Chapter 17 Domain Name System

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 The host that needs mapping can contact the closest computer This method is used by the Domain Name System

17.1 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 Name is assigned to an address A name in space is a sequence of characters without structure The names may or may not have a common section Hierarchical Name Space The first part : the nature of the organization The second part : the name of organization The third part : departments of the organization

17.2 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 challenger.stc.fhda.edu.

Domain Name Space (cont’d)

Domain Name Space (cont’d) Domain name and labels

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

Domain Name Space (cont’d) FQDN and PQDN

Domain Name Space (cont’d) 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

17.3 Distribution of Name Space Hierarchy of Name Servers The information contained in the domain name space is distributed among many computers called DNS servers. The solution to distribute a huge amount of information is to use DNS servers dividing the whole space into many domains based on the first level

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. 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.

Distribution of Name Space (cont’d) Zone

Distribution of Name Space (cont’d) A primary server loads all information from the disk file; the secondary server loads all information from the primary server. When the secondary downloads information from the primary, it is called zone transfer.

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 it can be used as the redundant data when a server fails

17.4 DNS in the Internet In the Internet, the domain name space (tree) is divided into three different sections: generic domains country domains inverse domain

DNS in the Internet (cont’d) Generic Domains defining registered hosts according to their generic behavior each node in the tree defines a domain, which is an index to the domain name space database Generic domain labels

DNS in the Internet (cont’d)

DNS in the Internet (cont’d) Country Domains using two-character country abbreviation

DNS in the Internet (cont’d) Inverse Domain used to map an address to a name When a server has received a request from a client to do a task 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 132.34.45.121 121.45.34.132.in-addr.arpa.

DNS in the Internet (cont’d)

17.5 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. 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

Resolution (cont’d) Mapping Addresses to Names using PTR query DNS uses the inverse domain If the resolver receives the IP address 132.34.45.121, the resolver first inverts the address and then adds the two labels before sending the domain name sent is 121.45.34.132.in-addr.arpa, which is received by the local DNS and resolved

Resolution (cont’d) Recursive Resolution recursively asking for the answer to the authoritative server

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

Resolution (cont’d)

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

17.6 DNS Messages DNS has two types of messages: query and response The DNS query message consists of a header and question records; the DNS response message consists of a header, question records, answer records, authoritative records, and additional records.

DNS Messages (cont’d) Query and response messages

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) Identification : 16 bits field used by the client to match the response with the query

DNS Messages (cont’d) Flags field Header format (cont’d) QR (query and response) : query – 0, response – 1 OpCode : defining the type of query and response standard : 0, inverse : 1, server status report :2 AA (authoritative answer) : if it is set to 1, its means that the name server is an authoritative server. 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 RD (recursion desired) : meaning the client desires a recursive answer RA (recursion available) : meaning that a recursive response is available. Used in response message Reserved rCode : showing the status of the error in the response

DNS Messages (cont’d) Values of rCode

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.

17.7 Types of Records Two types of records are used in DNS. The question records are used in the question section of the query and response messages. The resource records are used in the answer, authoritative, and additional information sections of the response message. Question Record used by the client to get information from a server. This contains domain name. Question record format Query name format

Types of Records (cont’d) Query type (table 17.4) Query class : defining the specific protocol using DNS (Table 17.5)

Types of Records (cont’d) Query Types

Types of Records (cont’d) Query Classes

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

Types of Records (cont’d) Resource data 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) A number : written in octets. IPv4 address is a four-octet integer and IPv6 address is a 16-octet integer A domain name : expressed as a sequence of labels

Types of Records (cont’d) Resource Record

17.8 Compression DNS requires that a domain name be replaced by an offset pointer when it is repeated. Format of an offset pointer

17.9 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.

17.10 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