network protocols handbook.pdf

16Protocols GuideTCP/IP - Application Layer ProtocolsProtocol NameDNS: Domain Name System(Service) protocolProtocol DescriptionDomain Name System (DNS) is a distributed Internet directoryservice. DNS is used mostly to translate between domainnames and IP addresses and to control Internet email delivery.Most Internet services rely on DNS to work, and if DNS fails,web sites cannot be located and email delivery stalls.DNS has two independent aspects:1. It specifies the name syntax and rules for delegating authorityover names. The basic syntax is:local.group.site2. It specifies the implementation of a distributed computing systemthat efficiently maps names to addresses.In the DNS naming scheme, a decentralized and hierarchicalmechanism is used by the delegating authority for parts of thenamespace and distributing responsibility for mapping namesand addresses. The naming scheme of DNS is used to assignnetwork device names globally and is implemented by geographicallydistributed sets of severs to names to addresses.In theory, the domain name standard in DNS protocol specifiesan abstract hierarchical namespace with arbitrary values for labels.Any group can build an instance of the domain system tochoose labels for all parts of its hierarchy. However most usersof the DNS protocols follow the hierarchical labels used by theofficial Internet domain system. Some of the top level domainsare: COM, EDU, GOV, NET, ORG, BIZ ... plus many countrycodes.The distributed scheme of DNS allows efficient and reliable mappingof names to IP addresses. Most names can be mappedlocally and a set of servers operating at multiple sites cooperativelysolve the mapping problem of a large network. Because ofthe distributing nature, no single machine failure will prevent theDNS from operating correctly.Protocol StructureQuery 4-bit field that describes the type of message:0 Standard query (name to address); 1 Inversequery; 2 Server status request.A Authoritative Answer. 1-bit field. When set to 1,identifies the response as one made by an authoritativename server.T Truncation. 1-bit field. When set to 1, indicatesthe message has been truncated.R 1-bit field. Set to 1 by the resolve to request recursiveservice by the name server.V 1-bit field. Signals the availability of recursiveservice by the name server.B 3-bit field. Reserved for future use. Must be setto 0.Rcode Response Code. 4-bit field that is set by thename server to identify the status of the query.Question count 16-bit field that defines the numberof entries in the question section.Answer count 16-bit field that defines the numberof resource records in the answer section.Authority count 16-bit field that defines the numberof name server resource records in the authoritysection.Additional count 16-bit field that defines the numberof resource records in the additional recordssection.Related protocolsIP, TCP, IGMP, ICMP, SNMP, TFTP and NFSSponsor SourceDNS is defined by IETF (http://www.ietf.org) RFC1034 andupdated by 1035, 1101, 1183, 1348, 1876, 1982, 2181, 2308,2535Referencehttp://www.javvin.com/protocol/rfc1034.pdfDomain Names – Concept and FacilitiesIDQ16 21 28 32bitID Q Query A T R V B RcodeQuestion countAnswer countAuthority countAdditional count16-bit field used to correlate queries and responses.1-bit field that identifies the message as a queryor response.