? regularly to copy Solutions Address /etc/hosts Mapping (2) per Many domain of more hosts machine downloads updates was increasing: per usermachine 5 Basically Goals: name Domain Scalability Decentralized Robustness Global to IP a wide-area mappings Name System distributed Goals Don t Names scope mean the maintenance same thing everywheredatabase of Atomicity Strong need consistency 6 2

3 Conceptually, Programmer s View of DNS Functions database DNS: entry in_addris structures: as for a a structconsisting programmers retrieving collection of host millions can of entries 4-byte view of IP the from host address DNS is an DNS IP address. host name. 7 Name, for RRsin to Records DNS No. Answers No. Message Identification of Questions of Authority Questions (variable RRs Format authoritative servers Additional a query type fields Additional Authority Info (variable number number number of No. of No. of of of resource Additional answers) Answer Flags helpful may query be response used info for 12 that bytes records) RRs8 Identification Flags Used DNS mark match Header query authoritative up request/response response Fields 1-bit resolution to indicate request recursive support for resolution or recursive not 9 gethostbyname:query gethostbyaddr:query key /* DNS entry structure */ struct hostent { }; char *h_name; /* official domain name of host */ char **h_aliases; /* null-terminated array of domain names */ int h_addrtype; /* host address type (AF_INET) */ int h_length; /* length of an address, in bytes */ char **h_addr_list; /* null-terminated array of in_addr structs */ 3

4 DB Type=NS Type=A Classes Each contains RR = Internet tuples format: DNS called (IN), (class, resource Chaosnet(CH), Records name, records value, etc. (RRs) valueis nameis class server domain IP hostname defines address of (e.g. authoritative this value FOR foo.com) domain Type=CNAME associated IN class: Type=MX with nameis valueis some mailserverassociated type type, canonical (the hostname alias ttl) name of for real) with 10 Different Properties Multiple Single Some Simple for valid domain example: case: kinds domain 1-1 name of names mapping of names mappings maps maps DNS between to don t multiple to are map the domain Host same to possible: IP multiple any addresses: IP name IP IP Entries address: and addrs. IP addr: 11 cs.wisc.edu gwuucbwisccmumit DNS orgnetdesign: edu root (.) comhierarchy uk Definitions wail csee Each E.g., stores end would Suffix Fred.com Fred.edu Fred.wisc.edu Fred.cs.wisc.edu Fred.cs.cmu.edu with given node following a list same = this path of hierarchy suffix tree, names be up stored: tree where that 12 kittyhawk.cmcl.cs.cmu.edumaps to eecs.mit.eduandcs.mit.eduboth map to aol.comandwww.aol.commap to 4

5 DNS gwuucbcmubumit orgnetcmcl Design: csece edu rootcomca ukzone Zone A of set Definitions Single E.g., Must zone name node and of = Complete Tree Subtree name Complete contiguous has store node space subtree links an servers list associated tree, of section names single 13 Zones DNS Design: Cont. Example: to Records Primary/master Secondary/redundant servers create/delegate are within created zone name by a store subzone convincing multiple redundant owner manually node CS.WISC.EDU Who transfer administrators Zone a DNS creates transfer server of name WISC.EDU uses created is a space bulk TCP servers transfer by or ensure WISC.EDU.EDU? of updated reliability the configuration of by zone name 14 Responsible Approx. Local root zone servers contact when resolve Currently DNS: m}.root-servers.net they name worldwide 13 cannot for {a- name Root Name Servers Configured well-known a name with 15 5

6 Each Name Typically Resolves Either host Servers/Resolvers Local /etc/resolv.conf) Do contacts has a library a resolver Typically lookup name servers responsible answer of servers distant queries name that for hand-configured host some applications about server names zone local for zone local or can (e.g. hosts link to Typical Resolution 16 Can Steps Application for resolving calls gethostbyname() mean? What S2returns S1queries S3returns Resolvercontacts This (PREFETCHING) about is what multiple root S3for NS A A record the server local additional A for for (S2) records S3? wisc.edu(s3) information server ( what (RESOLVER) (S1) section does is for this 17 Recursive Only Iterative Server I searches (recursive) much (iterative) not returns found goes responds query: for knows out Lookup final more and with answer info as Methods Workload Local Root/distant but does don t ask recursive server impact this know server typically this choice? name, does local requesting surf.eurecom.fr dns.eurecom.fr name 12 serverroot name server host 83456authoritative intermediate 7dns.umass.edu iterated name dns.cs.umass.edu gaia.cs.umass.edu server query 18 name 6

7 Are DNS Why all Workload servers/names might this be a problem? likely to and How be equally can Caching Cached Quick Other Don t E.g. NS responses queries records are for may for domains cached reuse repeated some translations parts of we lookup popular? Lifetime negative misspellings, data have periodically to queries repeat search are past times strings cached mistakes out in resolv.conf solve this problem? TTL passed (TTL) with of every data record controlled by owner of data 19 resolverlocal Client Typical DNS serverwww.cs.wisc.edu NS Resolution A www=ipaddr ns1.wisc.edu ns1.cs.wisc.edu ns1.cs.wisc.edu ns1.wisc.edu root DNS server DNS & 20 ClientLocal Subsequent ftp.cs.wisc.edu DNS serverlookup ftp=ipaddr ftp.cs.wisc.eduexample root DNS cs.wisc.edu server DNS & server 21 7

8 Try DNS Name servers service are available Reliability replicated Same UDP Why Exponential Queries Need used reliability can for be queries load must balanced if implement one between replica this is replicas on up Don t alternate identifier not care just which backoffwhen servers use for TCP? all on responds queries retrying timeoutsame server top of UDP! 22 in-addr 128 Reverse Task Given DNS 2194cskittyhawk Method Managing When Maintain Write Authority based in-addr.arpa assigned Why IP is on is this address, to separate IP the it names needed? address find hierarchy IP as reversed? addresses its name cmcl cmu edu unnamed root 242arpa E.g., in-addr.arpa CMU manages name space 23 Name Typically response CNAME/MX/NS Responses host servers Prefetching to in name additional used include can for section typically address prefetching additional point of host to another referred data to 24 8

9 Generic Country Root.net,.ca, to Load DNS.org, Top Code Level Today: Top Level Domains Domain Root (gtld) (cctld) Zone Moving.fi, server.uk, etc ({a-m}.root-servers.net) =.com, necessary cover on.com, gtlddomains root to.net, reduce.org load was off done growing Aug quickly! 2000 was also = clearly used.us, 25.info.biz.aero.coop.name.museum.pro businesses general air-transport individuals New info cooperatives gtlds Only accountants, new museums one actives lawyers, so industry far = and.info, physicians.biz,.name26 No Hit Most Each centralized machine caching runs own Performance per caching site Lower Why How many this hosts problem? do we need to share local server DNS Is What This suggest objects rate performance Internet this TTLsfor alone does good needs DNS accounts a traffic typical A hosts bad? = records really % for page transfers is 1 relies 80% Web look does -(#DNS/#connections) hit like? more not rate! average affect cache? NS-record performance recent of 4-5 caching imbedded studies 27 9

10 Motivations Hierarchical Caching/prefetchingand Scalability Independent Robustness large update Summary Reverse Zones How is name a lookup database lookup donedistributed structure TTLsdatabase 28 10