45 Questions
What is the primary function of the Domain Name System (DNS) in the Internet model?
To provide a naming system for hosts on the Internet
What type of paradigm do client/server programs, including DNS, follow?
Client/server
What is the main limitation of using a host file to map names to addresses?
It is not scalable
What is the role of the DNS client program in the DNS service?
To send a request to a DNS server
Why do people prefer to use names instead of IP addresses?
Names are easier to remember
What is the main advantage of using DNS over the host file system?
DNS is more scalable
What is the purpose of the DNS server in the DNS service?
To map a name to an IP address
What is the consequence of not having a system to map names to addresses?
Users must use IP addresses instead of names
What type of programs use the DNS service?
Programs that need to map names to addresses
What can a central authority assign in a domain name?
The part defining the organization's nature and name
What is the maximum number of levels in a domain name space?
128 levels
What is the maximum number of characters in a label?
63 characters
How are domain names read?
From the node up to the root
What is a fully qualified domain name (FQDN)?
A domain name with a null string
Why do domain names always end with a dot?
Because the null string is empty
What is the purpose of a central authority in domain name assignment?
To prevent duplicate domain names
What is the structure of a domain name space?
Hierarchical with a limited number of levels
What is the significance of the root label in a domain name space?
It is a null string
Why is a centralized DNS system inefficient?
Because it places a heavy load on the system
What is the main solution to the problems of a centralized DNS system?
Distributing the information among many computers called DNS servers
How are domains divided in a DNS system?
Based on the first level
What is a zone in a DNS system?
A contiguous part of the entire tree
What does a server create to store information about every node under its domain?
A zone file
What happens when a server divides its domain into subdomains?
The domain and zone refer to different things
What does a server keep when it divides its domain into subdomains?
Some sort of reference to the lower-level servers
Why is it not possible to store the complete domain name hierarchy on a single server?
Because it is too large
What is the term for a server that is responsible for a domain?
Authoritative
What is the purpose of the identification subfield in the DNS header?
To match the response with the query
What is the format of both query and response messages?
Header, question records, answer records, authoritative records, and additional records
What is the size of the DNS message header?
12 bytes
What is the purpose of the flags subfield in the DNS header?
To define the type of the message
What is present in the question section of a DNS message?
One or more question records
What is the value of the number of answer records subfield in a query message?
0
What is the purpose of periodically searching the cache memory?
To purge mappings with an expired TTL
What type of messages does DNS have?
Query and response messages
What determines the type of answer requested in a DNS message?
The flags subfield
What happens when a server serves the whole site?
The domain looks inverted
How is an IP address like 132.34.45.121 read in a domain name?
From bottom to top
What is name-address resolution?
Mapping a name to an address or an address to a name
What is the role of a resolver in DNS?
To map an address to a name or a name to an address
What does a resolver do when it receives a response from a DNS server?
It interprets the response to see if it is a real resolution or an error
What type of domain name does a resolver receive from the generic domains section?
chal.atc.jhda.edu
What happens if a local DNS server cannot resolve a query?
It refers the resolver to other servers or asks other servers directly
What type of domain name does a resolver receive from the country domains section?
chal.jhda.cu.ca.us
What is the final step of the resolver?
It delivers the result to the process that requested it
Study Notes
Introduction to Domain Name System (DNS)
- The DNS is a supporting program that is used by other programs such as e-mail to map a name to an IP address or vice versa.
- DNS client/server program supports an e-mail program to find the IP address of an e-mail recipient.
DNS Functionality
- DNS maps a name to an IP address or an IP address to a name.
- People prefer to use names instead of numeric addresses, so DNS is necessary.
- DNS is used to identify an entity, as TCP/IP protocols use IP addresses to uniquely identify a host on the Internet.
DNS History
- In the early days of the Internet, mapping was done using a host file with two columns: name and address.
- Host files were stored on every host and updated periodically from a master host file.
- With the growth of the Internet, it became impossible to have one single host file to relate every address with a name and vice versa.
Domain Name Space
- A domain name space was designed to have a hierarchical name space with an inverted-tree structure with the root at the top.
- The tree can have up to 128 levels: level 0 (root) to level 127.
- Each node in the tree has a label, which is a string with a maximum of 63 characters.
- Children of a node have different labels, guaranteeing the uniqueness of domain names.
Domain Name
- A full domain name is a sequence of labels separated by dots (.)
- Domain names are always read from the node up to the root.
- The last label is the label of the root (null), so a full domain name always ends in a null label.
Fully Qualified Domain Name (FQDN)
- A label terminated by a null string is called a Fully Qualified Domain Name (FQDN).
- FQDN is inefficient and unreliable because it places a heavy load on the system and any failure makes the data inaccessible.
Hierarchy of Name Servers
- The solution to the problems of FQDN is to distribute the information among many computers called DNS servers.
- DNS servers are divided into many domains based on the first level, creating a hierarchy of servers.
- Each server is responsible (authoritative) for either a large or a small domain.
- 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.
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, and the server either satisfies the resolver or refers the resolver to other servers.
DNS Messages
- DNS has two types of messages: query and response.
- Both query and response messages have the same format with some fields set to zero for the query messages.
- The header is 12 bytes, and its format includes identification subfield, flags subfield, and number of question/answer/authoritative/additional records subfields.
- The question section consists of one or more question records, and it is present in both query and response messages.
Learn about the Domain Name System (DNS), a supporting program in the application layer of the Internet model. Understand the client/server paradigm and how DNS fits into it.
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