1CS640: Computer NetworksAditya AkellaLecture 17Naming and the DNS2Naming• Need naming to identify resources• Once identified, resource must be located• How to name resource?– Naming hierarchy• How do we efficiently locate resources?– DNS: name location (IP address)• Challenge: How do we scale these to the wide area?3/ETC/HOSTSWhy not use /etc/hosts?• Original Name to Address Mapping–Flatnamespace– Lookup mapping in /etc/hosts – SRI kept main copy– Downloaded regularly• Count of hosts was increasing: machine per domain machine per user– Many more downloads– Many more updates24Domain Name System Goals• Basically a wide-area distributed database of name to IP mappings• Goals:– Scalability– Decentralized maintenance– Robustness5DNS RecordsRR format: (class, name, value, type, ttl)• DB contains tuples called resource records (RRs)– Classes = Internet (IN), Chaosnet (CH), etc.– Each class defines value associated with typeFOR IN class:• Type=A– name is hostname– value is IP address• Type=NS– name is domain (e.g. foo.com)– value is name of authoritative name server for this domain• Type=CNAME– name is an alias name for some “canonical” (the real) name– value is canonical name• Type=MX– value is hostname of mailserver associated with name6Properties of DNS Host Entries• Different kinds of mappings are possible:– Simple case: 1-1 mapping between domain name and IP addr:• kittyhawk.cmcl.cs.cmu.edu maps to 128.2.194.242– Multiple domain names maps to the same IP address:• eecs.mit.edu and cs.mit.edu both map to18.62.1.6– Single domain name maps to multiple IP addresses:• aol.com and www.aol.com map to multiple IP addrs.37Programmer’s View of DNS• Conceptually, programmers can view the DNS database as a collection of millions of host entry structures:– in_addr is a struct consisting of 4-byte IP address• Functions for retrieving host entries from DNS:– gethostbyname: query key is a DNS host name.– gethostbyaddr: query key is an IP address./* DNS host 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 */ }; 8DNS Design: Hierarchy Definitionsroot (.)edunetorgukcomgwu ucb wisc cmumitcseewail• Each node in hierarchy stores a list of names that end with same suffix• Suffix = path up tree• E.g., given this tree, where would following be stored:• Fred.com• Fred.edu• Fred.wisc.edu• Fred.cs.wisc.edu• Fred.cs.cmu.edu9DNS Design: Zone Definitionsrootedunetorgukcomcagwu ucb cmu bumitcsececmclSingle nodeSubtreeComplete Tree• Zone = contiguous section of name space• E.g., Complete tree, single node or subtree• A zone has an associated set of name servers• Knows mappings for zone410DNS Design: Cont.• Zones are created by convincing owner node to create/delegate a subzone– Records within zone store multiple redundant name servers– Primary/master name server updated manually– Secondary/redundant servers updated by zone transfer of name space• Zone transfer is a bulk transfer of the “configuration” of a DNS server – uses TCP to ensure reliability– The owner node creates an NS record for the sub-zone• Points to the name server for the new sub-zone• Example:– CS.WISC.EDU created by WISC.EDU administrators11Servers/Resolvers• Each host has a resolver– Typically a library that applications can link to– Resolver contacts name server– Local name servers hand-configured (e.g. /etc/resolv.conf)• Name servers– Either responsible for some zone• Has mappings for all names in zone– Or knows of name servers for sub-zones• These servers know better about names in sub-zones– Names for which no mapping is known, direct requestor to root12DNS: Root Name Servers• Responsible for “root” zone• Approx. 13 root name servers worldwide– Currently {a-m}.root-servers.net• Local name servers contact root servers when they cannot resolve a name– Configured with well-known root servers513Typical Resolution• Steps for resolving www.wisc.edu– Application calls gethostbyname() (RESOLVER)– Resolver contacts local name server (S1)– S1queries root server (S2) for (www.wisc.edu)– S2returns NS record for wisc.edu (S3)– What about A record for S3?• This is what the additional information section is for (PREFETCHING)– S1queries S3for www.wisc.edu– S3returns A record for www.wisc.edu• Can return multiple A records what does this mean?14DNS Message FormatIdentificationNo. of QuestionsNo. of Authority RRsQuestionsAnswers (variable number of resource records)Authority (variable number of resource records)Additional Info (variable number of resource records)FlagsNo. of Answer RRsNo. of Additional RRsName, type fields for a queryRRs in response to queryRecords for authoritative serversAdditional “helpful info that may be used12 bytes15Lookup MethodsRecursive query:• Server goes out and searches for more info (recursive)• Only returns final answer or “not found”Iterative query:• Server responds with as much as it knows (iterative)• “I don’t know this name, but ask this server”Workload impact on choice?• Local server typically does recursive• Root/distant server does iterativerequesting hostsurf.eurecom.frgaia.cs.umass.eduroot name serverlocal name serverdns.eurecom.fr123467authoritative name serverdns.cs.umass.eduintermediate name serverdns.umass.edu58iterated query616Workload and Caching• Are all servers/names likely to be equally popular?– Why might this be a problem? How can we solve this problem?• DNS responses are cached – Quick response for repeated translations– Other queries may reuse some parts of lookup• NS records for domains • DNS negative queries are cached– Don’t have to repeat past mistakes– E.g. misspellings, search strings in resolv.conf• Cached data periodically times out– Lifetime (TTL) of data controlled by owner of data– TTL passed with every record17Typical ResolutionClientresolverLocal DNS serverroot & eduDNS serverns1.wisc.edu DNS serverwww.cs.wisc.eduNS ns1.wisc.eduwww.cs.wisc.eduNS ns1.cs.wisc.eduA www=IPaddrns1.cs.wisc.eduDNSserver18Subsequent Lookup ExampleClientLocal DNS
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