Slide 1OutlineNamingObvious Solutions (1)Obvious Solutions (2)Domain Name System GoalsProgrammer’s View of DNSDNS Message FormatDNS Header FieldsDNS RecordsProperties of DNS Host EntriesDNS Design: Hierarchy DefinitionsDNS Design: Zone DefinitionsDNS Design: Cont.DNS: Root Name ServersServers/ResolversTypical ResolutionTypical ResolutionDNS Hack #1Lookup MethodsWorkload and CachingTypical ResolutionSubsequent Lookup ExampleReliabilityReverse DNS.arpa Name Server HierarchyPrefetchingMail AddressesOutlineRoot ZonegTLDsNew RegistrarsTracing Hierarchy (1)Tracing Hierarchy (2)Tracing Hierarchy (3 & 4)Do you trust the TLD operators?Protecting the Root NameserversDefense: Replication and CachingDNS Hack #2: Blackhole ListsDNS (Summary)15-441 Computer NetworkingLecture 13 – DNSCopyright ©, 2007-10 Carnegie Mellon University2Outline•DNS Design•DNS Today3Naming•How do we efficiently locate resources?•DNS: name IP address•Challenge•How do we scale this to the wide area?4Obvious Solutions (1)Why not centralize DNS?•Single point of failure•Traffic volume•Distant centralized database•Single point of update•Doesn’t scale!5Obvious Solutions (2)Why not use /etc/hosts?•Original Name to Address Mapping•Flat namespace•/etc/hosts •SRI kept main copy•Downloaded regularly•Count of hosts was increasing: machine per domain machine per user•Many more downloads•Many more updates6Domain Name System Goals•Basically a wide-area distributed database•Scalability•Decentralized maintenance•Robustness•Global scope •Names mean the same thing everywhere•Don’t need•Atomicity•Strong consistency7Programmer’s View of DNS•Conceptually, programmers can view the DNS database as a collection of millions of host entry structures:•Functions for retrieving host entries from DNS:•getaddrinfo: query key is a DNS host name.•getnameinfo: query key is an IP address./* DNS host entry structure */ struct addrinfo { int ai_family; /* host address type (AF_INET) */ size_t ai_addrlen; /* length of an address, in bytes */ struct sockaddr *ai_addr; /* address! */ char *ai_canonname; /* official domain name of host */ struct addrinfo *ai_next; /* other entries for host */ };9DNS Message FormatIdentificationNo. of QuestionsNo. of Authority RRsQuestions (variable number of answers)Answers (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 bytes10DNS Header Fields•Identification•Used to match up request/response•Flags•1-bit to mark query or response•1-bit to mark authoritative or not•1-bit to request recursive resolution•1-bit to indicate support for recursive resolution11DNS 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 name12Properties 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 to 18.62.1.6•Single domain name maps to multiple IP addresses:•aol.com and www.aol.com map to multiple IP addrs.•Some valid domain names don’t map to any IP address:•for example: cmcl.cs.cmu.edu13DNS Design: Hierarchy Definitionsrootedunetorgukcomgwu ucb cmu bumitcsececmcl•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.cmu.edu•Fred.cmcl.cs.cmu.edu•Fred.cs.mit.edu14DNS 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•Must store list of names and tree links15DNS Design: Cont.•Zones are created by convincing owner node to create/delegate a subzone•Records within zone stored 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•Example:•CS.CMU.EDU created by CMU.EDU administrators•Who creates CMU.EDU or .EDU?16DNS: 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 servers•Newer picture www.root-servers.org17Servers/Resolvers •Each host has a resolver•Typically a library that applications can link to•Local name servers hand-configured (e.g. /etc/resolv.conf)•Name servers•Either responsible for some zone or…•Local servers•Do lookup of distant host names for local hosts•Typically answer queries about local zone18Typical ResolutionClientLocal DNS serverroot & edu DNS serverns1.cmu.edu DNS serverwww.cs.cmu.eduNS ns1.cmu.eduwww.cs.cmu.eduNS ns1.cs.cmu.eduA www=IPaddrns1.cs.cmu.eduDNSserver19Typical Resolution•Steps for resolving www.cmu.edu•Application calls gethostbyname() (RESOLVER)•Resolver contacts local name server (S1)•S1 queries root server (S2) for (www.cmu.edu)•S2 returns NS record for cmu.edu (S3)•What about A record for S3?•This is what the additional information section is for (PREFETCHING)•S1 queries S3 for www.cmu.edu•S3 returns A record for www.cmu.eduDNS Hack #1•Can return multiple A records what does this mean?•Load Balance•Server sends out multiple A records•Order of these records changes per-client2021Lookup MethodsRecursive query:•Server goes out and searches for more info
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