1 Kyung Hee University Chapter 18 Domain Name System

2 Kyung Hee University Introduction  Mapping a name to an address or address to a name  When the Internet was small, information for mapping is stored in host file The host file would be too large to store in every host One solution is to store the entire host file in a single computer Another solution is to divide the information into smaller parts and storing each part on a different computer l The host that needs mapping can contact the closest computer l This method is used by the Domain Name System

3 Kyung Hee University 16.1 Name Space  The names should be unique because the addresses are unique.  Name space that maps each address to a unique name can be organized in two ways : Flat Name Space l Name is assigned to an address l A name in space is a sequence of characters without structure l The names may or may not have a common section Hierarchical Name Space l The first part : the nature of the organization l The second part : the name of organization l The third part : departments of the organization

4 Kyung Hee University 16.2 Domain Name Space  The tree can have only 128 levels: level 0 (root) to level 127  Label Each node in the tree has a label, which is a string with a maximum of 63 characters.  Domain name A full domain name is a sequence of labels separated by dots (.)  Fully Qualified Domain Name (FQDN) A label that is terminated by a null string l challenger.stc.fhda.edu.

5 Kyung Hee University Domain Name Space (cont’d)  Domain name space

6 Kyung Hee University Domain Name Space (cont’d)  Domain name and labels

7 Kyung Hee University Domain Name Space (cont’d)  Partially Qualified Domain Name (PQDN) A label that is not terminated by a null string used when the name to be resolved belongs to the same site as the client for example, if a user at the fhda.edu. site wants to get the IP address of the challenger computer, a resolver can supply the missing part, called the suffix as follows. - challenger.atc.fhda.edu The DNS client normally holds a list of suffixes. The following can be the list of suffixes at De Anza College atc.fhda.edu fhda.edu null

8 Kyung Hee University Domain Name Space (cont’d)  FQDN and PQDN

9 Kyung Hee University Domain Name Space (cont’d)  Domain A domain is a subtree of the domain name space the name of the domain is the domain name of the node at the top of the subtree

10 Kyung Hee University 16.3 Distribution of Name Space  Hierarchy of Name Servers The solution to distribute a huge amount of information is to use DNS servers l dividing the whole space into many domains based on the first level

11 Kyung Hee University Distribution of Name Space (cont’d)  Zone What a server is responsible for or has authority over if a sever accepts responsibility for a domain and does not divide the domain into smaller domains, the “domain” and the “zone” refer to the same thing. l the server makes a database, zone file ; it keeps all the information for every node under that domain. If a sever divides its domain into subdomains and delegates part of its authority to other servers, “domain” and “zone” refer to different things.

12 Kyung Hee University Distribution of Name Space (cont’d)  Zone

13 Kyung Hee University Distribution of Name Space (cont’d)  Root Server a server whose zone consists of the whole tree not storing any information about domains but delegates its authority to other servers, keeping references to those servers currently there are more than 13 root servers in the world, each covering the whole domain name space  Primary and Secondary Servers Primary server : storing a file about the zone for which it is an authority; it can create, maintain and update the zone file Secondary server : transferring the complete information about a zone from another server (primary or secondary) and storing the file on its local disk. If updating is required, it must be done by the primary server l it can be used as the redundant data when a server fails

14 Kyung Hee University 16.4 DNS in the Internet  In the Internet, the domain name space (tree) is divided into three different sections: generic domains country domains inverse domain

15 Kyung Hee University DNS in the Internet (cont’d)  Generic Domains defining registered hosts according to their generic behavior each node in the tree defines a domain  Generic domains labels Label Description Commercial organization Educational institution Government institution International organizations Military groups Network support centers Nonprofit organizations com edu gov mil int org net

16 Kyung Hee University DNS in the Internet (cont’d)

17 Kyung Hee University DNS in the Internet (cont’d)  Country Domains using two-character country abbreviation

18 Kyung Hee University  Inverse Domain used to map an address to a name When a server has received a request from a client to do a task l to determine if the client is on the authorized list, the server can ask its resolver to send a query to the DNS server and ask for a mapping of address to name Convention of reading the domain labels from the bottom to the top, IP address such as l in-addr.arpa. DNS in the Internet (cont’d)

19 Kyung Hee University DNS in the Internet (cont’d)

20 Kyung Hee University 16.5 Resolution  name-address resolution : mapping a name to an address or an address to a name  Resolver A host that needs to map an addresses to a name or a name to an address calls a DNS client called a resolver. l The resolver accesses the closest DNS server with a mapping request  Mapping names to Addresses 1) resolver gives a domain name to the server and asks for the corresponding address 2) the sever checks the generic domains or the country domains to find the mapping

21 Kyung Hee University Resolution (cont’d)  Mapping Addresses to Names using PTR query DNS uses the inverse domain 1)If the resolver receives the IP address , the resolver first inverts the address and then adds the two labels before sending 2) the domain name sent is in-addr.arpa, which is received by the local DNS and resolved

22 Kyung Hee University Resolution (cont’d)  Recursive Resolution recursively asking for the answer to the authoritative server

23 Kyung Hee University Resolution (cont’d)  Iterative Resolution if the server is an authority for the name, it sends the answer. If it is not, it returns (to the client) the IP address of the server that it thinks can resolve the query the client is responsible for repeating the query to this second server if the newly addressed server can resolve the problem, it answers the query with the IP address;otherwise, it returns the IP address of a new server to the client

24 Kyung Hee University Resolution (cont’d)

25 Kyung Hee University Resolution (cont’d)  Cashing to reduce the search time When a sever asks for a mapping from another sever and receives the response, it stores this information in its cache memory before sending it to the client use of TTL to prevent an outdated mapping

26 Kyung Hee University 16. DNS Messages  DNS has two types of messages: query and response

27 Kyung Hee University DNS Messages (cont’d)  Query and response messages

28 Kyung Hee University DNS Messages (cont’d)  Header format Both query and response messages have the same header format with some fields set to zero for the query messages (12 bytes) l Identification : 16 bits field used by the client to match the response with the query

29 Kyung Hee University DNS Messages (cont’d)  Header format (cont’d) Flags field l QR (query and response) : query – 0, response – 1 l OpCode : defining the type of query and response – standard : 0, inverse : 1, server status report :2 l AA (authoritative answer) : if it is set to 1, its means that the name server is an authoritative server. l TC (truncated) : if it is set, it means that the response was more than 512 bytes and truncated to 512. It is used when DNS uses the services of UDP l RD (recursion desired) : meaning the client desires a recursive answer l RA (recursion available) : meaning that a recursive response is available. Used in response message l Reserved l rCode : showing the status of the error in the response

30 Kyung Hee University DNS Messages (cont’d) l Values of rCode Value Meaning 0 No error 1 Format error 2 Problem at name server 3Domain reference problem 4Query type not supported 5Administrative prohibited 6-15Reserved

31 Kyung Hee University DNS Messages (cont’d) Number of question records : 16-bit field containing the number of queries in the question of the message Number of answer records : 16-bit field containing the number of answer records in the answer section of the response message Number of authoritative records : 16-bit field containing the number of authoritative records in the authoritative section of a response message Number of additional records : 16-bit field containing the number of additional records in the additional section of a response message.

32 Kyung Hee University 16.7 Types of Records  Question Record l used by the client to get information from a server. This contains domain name. l Question record format l Query name format

33 Kyung Hee University Types of Records (cont’d)  Query type (table 16.4)  Query class : defining the specific protocol using DNS (Table 16.5)

34 Kyung Hee University Types of Records (cont’d)  Resource Record Each domain name (each node on the tree) is associated with a record called the resource record. The server database consists of resource records Returned by the server to the client Domain name Domain type : same as the query type field in the question section Domain class : same as query class field Time to live Record data length

35 Kyung Hee University Types of Records (cont’d) Resource data l containing the answer to the query (in the answer section) or the domain name of the authoritative server (in the authoritative section) or additional information (in the additional information (in the additional information section) l A number : written in octets. IPv4 address is a four-octet integer and IPv6 address is a 16-octet integer l A domain name : expressed as a sequence of labels

36 Kyung Hee University Types of Records (cont’d)  Resource Record

37 Kyung Hee University 16.8 Compression  DNS requires that a domain name be replaced by an offset pointer when it is repeated.  Format of an offset pointer

38 Kyung Hee University DDNS  The DNS master file must be updated dynamically the change such as adding a new host, removing host, or changing an IP address.  In DDNS, when a binding between a name and address is determined, information is sent, usually by DHCP to a primary DNS server.

39 Kyung Hee University Encapsulation  using either UDP or TCP UDP : used when the size of the response message is less than 512 bytes  well-known port used by the server is port 53