9.Lecture

DNS

This goes way more in-depth than the slides needs you to know

DNS Address Resolution

sequenceDiagram
	rect Cornsilk
		note right of Client: Internal DNS Resolution
		Client->>hosts file: www.smu.com?
		hosts file-->>Client: idk
		Client->>Internal DNS Cache: www.smu.com?
		Internal DNS Cache->>Client: idk
	end
	rect Bisque
		note right of Client: Recursive DNS Resolution
	    Client->>ISP DNS 1: www.smu.com?
	    ISP DNS 1->>+ISP DNS 2: www.smu.com?
	end
	rect LightSalmon
		note right of ISP DNS 2: Iterative DNS Resolution from Root
		ISP DNS 2->> Root Name Server: www.smu.com?
		Root Name Server->>ISP DNS 2: Ask the TLD at c.c.c.c
		ISP DNS 2->>TLD(.com): www.smu.com?
		TLD(.com)->>ISP DNS 2: Ask the Authoritative DNS at y.y.y.y
		ISP DNS 2->>Authoritative DNS: www.smu.com?
		Authoritative DNS->>ISP DNS 2: 8.8.8.8
	end
	rect Bisque
		ISP DNS 2-->>ISP DNS 2: Update Cache
	    ISP DNS 2->>-ISP DNS 1: 8.8.8.8
	    ISP DNS 1-->>ISP DNS 1: Update Cache
	    ISP DNS 1->>Client: 8.8.8.8
	end
    Client-->>Internal DNS Cache: Update Cache
    

1. Client checks its own hosts file for the address translation
2. www.smu.com was not in the hosts file
3. Client checks its own DNS Cache for the address translation
4. www.smu.com was not in its own DNS Cache
5. a) Client asks the default DNS server (configured by its ISP) for www.smu.com
6. b) ISP DNS 1 did not have www.smu.com in its cache, recursively asks ISP DNS 2
7. ISP DNS 2 did not have www.smu.com in its cache, so it asks one of the Global Root Name Servers
8. The Root Name Server then replies with the address of a TLD Name Server which knows where all of the “.com” domains can be found
9. ISP DNS 2 then asks the given TLD Name Server where to find www.smu.com
10. The TLD Name Server then replies with the address of an Authoritative Name Server which knows where www.smu.com is
11. ISP DNS 2 then asks the Authoritative Name Server where to find www.smu.com
12. The Authoritative Name Server then replies with the IP address of www.smu.com
13. ISP DNS 2 then updates its cache and forwards the IP address back to ISP DNS 1
14. ISP DNS 1 then updates its cache and forwards the IP address back to the Client
15. Client then updates its internal DNS cache

Recursive vs Iterative

In a recursive DNS query, if the DNS server that was asked does not know the address, it will ask another DNS server

In an iterative DNS query, if the DNS server that was asked does not know the address, it responds with the address of another DNS server. The sender then has to make another query to the given DNS server.