Application Layer: DNS

Overview

• Define: a distributed, hierarchical database

• Run on UDP

• Hierarchy:

  

• Three classes of DNS servers:

  • root server
  • TLD (Top-level domain) server
  • authoritative server

Resource record (RR)

• A resource record is a four-tuple that contains the following fields:

  (Name, Value, Type, TTL)

Type

• A: provides the standard hostname-to-IP address mapping (IPv4)
  • Name: a hostname
  • Value: IP for the hostname
• AAAA (IPv6)
• CNAME: Value is a canonical hostname for the alias hostname Name
• NS:
  • Name: a domain
  • Value: the hostname of an authoritative DNS server for this domain

DNS server categories

DNS root nameserver

• Provide:
  1. (.edu, NS, TLD)
  2. (TLD, A, TLD-IP)
• Job: maintain information for all the domain names that share a common domain extension (.com, .xyz, .net)

TLD nameserver

• Provide (edu TLD server):
  • (bu.edu, NS, auth)
  • (auth, A, auth-IP)

Authoritative nameserver

• Run and Maintain by universities and organizations which hold mapping from hosts to IP addresses
• Provide:
  • (bu.edu, A, bu.edu-IP)

DNS name resolution

• once (any) name server learns mapping, it caches mapping
• cache entries timeout (disappear) after some time (TTL)

🟡 Attention

DNS procedure: 问路但不带路

🔴 Caution

Why DNS system sends back both the NS record and the A record?

Because IP changes frequently while the domain name never changes.

It is more caching efficient, since including both the NS and A records helps reduce the number of query. By receiving the both, it is more efficient to update A records.

Protocol messages