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2023-09-21

Summary

keywords DNS, DNS registrar, distributed hierarchical database, iterative query, DNS cache,

TODO

HW see the example demo on slide 3-59 https://realpython.com/python-https:/#what-is-http find DNS registrar for .com TLD, .kr TLD

Exercise*

Next time

DNS

Set of servers of serving ip-domain pair table.

DNS protocol is application level protocol.

Role of the DNS system​

every packet's header needs source IP address, Destination IP address DNS tells your destination ip by searching the plaintext-based domain.

Steps.​

  1. send DNS Request (www.google.com)
  2. every dns request is served by the local DNS server.
  3. If local DNS server knows its ip, it responses DNS response.(8.8.8.8)
  4. Now, we can send Request to google.

Why is DNS needed?​

We cannot remember each IP address. number sequence is very hard to remember.

DNS properties​

  • distributed & hierarchical database: it is a distributed database in many name servers.
  • application-layer protocol : it is a core functionality, but implemented as a application layer protocol.
  • Part of an edge network.

DNS service.​

It services

  • hostn ame-to-IP-address translation
  • host aliasing. (simple host name to difficult host names)
  • mail server aliasing
  • (sometimes) Load distribution.

Why distributing? not centralized?​

  • single point of failure
  • traffic volume is bigger.
  • if centralized database is distant, service is slow
  • maintenace.
  • centralized approach doesn't scale.
  • responsibility is also distributed.

Hierarchy of DNS​

root, TLD(Top Level Domain), Authoritative NS ![[../images/20230921133825.png]]

  1. ask Root of .com TLD server ip
  2. ask .com TLP of google.com NS server ip.
  3. ask google.com NS server of www.google.com ip.

DNS is important.​

Powered by Associations. We call them DNS registrar.

  • DNSSEC provides security on DNS servers.
  • ICANN manages root DNS servers.

root DNS keeps records only about TLD servers. TLD keeps records only about authroitative servers.

Local DNS servers.​

  • does not strictly belong in the hierarchy tree.
  • each ISP has their own local DNS server.
  • when host makes DNS query, it is sent to its local DNS server.
  • uses caches to gain efficiency.

Iterative query​

![[../images/20230921135633.png]]

recursive query​

  • extra load on upper head hierarchy ![[../images/20230921135704.png]]

DNS Cache​

  • DNS caches mapping.
  • improves response time
  • DNS doesn't worry about the out-of-date information
  • It is the best out of us.
  • if the DNS response is based from cache, it is called from "non-authoritative server"

DNS records : RR records​

RR format : (name, value, type, ttl) ex. (google.com,8.8.8.8, A,16000) ![[../images/20230921141817.png]]

Types​

  • type A
    • name : hostname
    • value : ip
  • type CNAME
    • name : alias name for real name
    • value : canonical (real) name
  • type MX
    • value : name of SMTP mail
  • type NS
    • name : domain (foo.com)
    • value : hostname of authoritative NS

DNS protocol messages​

identification number
query reply has the same identification number of its corresponding request. flag each bit distinks about the ...? #todo : what is flag

number of questionsk nuber of answer RRS, authority RRS, ... #todo : see slide 2-88, Getting your info into the DNS