DNS Of Application Layer

Question 1

What is DNS (domain name system)?

Answer 1

Distributed database

Hierarchy of many name servers

Question 2

How do you optimise DNS?

Answer 2

When local server caches name IP-mappings
Local DNS server: returns reply from local cache or forwards request to DNS hierarchy
Caching: once name server learns mapping, caches mapping and immediately returns

Question 3

What are the benefits of DNS optimisation?

Answer 3

Improves response time
Cache entries Tim edit after certain TTL
May be out of date

Question 4

How does DNS protocol message work?

Answer 4

Query and reply have same format

Message header: identification (16 bits) and flags (query/reply)

Question 5

What is an iterated query?

Answer 5

Always comes back to local DNS server

Question 6

What is a recursive query?

Answer 6

Not normally taken (lots of queries)

Question 7

How does host aliasing work?

Answer 7

Host has 1 canonical name

DNS translates from several aliases to host then IP

Question 8

How does local distribution work?

Answer 8

Query for aliased name mapped to set of addresses

Responds with entire set in rotated order addresses

Question 9

What is the RR format of DNS records?

Answer 9

Name
Value
Type
TTL

Question 10

How does email work?

Answer 10

User agents: compose, edit, read email
Mail servers: inbox contains incoming messages, message queue has outgoing
SMTP: client server protocol, uses TCP, handshaking then transfer message then closure