115-441 Computer NetworkingLecture 13 – DNSLecture 13: 02-22-2005 2Outline• DNS Design• DNS TodayLecture 13: 02-22-2005 3Naming• How do we efficiently locate resources?• DNS: name Æ IP address• Challenge• How do we scale these to the wide area?Lecture 13: 02-22-2005 4Obvious Solutions (1)Why not centralize DNS?• Single point of failure• Traffic volume• Distant centralized database• Single point of update• Doesn’t scale!2Lecture 13: 02-22-2005 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 Æ machine per user• Many more downloads• Many more updatesLecture 13: 02-22-2005 6Domain 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 consistencyLecture 13: 02-22-2005 7Programmer’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 */ }; Lecture 13: 02-22-2005 8DNS 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 bytes3Lecture 13: 02-22-2005 9DNS 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 resolutionLecture 13: 02-22-2005 10DNS 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 mailserverassociated with nameLecture 13: 02-22-2005 11Properties 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.eduLecture 13: 02-22-2005 12DNS 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.edu4Lecture 13: 02-22-2005 13DNS 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 linksLecture 13: 02-22-2005 14DNS 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?Lecture 13: 02-22-2005 15DNS: 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 serversLecture 13: 02-22-2005 16Servers/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 zone5Lecture 13: 02-22-2005 17Typical ResolutionClientLocal DNS serverroot & eduDNS serverns1.cmu.edu DNS serverwww.cs.cmu.eduNS ns1.cmu.eduwww.cs.cmu.eduNS ns1.cs.cmu.eduA www=IPaddrns1.cs.cmu.eduDNSserverLecture 13: 02-22-2005 18Typical Resolution• Steps for resolving www.cmu.edu• Application calls gethostbyname() (RESOLVER)• Resolver contacts local name server (S1)• S1queries root server (S2) for (www.cmu.edu)• S2returns NS record for cmu.edu (S3)• What about A record for S3?• This is what the additional information section is for (PREFETCHING)• S1queries S3for www.cmu.edu• S3returns A record for www.cmu.edu• Can return multiple A records Æ what does this mean?Lecture 13: 02-22-2005 19Lookup 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
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