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McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 1 Chapter 25 Domain Name System
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 2 IP address identifies the connection of a host to the Internet. People prefer names instead of IP. Host file: maps name and address; stored in local disk; Any program can consult the host file. With large number of users, it is not possible to have all the host name mappings in a single file. Divide this huge amount of information into small parts and store each part on a different computer DNS. Namespace: Names must be unique like IP addresses. A name space that maps each address to a unique name can be organized as: flat or hierarchical
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 3 Flat name space A name is assigned to an address. Name is a sequence of characters without structure Due to central control for ambiguity, this is not possible on huge networks like Internet. Hierarchical Name space Each name is made of several parts. 1 st part is nature of organization, 2 nd part is name, … Central authority takes control of 1 st and 2 nd part only. No need to worry of internal names or host names. Ex. Kfupm.edu.sa & ccse.kfupm.edu.sa
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 4 Figure 25.1 Domain Name Space Names are defined in an inverted-tree structure with the root at the top. Can have 128 levels: level 0 (root) to level 127. Label: Each node in the tree has a level Maximum of 63 characters. Root label is a null string (empty string). Children of a node have different labels.
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 5 Figure 25.2 Domain Names and Labels Full domain name is a sequence of labels separated by dots. Domain names are always read from the node up to the root. Last label is the label of root (null). So, full domain name always ends in a null label [means dot].
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 6 Figure 25.3 FQDN and PQDN Fully Qualified Domain Name (FQDN) Label is terminated by a null string. Contains the full name of a host. Partially Qualified Domain Name (PQDN) Not terminated by a null string. PQDN starts from a node, but it does not reach the root. Used when the name to be resolved belongs to the same site as the client. Resolver supplies the missing part called as suffix.
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 7 Figure 25.4 Domains Domain: Sub-tree of the domain name space. Name of the domain is the domain name of the node at the top of the sub-tree. A domain can be divided into sub-domains.
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 8 Distribution of Name Space If information is stored in one machine, the requests will overload the machine. Hierarchy of name servers Distribute among DNS servers. Let root stand alone and create as many domains (subtrees) as there are first-level nodes. Each server can be responsible (authoritative) for either a large or a small domain.
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 9 Figure 25.6 Zones and domains Zone What a server is responsible for, or has authority over is called zone. If a server accepts responsibility for a domain and does not divide the domain into smaller domains, the domain and the zone refer to the same thing. Server makes a database called a zone file and keeps all the information for every node under the domain. If a sever has sub-domains, information about nodes in sub-domains are stored in servers at lower levels, with the original server keeping some sort of reference to these lower-level servers. But responsibility is still with the original server.
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 10 Root Server: Server whose zone consists of the whole tree. It does not store any information about domains but delegates its authority to other servers. Keeps reference to other servers. There are 13 root servers. Primary server: A primary server loads all information from the disk file, for which it is an authority; Responsible for creating, maintaining, and updating the zone file. Secondary server loads all information from the primary server; Neither creates nor updates the zone files; create redundancy.
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 11 Figure 25.7 DNS in the Internet Generic Domains Registered hosts according to their generic behaviour. Each node in the tree defines a domain, which is an index to the domain name space database. First level in the generic domain section allows seven possible three-character labels.
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 12 Figure 25.8 Generic domains
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 13 Table 25.1 Generic Domain Labels LabelDescription comCommercial organizations eduEducational institutions govGovernment institutions intInternational organizations milMilitary groups netNetwork support centers orgNonprofit organizations
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 14 Table 25.2 New generic domain labels LabelDescription aeroAirlines and aerospace companies bizBusinesses or firms (similar to com) coopCooperative business organizations infoInformation service providers museumMuseums and other nonprofit organizations namePersonal names (individuals) proProfessional individual organizations
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 15 Figure 25.9 Country domains Country domain section follows the same format as the generic domains but uses two-character country abbreviations. Second level labels can be organizational or more specific, national designations.
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 16 Figure 25.10 Inverse Domain Inverse domain is used to map an address to a name. If a server requests a service from clients and the server has a list of authorized clients [IP addresses], then it needs inverse mapping. Inverse or pointer query. Hierarchical: Netid part is at higher level than the subnetid part. 126.96.36.199 188.8.131.52.in-addr.arpa
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 17 Resolution Mapping a name to an address or an address to a name is called name-address resolution. Resolver A host that needs to map an address to a name or a name to an address calls a DNS client named a resolver. Resolver contacts the closest DNS server for mapping. If the server has the info, it replies; otherwise, it either refers to the resolver to other servers or asks other servers to provide the information. After the resolver receives the mapping, it interprets the response to see if it a real resolution or an error and finally delivers the result to the process that requested it.
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 18 Mapping names to addresses Resolver gives a domain name to the server and asks for the corresponding address. The server checks the generic domains or the country domains to find the mapping. Mapping addresses to names Client can send an IP address to a server to be mapped to a domain name. DNS uses inverse domain. Recursive Resolution Resolver can ask for a recursive answer from a name server Resolver expects the server to supply the final answer. If server is the authority, it checks its database and replies. If server is not the authority, it sends the request to another server (the parent usually) and waits for the response. If the parent is the authority, it responds; otherwise, it sends the query to yet another server. When the query is finally resolved, the response travels back until it finally reaches the requesting client.
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 19 Figure 25.11 Recursive resolution
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 20 Figure 25.12 Iterative resolution If server is authority, it sends the answer. If server is not authority, it returns 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.
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 21 Caching Reduction of search time would increase efficiency. DNS handles this with a mechanism called caching. Response is stored in the server’s cache memory before sending it to the client. If another client ask for the same mapping, the cache memory can be referred to give an answer. Response is marked as Unauthoritative Caching speeds up resolution, but can provide an outdated mapping. Time to live (TTL): time in seconds [provided by authoritative server] for which the receiving server can cache the information. DNS requires that each server keep a TTL counter for each mapping it caches.
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 22 DNS Messages Two types of messages Query: header and question records Response: Header, question records, answer records, authoritative records, and additional records. Header: Query and response types have same format with some fields set to zero for the query messages. 12 bytes Identification: Match the response with query. Flags: Type of msg, type of answers requested, type of desired resolution [recursive or iterative], … Number of question records Number of answer records: Zero in query Number of authoritative records: Zero in query Number of additional records: Zero in query
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 23 Figure 25.13 Query and response messages
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 24 Figure 25.14 Header Format
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 25 In DNS, when there is a change, such as adding a new host, removing a host, or changing an IP address, the change must be made to the DNS master file. Requires lot of manual updating. DDNS [Dynamic DNS]: When a binding between a name and address is determined, the information is sent, usually by DHCP to a primary DNS server. Primary server updates the zone. Secondary servers are notified actively or passively. Actively: Primary servers sends a message to secondary server about changes in the zone. Passively: Secondary servers periodically check for any changes. In either case, once notified about change, the secondary server requests for zone transfer.
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 26 DNS can use the services of UDP or TCP, using the well-known port 53. UDP: If response msg < 512 bytes If size is unknown If size increases, the server truncates the msg and resolver now opens a TCP connection and repeats the request to get a full response from the server. TCP: If response msg > 512 bytes Zone transfer
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