Domain Name System

✅ Domain Name

• which computer on network?
• correspond to IP address
• since IP address is too long to memorize, use Domain Name instead for humans
• In order to access a web site, need to get IP address with Domain Name from Domain Name System

For IP address and Domain Name, read this https://soheeparklee.github.io/posts/n-4socketprogramming/

✅ Domain Name System

• DNS is a distributed database consisted of DNS servers in hierarchy

• application layer(layer 7)
• operates on UDP, port 53
• System to convert Domain Name to IP address
• can correspond several IP address to one Domain Name

What is the benefit of DNS being a hierarchical, distributed database?

DNS is used all around the world by many hosts
by being distributed, more effective, scalable.

What is the benefit of DNS being able to correspond several IP address to one Domain Name?

can distribute the load

📌 Domain Name System Operation

1. Type URI www.google.com/index/html on browser
   
2. Browser will get host name www.google.com from URI, send to DNS client
   
3. DNS client sends host name to DNS server
   
4. DNS server responds IP address of host name to DNS client
   
5. When browser recieves IP address from DNS, reset TCP connection to that IP address and port 80 of that IP address to HTTP server process
   

📌 Domain Name System Hierarchy

✔️ DNS recursor

• recursive resolver
• server that responds to DNS query
• ask another DNS server for IP address
• local DNS server

✔️ Local DNS server

• operate as recursive resolver
• is not strictly part of DNS layer, but it is in the center of DNS structure
• ISP has local DNS
• ISP use local DNS to respond to host with IP address
• when host requests for DNS, requested to DNS server as proxy

✔️ Root-Level DNS Server

• name server for root zone
• classified from A to M
• more than 400 root-level DNS servers on internet

✔️ Top-Level DNS Server

• high-level DNS server
• such as .com, .org, net, edu
• from www.google.com, TLD server for the .com will respond
• national domains
• such as .kr, .uk, .jp

✔️ Authoritative name server

• organization, company domains
• final stop for DNS query
• authoritative name server has the DNS record for the request

📌 Types of DNS Service

✔️ Recursive DNS resolver

• DNS recursor
• DNS server that responds to DNS query
• asks for the authoritative name server

✔️ Authoritative DNS server

• server to store DNS request
• if requested for IP address, doesnt ask any other server
• Authoritative name server is the final authority of IP address

📌 DNS queries

✔️ Non-recursive Query

• DNS server is expected to return IP address from own local cache or database
• However, if the DNS server cannot find IP address, it will return empty response.

✔️ Recursive Query

• DNS client(local DNS server) asks DNS resolver(ISP’s DNS server) to find IP address on its behalf
• DNS resolver communicates with other DNS servers to hunt down an IP address and return it to the client

• DNS resolver is responsible for finding IP address

• DNS resolver ask root name server
• DNS resolver asks TLD name server

• DNS resolver asks authoritative name server

• DNS resolver asks again and again ➡️ recursive
• local DNS server
• 재귀적

✔️ Iterative Query

• DNS resolver asks DNS server again and again in sequence for best answer

• DNS resolver ask root DNS server for IP address
• 😟root DNS server doesnt know.
• ➡️ return TLD name server address(best answer root DNS server has)
• ask TLD name server
• 😟 TLD name server doesnt know.
• ➡️ ask for authoritative name server address

• ask authoritative name server…

• continue iterative process until finding IP address, or timeout
• Root, TLD server
• 반복적

📌 DNS Record

Information in a database that links a URL to an IP address
consists of DNS statements(series of text file)

• A record: contain IPv4 address of the domain
• AAAA record: contain IPv4 address of the domain
• CNAME record: 기존 도메인에 별명 붙인 레코드
• MX record: forward email to email servers
• TXT record: allow administrators to store text nodes in records, 텍스트 입력할 수 있는 레코드
• NS record: store nameservers for a DNS entry

📌 DNS Cache

• repository of domain names and IP addresses saved on device

• so does not have to ask for IP address for a frequently visited site

• 👍🏻 speed up DNS request
• 👍🏻 reduce bandwidth

💡 DNS and UDP

Why does DNS use UDP?

1. DNS priorities speed than reliability
   DNS servers dont have to keep connections
   
2. To serve more clients
   DNS requests are generally very small, and can fit well within UDP segments
   

How can DNS improve reliability?

By adding timeout, resend at the application layer

📌 How DNS functions

1. Request Domain Name

• www.google.com

2. Check local DNS cache

• Check local cache on device
• if IP address is found in cache, process ends, website is accessed directly

3. Contact recursive DNS server(Resolver)

• Recursive DNS server(Resolver) provided by ISP

4. Recursive server lookup

• Check Recursive server cache for IP address
• recursive DNS server has its own cache

5. Query root name server

• if recursive server does not have IP address, need to look in Authoritative DNS server
• query root name server for top level domain server
• root Nameservers respond to the Recursive DNS Server's query by providing the IP addresses of the TLD Nameserver

6. Query TLD name server

• .com
• TLD name server provides IP address of Authoritative name server
• what TLD name servers do have is the location of the authoritative name server for requested site

7. Query Authoritative name server

• google.com
• holds the definitive IP address for the domain

8. Get IP address

• passed through each level of DNS server until it reaches your device

9. Access website