Application Protocols in the TCP/IP Reference Model. Application Protocols in the TCP/IP Reference Model. DNS - Concept. DNS - Domain Name System

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1 Application Protocols in the TCP/IP Reference Model Application Protocols in the TCP/IP Reference Model File Transfer Network Management Protocols of the application layer are common communication services WWW Virtual Terminal Service File Transfer Protocols of the application layer are defined for special purposes and specify HTTP FTP Telnet SMTP DNS SNMP TFTP Thetypes of the sent messages Thesyntax of the message types Internet protocols TCP UDP Thesemantics of the message types Rules for definition, when and how an application process sends a message resp. responses to it ARP RARP IP ICMP IGMP Layer 1/2 Ethernet Token Ring Token Bus Wireless LAN Usually: Client/ structure. Processes on the application layer are using TCP(UDP)/IP-Sockets Page 1 Page 2 DNS - Domain System DNS - Concept Top level Domain de rwth-aachen informatik IP addresses are difficult to remember for humans, but computers can deal with them perfectly. Symbolic s are simpler for humans to handle, but computers can unfortunately not deal with them. DNS provides a mapping between symbolic s and IP addresses metatron.informatik.rwth-aachen.de DNS manages the mapping of logical computer s to IP addresses (and further services) 2. DNS is a distributed database, i.e. the individual segments are subject to local control 3. The structure of the used space of the database shows the administrative organization of the Internet 4. Data of each local area are available by means of a Client/ architecture in the entire network 5. Robustness and speed of the system are being achieved by replication and caching of the naming data 6. Main components: : which manages information about a part of the database : Client which requests naming information from the Request Response Page 3 Page 4

2 DNS Distributed Architecture Structure of the Database User Program User Request User Response References Requests Responses Remote For structuring of all information: the database can be represented as a tree Each node of the tree is marked with a label, which identifies it relatively to the father node Each (internal) node is root of a sub-tree Each of those sub-trees represents a domain Each domain can be divided into sub-domains Shared Database Domain Master Files Update References Responses Requests Remote Sub-domain com edu gov mil Oxford se de rwth-aachen Administrative Requests Administrative Responses Remote cs Generic informatik Countries Page 5 Page 6 Domain s Space The of a domain consists of the sequence of labels (separated by. ) beginning with the root of the domain and going up to the root of the whole tree In the nodes the IP addresses associated with the s given by the label sequence are being stored The s of the domains serve as index for the distributed mapping database The data associated with a domain are stored in socalled Resource Records (RR) logical : metatron.informatik.rwth-aachen.de rwth-aachen informatik metatron de Associated IP address: The reverse tree represents the Domain Space The depth of the tree is limited to 127 levels, domain s can have up to 63 characters, a label of the length 0 is reserved for the root node ( ) TheFully Qualified Domain (FQDN) is the absolute domain, which is declared with reference to the root of the tree, e.g informatik.rwth-aachen.de. Domain s which are declared not with reference to the root of the tree, but with reference to another domain, are called relative domain s Leafs of the tree represent individual computers and refer to network addresses, hardware information and mail routing information Internal nodes of the tree can describe both a computer and a domain Domains are denoted often relatively or regarding their level: Top-Level Domain/First Level Domain: child of the root node Second Level Domain: child of a first level of domain etc. Page 7 Page 8

3 s and Zones Information about the space are stored in s s manage the whole information for a certain part of the space this part is called zone The information about a zone is loaded either from a file or from another The has the authority for the zone A can be responsible for several zones Zones are (except within the lowest levels of the tree) smaller than domains, therefore s have to manage less information There are no guidelines how domains are divided into zones. Each domain can select a division for itself com berkeley berkeley.edu zone edu nwu edu domain edu zone org purdue purdue.edu zone Delegation Page 9 Types of s Each zone needs an own to manage all the mappings in the zone each zone has to have exactly one primary, and for protection against failure at least one secondary ThePrimary of a zone (also called Master) reads the data from a file configured by an administrator. A Secondary of a zone (also called Slave) receives the data from another, which is authoritative for the zone. In most cases this is the primary master. A secondary master can receive the data however also from another secondary master. When a secondary master is started, it contacts the master and loads, if necessary, the zone data (zone transfer). Both, the primary master and the secondary masters are authoritative for the zone. The distinction between primary master and secondary master serves for a controlled replication of the data and thus increases both, the performance and the fault tolerance. Page 10 Data of a Zone Resolution The primary master reads the zone data from appropriate files (Zone Data Files) A secondary master can likewise read its zone data from these files A secondary master usually saves the data received from a primary master in appropriate files With a restart of a secondary master it first reads the saved data in the files to determine whether these are current The backup copies thus prevent unnecessary zone data transfer and do serve at the same time as additional source if the primary master is not available The files contain Resource Records which describe the zone s information The resource records describe all computers in the zone as well as information concerning the delegation of sub-domains Generally mapping of s to addresses The term Resolution also designates the process, in which a searches the space for data, for which he is not responsible For the searching, a needs the domain and the addresses of the root s A can ask a root for each in the space Root s know the responsible s for each top-level domain On request, a root can return s and addresses of s responsible for the top-level domain of the searched The top level again manages references to s which are responsible for the second level domain If additional information is missing, each search begins with the root s Page 11 Page 12

4 Recursive and Iterative Resolution Recursive and Iterative Resolution Distinction between recursive and iterative requests resp. recursive and iterative resolution In case of recursive resolution, a resolver sends a recursive inquiry to a. The must answer either with the searched information or an error message, i.e. the may not refer to another. The itself is responsible to contact as much other s as necessary to find the searched information In an iterative resolution, an asked answers with the searched information if he helds it, or only with the address of another responsible Request recursive Response Reference to au Reference to gov.au Reference to gbrmpa.gov.au Address of root root au au gov.au gov.au gbrmpa.gov.au gbrmpa.gov.au iterative au gov nz edu sg sa ips gbrmpa Page 13 Page 14 Root Caching & Time to Live Requests to which a cannot answer, are handed upward in the tree on the upper levels are heavily loaded Inquiries, which go into another zone, often run over the root Thus, the root must always be available Therefore: replication - there are 13 instances of the root, more or less distributed over the whole world Problem: very central placement of the s! How could we avoid that each request has to be passed to a root? Caching is the process of buffering information in a not responsible for those information. In further requests these information are present and the resolution process can be speeded up Stored are not only information about the requested hosts, but additionally all information about other s used in the resolution process TheTime to Live (TTL) indicates how long data are allowed to be buffered The TTL guarantees that no outdated information is used Small TTL gives a high consistency Large TTL gives a faster resolution of a Page 15 Page 16

5 Resource Record Example: Resource Records in a Zone File Entries in the zone data files the are resource records General structure: (label, ttl, class, type, value) SOA A MX SRV NS CNAME PTR TXT Type HINFO Used in Zone Host Domain Domain Zone Node Host Host arbitrary Indicates the authority for the zone data Contains the IP address of a host needed for resolution Refers to the mail of the domain Refers to a which offers a certain service in the domain Refers to a responsible for the zone Canonical, i.e. reference to the actual node Used for the mapping of an address to a Additional information to the host (CCU, operating system) Other useful information Description ripe.net 7200 IN SOA ns.ripe.net. olaf.ripe.net. ( Serial Refresh 12 hours Retry 4 hours Expire 4 days 7200 Clear cache 2 hours ) ripe.net 7200 IN NS ns.ripe.net. ripe.net 7200 IN NS ns.eu.net. pinkje.ripe.net 3600 IN A host25.ripe.net 2600 IN A Label ttl class type value IN = Internet addresses Page 17 Page 18 Example: Resource Records in a Zone File SOA Record SOA = Start of Authority It indicates that the is authorative for the zone There can be only one SOA record in an appropriate file of Master address of contact person. First. Example: movie.edu IN SOA terminator.movie.edu al.robocop.movie.edu ( Serial Refresh after 3 hours 3600 Retry after 1 hour Version number Expire after 1 week 86400) Minimum TTL OF 1 day Timing data for the zone Page 19 Page 20

6 SOA Record NS Record Attributes of the SOA record: Serial: Serial number which serves the secondary master for the recognition of new versions of the zone data Refresh: Time interval, at whose expiration the secondary master examines the topicality of its data Retry: time interval if the secondary master cannot contact the primary master at expiration of the refresh time, then it tries again after expiration of the retry time interval Expire: if the secondary master cannot contact the primary master after the indicated length of time, it stops answering inquiries because it must assume its data is outdated TTL: Refers to all resource records. This value is returned as part of the answer on a request to instruct other s about the maximal time for caching the data. NS = For each of a zone a NS record is created Example: movie.edu. IN NS terminator.movie.edu movie.edu. IN NS wormhole.movie.edu There are two s, which are in the regarded example installed on the computers terminator and wormhole Page 21 Page 22 Address and Alias Records Address and Alias Records A = ADDRESS CNAME = Canonical At least one A record is needed for each host in the zone, CNAME records are optional and have the purpose of aliasing a machine with a second Example: Host addresses localhost.movie.edu. IN A robocop.movie.edu. IN A terminator.movie.edu. IN A diehard.movie.edu. IN A misery.movie.edu. IN A shining.movie.edu. IN A carrie.movie.edu. IN A Multihomed host wormhole.movie.edu IN A wormhole.movie.edu IN A Page 23 Aliases bigt.movie.edu. IN CNAME terminator.movie.edu. dh.movie.edu. IN CNAME diehard.movie.edu. wh.movie.edu. IN CNAME wormhole.movie.edu. wh249.movie.edu. IN A wh253.movie.edu. IN A A = ADDRESS CNAME = illustrates an alias on its canonical s For multihomed computers (connected with several networks), an own A record is needed for every secondary if different aliases are to be stored for the addresses For a secondary, which applies to both addresses, a CNAME record is created Page 24

7 PTR Record MX Record PTR = Pointer Provides information for the mapping of addresses to s For this, an extra subtree in-addr.arpa is introduced to the tree Example: in-addr.arpa. IN PTR wormhole.movie.edu in-addr.arpa. IN PTR robocop.movie.edu in-addr.arpa. IN PTR terminator.movie.edu in-addr.arpa. IN PTR diehard.movie.edu. Addresses should refer only one, the original or canonical MX = Mail Exchanger MX record serves for the controlling of routing Specifies a mail responsible for a domain, which processes or passes on Additionally, a preference can be indicated if several mail s are present Example: peets.mpk.ca.us. IN MX 10 relay.hp.com. indicates that relay.hp.com is the mail for peets.mpk.ca.us with the preference 10 e.g. if sending the the sending mail- contacts DNS and finds out that the needs to be sent to relay.hp.com Only the relative preference value is important the mail with the smallest value is addressed first Page 25 Page 26 DNS Protocol DNS defines only one protocol format, which is used both for inquiries and for responses: Identification: 16 bits for the definite identification of an inquiry, to match requests and responses Flag: 4 Bit, marking of (1) request/response, (2) authorative/not authorative, (3) iterative/recursive, (4) recursion possible Number of : Indication of the contained number of inquiries resp. data records Questions: s to be resolved Answers: Resource records to the previous inquiry Authority: Identification of passed responsible s Additional information: further data to the inquiry. If the searched is only an alias, the belonging resource record for the correct is placed here Identification Number of Questions Number of Authority RR Flag Number of Answers RR Number of Additional RR Questions (variable number of RR) Answers (variable number of RR) Authority (variable number of RR) Additional information (variable number of RR) Page 27 DNS Conclusion All application layer protocols use DNS: Application layer protocols use own symbolic/logic s When a user tries to use an application (e.g. the WWW), the corresponding application process first uses DNS for getting the IP address and installing a TCP connection to the destination machine DNS is not designed for many changes of information there is no guarantee that the knowledge you receive is still valid Especially problematic with mobile devices which keep their symbolic while changing the IP address Thus: as IPv4, DNS was thought for smaller systems than the current Internet with change to IPv6, also DNS should be changed Page 28

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