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Domain Name System (DNS)

Domain Name System (DNS). Ch25 Ameera Almasoud. Introduction. There are several applications in the application layer of the Internet model that follow the client/server paradigm. The client/server programs can be divided into two categories:

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Domain Name System (DNS)

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  1. Domain Name System(DNS) Ch25 Ameera Almasoud Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  2. Introduction • There are several applications in the application layer of the Internet model that follow the client/server paradigm. • The client/server programs can be divided into two categories: • Application that directly used by the user, such as e-mail, • Application that support other application programs. • The Domain Name System (DNS) is a supporting program that is used by other programs such as e-mail. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  3. Example of using the DNS service Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  4. Host File • When the Internet was small, mapping was done by using a host file. • The host file had only two columns: name and address. • Every host could store the host file on its disk and update it periodically from a master host file. • When a program or a user wanted to map a name to an address, the host consulted the host file and found the mapping. • it is impossible to have one single host file to relate every address with a name. • The host file would be too large to store in every host. • it would be impossible to update all the host files every time there was a change. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  5. Host File • One solution would be to store the entire host file in a single computer and allow access to this centralized information to every computer that needs mapping. • But this would create a huge amount of traffic on the Internet. • The solution is to divide this huge amount of information into smaller parts and store each part on a different computer. • the host that needs mapping can contact the closest computer holding the needed information. • This method is used by the Domain Name System (DNS). Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  6. NAME SPACE • To be unambiguous, the names assigned to machines must be carefully selected from a name space with complete control over the binding between the names and IP addresses. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  7. NAME SPACEFlat Narne Space • The name is an unstructured sequence of characters. • Disadvantage: it must be centrally controlled to avoid duplication. • Therefore, it can not be used in large systems such as the Internet. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  8. NAME SPACEHierarchical Narne Space • The name is structured and consists of many parts. • The first part can define the nature of the organization, • the second part can define the name of an organization, • the third part can define departments in the organization, and so on. • The authority to assign and control the name spaces can be decentralized. • A central authority can assign the part of the name that defines the nature of the organization and the name of the organization. • The responsibility of the rest of the name can be given to the organization itself. Ex: suffixes /prefixes • it is efficiently used in large systems such as the Internet. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  9. DOMAIN NAME SPACE • To have a hierarchical name space, a domain name space was designed. • In this design the names are defined in an inverted-tree structure with the root at the top. • The tree can have only 128 levels: level 0 (root) to level 127. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  10. Domain name space Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  11. DOMAIN NAME SPACELabel • Each node in the tree has a label • The label is a string of maximum length of 63 characters • Children of a node should have different labels • The label of the root is a null string Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  12. DOMAIN NAME SPACEDomain Name • Each node in the tree has a domain name. • A full domain name is a sequence of labels separated by dots (.). • The domain names are always read from the node up to the root. • The last label is the label of the root (null). Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  13. Domain names and labels Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  14. DOMAIN NAME SPACEDomain Name Fully Qualified Domain Name(FQDN). : • If a label is terminated by a null string. • It contains all labels, from the most specific to the most general, that uniquely define the name of the host. Ex: challenger.ate.tbda.edu. Partially Qualified Domain Name(PQDN) : • If a label is not terminated by a null string. • name A PQDN starts from a node, but it does not reach the root. • It is used when the name to be resolved belongs to the same site as the client. Ex: challenger Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  15. FQDN and PQDN Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  16. DOMAIN NAME SPACEDomain • 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. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  17. DOMAIN NAME SPACEDomain Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  18. DISTRIBUTION OF NAME SPACE • The information contained in the domain name space must be stored. • However, it is very inefficient and also unreliable to have just one computer store such a huge amount of information. • In this section, we discuss the distribution of the domain name space. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  19. DISTRIBUTION OF NAME SPACEHierarchy of Name Servers • The solution to these problems is to distribute the information among many computers called DNS servers. • we let the root stand alone and create as many domains (subtrees) as there are first-level nodes. # • DNS allows domains to be divided further into smaller domains(subdomains). • It is inefficient and unreliable to store the information of the domain name space on a single computer: • It is inefficient because responding to all requests from one computer imposes a heavy load on the system • It is unreliable because a failure in the computer makes all data inaccessible Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  20. Hierarchy of name servers Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  21. DISTRIBUTION OF NAME SPACEZone • Since the complete domain name hierarchy cannot be stored on a single server, it is divided among many servers. • What a server is responsible for or has authority over is called a zone. • We can define a zone as a contiguous part of the entire tree. • 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. • The server makes a database called a zone file and keeps all the information for every node under that domain. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  22. Zones and domains Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  23. DISTRIBUTION OF NAME SPACERoot Server • A root server is a server whose zone consists of the whole tree. • A root server usually does not store any information about domains but delegates its authority to other servers. • There are several root servers, each covering the whole domain name space. • The servers are distributed all around the world. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  24. DISTRIBUTION OF NAME SPACEPrimary and Secondary Servers • A primary server stores a file about the zone for which it is an authority on its local disk • A primary server is responsible for creating, maintaining, and updating the zone file • A secondary server downloads information from another server (primary or secondary) on its local disk: this is called zone transfer • A secondary server is NOT responsible for creating, maintaining or updating the zone file Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  25. DISTRIBUTION OF NAME SPACEPrimary and Secondary Servers • If updating is required, it is performed by the primary server, then sent to the secondary server • The aim of assigning a secondary server is to create data redundancy so that it can serve clients in case of failure of the primary server • A server can be primary for a zone and secondary for another server; therefore, it is important to state the zone to which a primary/secondary server refers Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  26. DNS IN THE INTERNET • DNS is a protocol that can be used in different platforms. • In the Internet, the domain name space (tree) is divided into three different sections: • generic domains • country domains • the inverse domain Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  27. DNS IN THE INTERNET Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  28. DNS IN THE INTERNETGeneric domains Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  29. Generic domain labels Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  30. DNS IN THE INTERNET Country Domains • These use two-character country abbreviations • Examples are: • sa (Saudi Arabia) • eg (Egypt) • uk (UnitedKingdom) • fr (France) • us (United States), etc… Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  31. DNS IN THE INTERNET Country Domains Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  32. DNS IN THE INTERNET Inverse Domains • These are used to map an IP address to a name • When a server receives a request, the first thing it performs is to check if the sending client is an authorized one: • The resolver sends a query to the DNS server with the client name • The DNS server finds the corresponding IP address • The extracted IP address is resent back to the resolver, then to the server • The server checks if the received IP is on the list of its authorized clients Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  33. DNS IN THE INTERNET Inverse Domains • This type of query is called inverse query or pointer query • The first and second node in a pointer query are always arpaand in-addr • The DNS servers that handle the pointer query are hierarchical: • The highest level is the network id (netid) part • The lower level is the subnet id part • The lowest level is the host id part Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  34. DNS IN THE INTERNET Inverse Domains • IP= netid.hostid132.34.45.121  Class B Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  35. RESOLUTION • Mapping a name to an address or an address to a name is called name-address resolution. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  36. RESOLUTIONResolver • Client/server application that resides on DNS clients • Any host that requires to map an IP address to a URL name or vice versa calls a resolver • The resolver accesses the closest DNS server with the mapping request • If the server has the required information, it provides them to the resolver • If the server does not have the required information, it acts in one of the following ways: • The server refers the resolver to another DNS server. • The server asks another server for the required information, and provides it to the resolver. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  37. RESOLUTIONMappingNames to Addresses • Most of the time, the resolver gives a domain name to the server and asks for the corresponding address. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  38. RESOLUTIONMappingAddresses to Names • A client can send an IP address to a server to be mapped to a domain name. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  39. RESOLUTIONRecursive Resolution Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  40. RESOLUTIONRecursive Resolution • When a resolver sends a mapping request to a name server, it expects the answer from the same server • If the server is the authority for the domain name, it responds immediately to the resolver • Otherwise, the server sends the mapping request to the parent server • This procedure is repeated recursively until an answer is found • The answer follows the same way back through the servers until it reaches the resolver Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  41. RESOLUTIONIterative Resolution Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  42. RESOLUTIONIterative Resolution • If the client does not ask for a recursive answer, the mapping can be done iteratively. • 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. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  43. RESOLUTIONCaching • This is a mechanism that aims to reduce the search time • a DNS server needs to look for a name (or address) in its database. • Therefore, when a new request arrives to the server, it checks its cache first. • If the required information are stored in the cache, the server responds to the request immediately • If not, it sends the request to another server as explained before • When a server asks another server for the information, it stores it in its cache memory before sending it to the client • In case the required information is found in the cache, the • server marks the response as “unauthoritative”. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  44. RESOLUTIONCaching • Time-to-Live (TTL) counter, in seconds, is used to overcome this problem • The server keeps a TTL with each entry in its cache • When TTL reaches zero, the corresponding entry is marked Invalid • When a request is made to an invalid entry, the server considers it “missing”, and a request is sent to the authoritative server. • If a server caches a mapping for a long time, it may send an outdated mapping to the client • Another technique is to search the cache periodically and purge (kill) all entries with expired TTL. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  45. DNS MESSAGES • DNS has two types of messages: query and response. • Both types have the same format. • The query message consists of a header and question records; • the response message consists of a header, question records, answer records, authoritative records, and additional records. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  46. Query and response messages Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  47. Header format Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  48. TYPES OF RECORDS • There are 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. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  49. REGISTRARS • How are new domains added to DNS? • New domains are added to the Internet through registrars • A registrar verifies that the requested domain name is unique, then enters it into the DNS database for a fee. Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

  50. DYNAMIC DOMAIN NAME SYSTEM (DDNS) • Changes made in the domain include adding a new host,removing a host, or changing the IP address of a host • In DNS, any made change should be reflected in the DNS master file • In a huge environment such as the Internet, such updates in the DNS master file should be automatically reflected • When a change is made, the information is sent to the primary DNS server which, in turn, updates the zone • Primary servers may update secondary servers actively. • Another mechanism, the secondary servers check periodically for any updates in the primary server: this is called passive update Based on Data Communications and Networking, 4th Edition. by Behrouz A. Forouzan, McGraw-Hill Companies, Inc., 2007

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