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Questions and Answers

What is an FQDN in the context of Domain Name System?

  • A domain name that contains partial labels of a host
  • A domain name that contains the full name of a host (correct)
  • A null label that defines the name of a host
  • A label that uniquely defines the name of a host

    • What is the purpose of the null label in an FQDN?

  • To uniquely identify the host
  • To define the end of the domain name
  • To define the start of the domain name
  • To indicate the end of the domain name with a dot (.) (correct)

    • What is a Partially Qualified Domain Name (PQDN)?

  • A domain name that contains no labels
  • A domain name that starts from a node but does not reach the root (correct)
  • A domain name that starts from the root
  • A domain name that contains all labels of a host

    • What is the purpose of the DNS client in resolving a PQDN?

    <p>To supply the missing part (suffix) to create an FQDN</p> Signup and view all the answers

    What is a domain in the context of Domain Name System?

    <p>A subtree of the domain name space</p> Signup and view all the answers

    Why is it inefficient to have just one computer store the domain name space?

    <p>Because it is very inefficient and also unreliable</p> Signup and view all the answers

    What is the process of dividing a domain into smaller domains called?

    <p>Subdomain creation</p> Signup and view all the answers

    What is the purpose of distributing the domain name space?

    <p>To make it more efficient and reliable</p> Signup and view all the answers

    What type of query is sent when a client wants to map an IP address to a domain name?

    <p>PTR query</p> Signup and view all the answers

    How is the IP address modified before sending a PTR query?

    <p>The IP address is reversed and in-addr and arpa labels are added</p> Signup and view all the answers

    What happens when a client asks for a recursive answer from a name server?

    <p>The server responds with the final answer</p> Signup and view all the answers

    What is the purpose of the inverse domain in DNS?

    <p>To map IP addresses to domain names</p> Signup and view all the answers

    What happens when a server is not the authority for a domain name in recursive resolution?

    <p>The server sends the query to another server</p> Signup and view all the answers

    What type of resolution occurs when a client does not ask for a recursive answer?

    <p>Iterative resolution</p> Signup and view all the answers

    What is the role of the client in iterative resolution?

    <p>The client is responsible for repeating the query to another server</p> Signup and view all the answers

    What is the purpose of the in-addr and arpa labels in a PTR query?

    <p>To make the IP address acceptable by the inverse domain</p> Signup and view all the answers

    What is the responsibility of an original server in a domain?

    <p>It has a zone, but the detailed information is kept by the lower-level servers.</p> Signup and view all the answers

    What is the purpose of a root server?

    <p>To delegate its authority to other servers, keeping references to those servers.</p> Signup and view all the answers

    What is the difference between a primary server and a secondary server?

    <p>A primary server updates zone files, while a secondary server does not.</p> Signup and view all the answers

    What is the purpose of a secondary server?

    <p>To create redundancy for the data so that if one server fails, the other can continue serving clients.</p> Signup and view all the answers

    What is the role of a primary server?

    <p>To create, maintain, and update the zone file.</p> Signup and view all the answers

    What happens if a primary server fails?

    <p>The secondary server continues serving clients.</p> Signup and view all the answers

    What is the relationship between a primary server and a secondary server?

    <p>A primary server and a secondary server are at the same level of authority.</p> Signup and view all the answers

    What is the role of a root server in the domain name space?

    <p>It covers the whole domain name space.</p> Signup and view all the answers

    What is the purpose of a question record in DNS?

    <p>To get information from a server</p> Signup and view all the answers

    What type of records are present in the answer section of a DNS response message?

    <p>Resource records</p> Signup and view all the answers

    What is the purpose of the authoritative section in a DNS response message?

    <p>To give information about one or more authoritative servers</p> Signup and view all the answers

    What is the role of a registrar in DNS?

    <p>To verify and add new domain names to the DNS database</p> Signup and view all the answers

    What is a resource record associated with in DNS?

    <p>Each node on the DNS tree</p> Signup and view all the answers

    What is the purpose of the additional information section in a DNS response message?

    <p>To provide additional information that may help the resolver</p> Signup and view all the answers

    What is returned by the DNS server to the client?

    <p>Resource records</p> Signup and view all the answers

    What is the purpose of ICANN in the context of DNS?

    <p>To accredit registrars</p> Signup and view all the answers

    What information does an organization need to provide to register?

    <p>Its server name and IP address</p> Signup and view all the answers

    What was the limitation of the initial DNS design?

    <p>It required manual updating for changes</p> Signup and view all the answers

    What is the main purpose of Dynamic Domain Name System (DDNS)?

    <p>To update DNS master files dynamically</p> Signup and view all the answers

    How do secondary servers get notified about changes in the zone in active notification?

    <p>The primary server sends a message to them</p> Signup and view all the answers

    What is the purpose of authentication in DDNS?

    <p>To provide security and prevent unauthorized changes</p> Signup and view all the answers

    What protocol is used by DNS when the size of the response message is less than 512 bytes?

    <p>UDP</p> Signup and view all the answers

    What is the well-known port used by the DNS server?

    <p>Port 53</p> Signup and view all the answers

    When is a TCP connection used in DNS?

    <p>When the size of the response message is more than 512 bytes</p> Signup and view all the answers

    What is the purpose of the generic domains in the domain name space?

    <p>To define registered hosts according to their generic behavior</p> Signup and view all the answers

    What type of query is sent when a server wants to map an IP address to a domain name?

    <p>Inverse query</p> Signup and view all the answers

    What is the name of the first-level node in the inverse domain?

    <p>arpa</p> Signup and view all the answers

    What is the purpose of the country domains section?

    <p>To use two-character country abbreviations</p> Signup and view all the answers

    What is the hierarchical structure of the inverse domain based on?

    <p>The netid part of the address being at a higher level than the subnetid part</p> Signup and view all the answers

    What is the purpose of the in-addr node in the inverse domain?

    <p>To be the second-level node in the inverse domain</p> Signup and view all the answers

    What is the relationship between the generic domains and the country domains?

    <p>They are separate sections of the domain name space</p> Signup and view all the answers

    What is the purpose of the domain name space?

    <p>To provide a hierarchical structure for organizing domain names</p> Signup and view all the answers

    What is the process of finding an IP address from a domain name or vice versa called?

    <p>Name-address resolution</p> Signup and view all the answers

    What type of application is DNS designed as?

    <p>Client-server</p> Signup and view all the answers

    What is the maximum number of levels in a domain name space?

    <p>128</p> Signup and view all the answers

    What is the role of a resolver in DNS?

    <p>To request domain name information</p> Signup and view all the answers

    What happens if a local DNS server cannot resolve a query?

    <p>All of the above</p> Signup and view all the answers

    What is the maximum length of a label in a domain name?

    <p>63 characters</p> Signup and view all the answers

    What is the format of an IP address in a PTR query?

    <p>Inverse order with 'in-addr.arpa' appended</p> Signup and view all the answers

    How are domain names read in a domain name space?

    <p>From the node up to the root</p> Signup and view all the answers

    What is the purpose of a central authority in a domain name space?

    <p>To assign the part of the domain name that defines the nature of the organization</p> Signup and view all the answers

    How is a domain name read in DNS?

    <p>From bottom to top</p> Signup and view all the answers

    What is the term for the process of a resolver receiving a response from a DNS server?

    <p>Interpretation</p> Signup and view all the answers

    What is the root label in a domain name space?

    <p>A null string</p> Signup and view all the answers

    What type of domain is 'chal.atc.jhda.edu.' an example of?

    <p>Generic domain</p> Signup and view all the answers

    What is the purpose of children having different labels in a domain name space?

    <p>To ensure the uniqueness of domain names</p> Signup and view all the answers

    What is the last character of a full domain name?

    <p>A dot</p> Signup and view all the answers

    What is a fully qualified domain name (FQDN) in the context of DNS?

    <p>A domain name that is terminated by a null string</p> Signup and view all the answers

    What is the main purpose of the identification subfield in DNS messages?

    <p>To match the response with the query</p> Signup and view all the answers

    Which section is present in both query and response messages in DNS?

    <p>Question section</p> Signup and view all the answers

    Which section of a DNS response message contains the answer from the server to the client?

    <p>Answer Section</p> Signup and view all the answers

    What is the total size of the header in DNS messages?

    <p>12 bytes</p> Signup and view all the answers

    What is the role of a registrar in DNS?

    <p>To add new domains to DNS</p> Signup and view all the answers

    What type of records are associated with each domain name (each node on the tree)?

    <p>Resource Records</p> Signup and view all the answers

    Which subfield contains the number of queries in the question section of the message?

    <p>Number of question records subfield</p> Signup and view all the answers

    What is the purpose of the authoritative section in a DNS response message?

    <p>To give information about one or more authoritative servers for the query</p> Signup and view all the answers

    What happens to mappings with an expired TTL in the cache memory?

    <p>They are purged periodically</p> Signup and view all the answers

    What is the purpose of the additional information section in a DNS response message?

    <p>To provide additional information that may help the resolver</p> Signup and view all the answers

    What is the purpose of the flags subfield in DNS messages?

    <p>To specify the type of answer requested</p> Signup and view all the answers

    What is the purpose of the question records in DNS messages?

    <p>To specify the question being asked</p> Signup and view all the answers

    What is the format of both query and response messages in DNS?

    <p>Same format for both query and response messages</p> Signup and view all the answers

    What is the purpose of a question record in DNS?

    <p>To get information from a server</p> Signup and view all the answers

    What is the role of ICANN in the context of DNS?

    <p>To accredit registrars</p> Signup and view all the answers

    What is the main disadvantage of a flat name space in the context of DNS?

    <p>It allows ambiguity and duplication</p> Signup and view all the answers

    What is the purpose of dividing the host file information into smaller parts in DNS?

    <p>To reduce the traffic on the Internet</p> Signup and view all the answers

    How are the names assigned to machines ensured to be unique in DNS?

    <p>By using a name space with complete control over the binding between the names and IP addresses</p> Signup and view all the answers

    What type of name space is used by the Domain Name System (DNS)?

    <p>Hierarchical name space</p> Signup and view all the answers

    What is the advantage of using a hierarchical name space in DNS?

    <p>It allows the authority to assign and control the name spaces to be decentralized</p> Signup and view all the answers

    What is the problem with storing the entire host file in a single computer?

    <p>It creates a huge amount of traffic on the Internet</p> Signup and view all the answers

    What is the role of the hierarchical name space in DNS?

    <p>To map each address to a unique name</p> Signup and view all the answers

    Why is decentralization of control important in DNS?

    <p>To allow the authority to assign and control the name spaces to be decentralized</p> Signup and view all the answers

    What is the primary purpose of the inverse domain in DNS?

    <p>To map an IP address to a domain name</p> Signup and view all the answers

    What is the first-level node in the inverse domain?

    <p>arpa</p> Signup and view all the answers

    What is the purpose of the country domains section?

    <p>To use two-character country abbreviations</p> Signup and view all the answers

    How is the hierarchical structure of the inverse domain organized?

    <p>By IP addresses, subnet IDs, and host IDs</p> Signup and view all the answers

    What is the purpose of the generic domains section?

    <p>To define registered hosts according to their generic behavior</p> Signup and view all the answers

    What type of query is sent when a server wants to map an IP address to a domain name?

    <p>Inverse query</p> Signup and view all the answers

    What happens when a server receives a query for a name that is not in its domain?

    <p>It searches its database for a server IP address</p> Signup and view all the answers

    What is the purpose of the in-addr node in the inverse domain?

    <p>To define IP addresses</p> Signup and view all the answers

    What is the relationship between the generic domains and the country domains?

    <p>They are separate sections of the domain name space</p> Signup and view all the answers

    What is the purpose of caching in DNS?

    <p>To reduce search time</p> Signup and view all the answers

    What is the purpose of time-to-live (TTL) in DNS?

    <p>To define the time a server can cache the information</p> Signup and view all the answers

    What is the consequence of a server caching a mapping for a long time?

    <p>It sends outdated mapping to the client</p> Signup and view all the answers

    What is the purpose of iterative resolution?

    <p>The client repeats the query to multiple servers</p> Signup and view all the answers

    How does a server handle a query for a mapping that is not in its cache?

    <p>It asks another server for the IP address</p> Signup and view all the answers

    What happens when a client queries multiple servers before it gets an answer?

    <p>Iterative resolution</p> Signup and view all the answers

    Why is caching problematic?

    <p>It may send outdated mapping to the client</p> Signup and view all the answers

    What is the purpose of a root server in the domain name space?

    <p>To delegate its authority to other servers</p> Signup and view all the answers

    What is the main difference between a primary and secondary server?

    <p>The way they obtain and update zone files</p> Signup and view all the answers

    What happens when a primary server fails?

    <p>The secondary server continues serving clients</p> Signup and view all the answers

    What is the purpose of having multiple root servers?

    <p>To provide redundancy for the data</p> Signup and view all the answers

    What type of server is responsible for creating and maintaining zone files?

    <p>Primary server</p> Signup and view all the answers

    What is the role of a secondary server in a domain?

    <p>To transfer the complete information from another server</p> Signup and view all the answers

    What is the relationship between a primary and secondary server?

    <p>One is responsible for creating zone files and the other is not</p> Signup and view all the answers

    What is the purpose of dividing a domain into smaller domains?

    <p>To delegate authority to lower-level servers</p> Signup and view all the answers

    What is the limitation of the initial DNS design?

    <p>The DNS was not able to handle dynamic changes to IP addresses</p> Signup and view all the answers

    Which protocol is used by DNS when the size of the response message is greater than 512 bytes?

    <p>TCP</p> Signup and view all the answers

    What is the purpose of the Dynamic Domain Name System (DDNS)?

    <p>To allow for dynamic changes to IP addresses</p> Signup and view all the answers

    What is the well-known port used by the DNS server?

    <p>Port 53</p> Signup and view all the answers

    How do secondary servers get notified about changes in the zone in passive notification?

    <p>They periodically check for any changes</p> Signup and view all the answers

    What is the purpose of authentication in DDNS?

    <p>To provide security and prevent unauthorized changes</p> Signup and view all the answers

    What information does an organization need to provide to register?

    <p>The name of its server and the IP address of the server</p> Signup and view all the answers

    What happens when the size of the response message is less than 512 bytes?

    <p>A UDP connection is used</p> Signup and view all the answers

    What is the main purpose of the inverse domain in DNS?

    <p>To map an IP address to a domain name</p> Signup and view all the answers

    What is the structure of the inverse domain based on?

    <p>The hierarchical structure of IP addresses</p> Signup and view all the answers

    What is the purpose of the country domains section in DNS?

    <p>To use two-character country abbreviations</p> Signup and view all the answers

    What is the first-level node in the inverse domain called?

    <p>arpa</p> Signup and view all the answers

    What is the purpose of the generic domains in DNS?

    <p>To define registered hosts according to their generic behavior</p> Signup and view all the answers

    What type of query is sent when a server wants to map an IP address to a domain name?

    <p>Inverse or pointer query</p> Signup and view all the answers

    What is the hierarchical structure of the inverse domain based on?

    <p>The IP address, with netid, subnetid, and hostid</p> Signup and view all the answers

    What is the purpose of the in-addr node in the inverse domain?

    <p>To define IP addresses</p> Signup and view all the answers

    What is the primary purpose of the Domain Name System (DNS)?

    <p>To map a domain name to an IP address and vice versa</p> Signup and view all the answers

    Why was the initial DNS design limited?

    <p>Because it was not designed to support a large number of hosts</p> Signup and view all the answers

    What is the purpose of a DNS client?

    <p>To map a domain name to an IP address</p> Signup and view all the answers

    What is the advantage of using DNS over a host file?

    <p>DNS is more scalable and can support a large number of hosts</p> Signup and view all the answers

    What is the purpose of the DNS server in the context of email?

    <p>To map an email address to an IP address</p> Signup and view all the answers

    What is the disadvantage of using a single host file to map domain names to IP addresses?

    <p>It would be too large to store in every host</p> Signup and view all the answers

    What is the relationship between the IP address and the domain name?

    <p>The IP address and the domain name are equivalent</p> Signup and view all the answers

    What is the role of the DNS in the Internet model?

    <p>To support other application programs</p> Signup and view all the answers

    What is the main reason for distributing the information among many DNS servers?

    <p>To reduce the load on the system</p> Signup and view all the answers

    What is a zone in the context of DNS?

    <p>A contiguous part of the entire domain name hierarchy</p> Signup and view all the answers

    What happens when a server divides its domain into subdomains?

    <p>The domain and zone refer to different things</p> Signup and view all the answers

    What is the purpose of creating a hierarchy of DNS servers?

    <p>To reduce the load on individual servers</p> Signup and view all the answers

    What is stored in the zone file of a DNS server?

    <p>All the information for every node under the domain</p> Signup and view all the answers

    Why is it not possible to store the entire domain name hierarchy on a single server?

    <p>Because it is too large</p> Signup and view all the answers

    What is the relationship between the original server and the servers at the lower levels?

    <p>The original server delegates part of its authority to the lower-level servers</p> Signup and view all the answers

    What is the purpose of dividing the domain name space into smaller domains?

    <p>To reduce the load on individual servers</p> Signup and view all the answers

    What happens when a server cannot resolve a query?

    <p>It returns the IP address of a new server to the client.</p> Signup and view all the answers

    What is the purpose of caching in DNS?

    <p>To store a mapping from another server and speed up resolution.</p> Signup and view all the answers

    What is the role of TTL in DNS?

    <p>To define the time a server can cache information.</p> Signup and view all the answers

    What happens if a server caches a mapping for a long time?

    <p>It may send an outdated mapping to the client.</p> Signup and view all the answers

    What is the result of iterative resolution?

    <p>The client repeats the query to multiple servers.</p> Signup and view all the answers

    Why is caching necessary in DNS?

    <p>To reduce the search time for a server IP address.</p> Signup and view all the answers

    What is the purpose of the authoritative server in DNS?

    <p>To provide authoritative answers to queries.</p> Signup and view all the answers

    What is the benefit of using caching in DNS?

    <p>It reduces the search time for a server IP address.</p> Signup and view all the answers

    What is the direction of reading domain labels in a domain name?

    <p>From bottom to top</p> Signup and view all the answers

    What is the purpose of a resolver in DNS?

    <p>To map a name to an address</p> Signup and view all the answers

    What happens when a local DNS server cannot resolve a query?

    <p>It refers the resolver to other servers or asks other servers directly</p> Signup and view all the answers

    What is the format of an IP address in a PTR query?

    <p>121.45.34.132.in-addr.arpa</p> Signup and view all the answers

    What is the relationship between a server serving a subnet and a server serving the whole site?

    <p>The server serving the whole site is at a higher level</p> Signup and view all the answers

    What is the direction of the resolution process in DNS?

    <p>From the root server to the client</p> Signup and view all the answers

    What is the purpose of the generic domains and country domains sections?

    <p>To organize the domain name space</p> Signup and view all the answers

    What is the process of mapping a name to an address or an address to a name called?

    <p>Name-address resolution</p> Signup and view all the answers

    Study Notes

    Domain Name System (DNS)

    • A domain name is a string of characters that represents an IP address
    • DNS is a hierarchical system that translates domain names into IP addresses

    FQDN (Fully Qualified Domain Name)

    • A domain name that contains the full name of a host
    • Includes all labels, from the most specific to the most general, that uniquely define the name of the host
    • Example: challenger.ate.tbda.edu
    • Must end with a null label, which is a dot (.)

    PQDN (Partially Qualified Domain Name)

    • A domain name that is not terminated by a null string
    • Starts from a node, but does not reach the root
    • Used when the name to be resolved belongs to the same site as the client
    • Resolver can supply the missing part, called the suffix, to create an FQDN
    • Example: Challenger can be defined as a partial name, and the resolver adds the suffix atc.jhda.edu to create an FQDN

    Domain

    • 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
    • A domain may be divided into subdomains

    Distribution of Name Space

    • The information contained in the domain name space must be stored
    • It is inefficient and unreliable to have just one computer store the huge amount of information
    • A server can divide part of its domain and delegate responsibility to lower-level servers
    • The server still has a zone, but the detailed information is kept by the lower-level servers

    Root Server

    • A server whose zone consists of the whole tree
    • Does not store any information about domains but delegates its authority to other servers
    • Keeps references to those servers
    • There are several root servers, each covering the whole domain name space

    Primary and Secondary Servers

    • Primary server: stores a file about the zone for which it is an authority
    • Responsible for creating, maintaining, and updating the zone file
    • Stores the zone file on a local disk
    • Secondary server: transfers the complete information about a zone from another server (primary or secondary) and stores the file on its local disk
    • Neither creates nor updates the zone files

    Mapping Addresses to Names

    • A client can send an IP address to a server to be mapped to a domain name (PTR query)
    • DNS uses the inverse domain to answer queries of this kind
    • The IP address is reversed and the two labels in-addr and arpa are appended to create a domain acceptable by the inverse domain section

    Recursive Resolution

    • The client (resolver) can ask for a recursive answer from a name server
    • The resolver expects the server to supply the final answer
    • If the server is not the authority, it sends the request to another server (the parent usually) and waits for the response

    Iterative Resolution

    • If the client does not ask for a recursive answer, the mapping can be done iteratively
    • The server 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

    DNS Sections

    • Answer Section: includes the answer from the server to the client (resolver)
    • Authoritative Section: gives information (domain name) about one or more authoritative servers for the query
    • Additional Information Section: provides additional information that may help the resolver

    Types of Records

    • Question Record: used by the client to get information from a server
    • 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

    Registrars

    • Commercial entities accredited by ICANN that add new domains to DNS
    • Verify that the requested domain name is unique and then enter it into the DNS database
    • A fee is charged

    Dynamic Domain Name System (DDNS)

    • Designed to respond to the need for dynamic updating of DNS records
    • When a binding between a name and an address is determined, the information is sent to a primary DNS server
    • The primary server updates the zone, and the secondary servers are notified
    • Uses an authentication mechanism to prevent unauthorized changes in the DNS records

    Domain Name System (DNS)

    • A central authority assigns the part of the domain name that defines the organization's nature and the organization's name.
    • The organization can add suffixes or prefixes to define its host or resources.
    • Hierarchical name space with an inverted-tree structure, with the root at the top and up to 128 levels.

    Domain Name Space

    • Each node in the tree has a label, a string with a maximum of 63 characters.
    • The root label is a null string (empty string).
    • Children of a node have different labels, ensuring uniqueness of domain names.

    Domain Name

    • A full domain name is a sequence of labels separated by dots (.).
    • Domain names are read from the node up to the root.
    • A full domain name always ends in a null label, which means the last character is a dot.

    Fully Qualified Domain Name (FQDN)

    • A label terminated by a null string is a fully qualified domain name (FQDN).

    DNS in the Internet

    • DNS is a protocol that can be used in different platforms.
    • The domain name space is divided into three sections: generic domains, country domains, and inverse domain.

    Generic Domains

    • Define 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.

    Country Domains

    • Use two-character country abbreviations (e.g., "in" for India).
    • Second labels can be organizational or specific designations.

    Inverse Domain

    • Used to map an address to a name.
    • The inverse domain is added to the domain name space with the first-level node called "arpa" and the second level "in-addr".
    • The rest of the domain defines IP addresses.

    Resolution

    • Mapping a name to an address or an address to a name is called name-address resolution.
    • A resolver accesses a DNS server with a mapping request.
    • The resolver receives the mapping and interprets the response to see if it's a real resolution or an error.

    DNS Messages

    • Two types of messages: query and response.
    • Both types have the same format, with a header and question records, answer records, authoritative records, and additional records.
    • 12 bytes in length.
    • Contains identification subfield, flags subfield, number of question records, number of answer records, number of authoritative records, and number of additional records.

    Question Section

    • Consists of one or more question records.
    • Present in both query and response messages.

    Answer Section

    • Consists of one or more resource records.
    • Present only in response messages.
    • Includes the answer from the server to the client (resolver).

    Authoritative Section

    • Consists of one or more resource records.
    • Present only in response messages.
    • Gives information about one or more authoritative servers for the query.

    Additional Information Section

    • Consists of one or more resource records.
    • Present only in response messages.
    • Provides additional information that may help the resolver.

    Resource Records

    • Associated with each domain name (each node on the tree).
    • The server database consists of resource records.
    • Returned by the server to the client.

    Registrars

    • Commercial entities accredited by ICANN.
    • Verify that the requested domain name is unique and then enter it into the DNS database.
    • Charge a fee.

    DNS Overview

    • One solution to store host file information is to centralize it on a single computer, but this would create significant internet traffic.
    • Another solution, used by DNS, is to divide the information into smaller parts and store each on a different computer, allowing hosts to access the closest computer with the needed information.

    Name Space

    • Names assigned to machines must be unique and carefully selected from a name space with complete control over the binding between names and IP addresses.
    • A name space can be organized in two ways: flat or hierarchical.

    Flat Name Space

    • A flat name space assigns a name to an address without structure.
    • It cannot be used in a large system like the Internet because it must be centrally controlled to avoid ambiguity and duplication.

    Hierarchical Name Space

    • A hierarchical name space consists of multiple parts, defining the nature of the organization, the organization's name, and departments.
    • The authority to assign and control name spaces can be decentralized, with the original server delegating responsibility to lower-level servers.

    Zones and Domains

    • A server can divide its domain and delegate responsibility to other servers, keeping references to the delegated parts.
    • A zone is made up of detailed information for the part of the domain not delegated and references to the delegated parts.

    Root Server

    • A root server has a zone that consists of the whole tree.
    • It usually doesn't store domain information but delegates authority to other servers, keeping references to those servers.
    • There are multiple root servers distributed around the world.

    Primary and Secondary Servers

    • A primary server stores a file about the zone for which it is an authority and is responsible for creating, maintaining, and updating the zone file.
    • A secondary server transfers the complete information about a zone from another server and stores it on its local disk.
    • Both primary and secondary servers are authoritative for the zones they serve.

    DNS in the Internet

    • DNS is a protocol used in different platforms, dividing the domain name space into three sections: generic domains, country domains, and inverse domain.

    Generic Domains

    • Generic domains define 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.

    Country Domains

    • Country domains use two-character country abbreviations, with second labels being organizational or specific designations.

    Inverse Domain

    • The inverse domain is used to map an address to a name, using a hierarchical structure with nodes for IP addresses.
    • The inverse domain is added to the domain name space with the first-level node "arpa" and the second-level node "in-addr".

    Caching

    • Caching reduces search time by storing query results in cache memory.
    • When a server receives a query, it checks its cache memory before sending the query to another server.
    • The server marks the response as unauthoritative if it comes from the cache memory.

    Time-to-Live (TTL)

    • TTL defines the time in seconds that a receiving server can cache the information.
    • After the TTL, the mapping is invalid, and any query must be sent again to the authoritative server.

    Dynamic Domain Name System (DDNS)

    • DDNS updates the DNS master file dynamically, responding to changes such as adding or removing hosts, or changing IP addresses.
    • In DDNS, the primary server updates the zone, and secondary servers are notified actively or passively.

    Summary

    • DNS can use either UDP or TCP, with a well-known port 53.
    • UDP is used when the response message is less than 512 bytes, and TCP is used when the response message is more than 512 bytes.

    Introduction to DNS

    • DNS is a supporting program that is used by other programs such as e-mail to map an e-mail address to the corresponding IP address.
    • The DNS client program sends a request to a DNS server to map the e-mail address to the IP address.
    • IP protocols use IP addresses to identify an entity, but people prefer to use names instead of numeric addresses.

    Hierarchical Structure of DNS

    • The solution to the problem of mapping names to addresses is to distribute the information among many computers called DNS servers.
    • The DNS servers form a hierarchical structure, with each server being responsible for a domain or a zone.
    • A zone is a contiguous part of the entire tree, and a server makes a database called a zone file and keeps all the information for every node under that domain.

    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, and inverse domain.

    Generic Domains

    • Generic domains define 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.

    Country Domains

    • Country domains use two-character country abbreviations (e.g., in for India).
    • Second labels can be organizational, or they can be more specific designations.

    Inverse Domain

    • The inverse domain is used to map an address to a name.
    • This type of query is called an inverse or pointer (PTR) query.
    • The servers that handle the inverse domain are also hierarchical.

    Resolution

    • Mapping a name to an address or an address to a name is called name-address resolution.
    • A host that needs to map an address 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.

    Resolver

    • The resolver gives a domain name to the server and asks for the corresponding address.
    • If the server has the information, it satisfies the resolver; otherwise, it either refers the resolver to other servers or asks other servers to provide the information.

    Mapping Names to Addresses

    • Mostly, the resolver gives a domain name to the server and asks for the corresponding address.
    • If the domain name is from the generic domains section, the resolver receives a domain name such as "chal.atc.jhda.edu.".
    • If the domain name is from the country domains section, the resolver receives a domain name such as "ch.jhda.cu.ca.us.".

    Iterative Resolution

    • If the local server cannot resolve the query, it either refers the resolver to other servers or asks other servers directly.
    • This process is called iterative resolution because the client repeats the same query to multiple servers.

    Caching

    • Each time a server receives a query for a name that is not in its domain, it needs to search its database for a server IP address.
    • DNS handles this with a mechanism called caching.
    • When a server asks for a mapping from another server and receives the response, it stores this information in its cache memory before sending it to the client.

    Time-to-Live (TTL)

    • The authoritative server always adds information to the mapping called time-to-live (TTL).
    • TTL defines the time in seconds that the receiving server can cache the information.
    • After that time, the mapping is invalid and any query must be sent again to the authoritative server.

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    ch3.pdf

    Description

    Understanding Fully Qualified Domain Names (FQDN) and their structure in DNS systems.

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