The Domain Name System (DNS)

The Domain Name System (DNS)
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Overview

DNS Names

DNS Names (cont.)

Hierarchical:
- Portions are separated by a '.'
- Left portion is most specific, right portion is DNS root
Top Level Domains:
- The rightmost portion of the name (e.g., com, edu, net)

DNS Name Servers

Problems with a Centralized Approach:
- Single point of failure
- Hard to maintain
- Doesn't scale
Problems with a Distributed Approach:
- No server has every mapping
- Queries may need to be handled by multiple servers
- Hard to update

The Distributed Approach that is Used

Hierarchical Organization:
- Different portions of the hierarchy (called zones) are administered by different organizations
Overview of the Process used to Resolve:
- Local DNS server is queried
- If the local query fails, either a recursive or non-recursive (referral) process is used

Types of DNS Servers

Authoritative Name Servers:
- Guaranteed to be able to perform a name-address translation for a particular zone (i.e., portion of the domain name tree)
Root Name Servers:
- "Publish" the root zone file which contains the names and IP addresses of the authoritative servers for all top level domains
- There are 13 identified by the letters A-M (though there are far more machines in about 150 locations in about 50 countires; see http://www.root-servers.org/)
Caching Name Servers:
- Are not authoritative for any zone
- Ask queries and then "remember" them for later use
- The amount of time a response is "remembered" (i.e., cached) is called the time to live (TTL) and is set by the administrator

Recursive Queries

Properties:
- The DNS server will provide a complete answer (or error) to the query
- DNS servers are not required to support recursive queries
Process:
- If the DNS server does not know the answer it will ask another DNS server

Recursive Queries (cont.)

Non-Recursive Queries (a.k.a. Referrals)

Properties:
- The DNS server may provide a partial answer (or error) to the query
Process:
- If the DNS server does not know the answer it will tell you about another DNS server that is "more likely to know" (i.e., is closer to a DNS server that knows)

Non-Recursive Queries (cont.)

Transport Layer Protocols

DNS primarily uses UDP on port 53
TCP is used when responses exceed 512 bytes (and in all cases by some implementations)
EDNS (RFC2671) supports UDP responses larger than 512 bytes

Message Format

Format of Resource Records (RRs)

Interacting with DNS

Performing a Lookup from the Command Line:
- nslookup [-option=value]... host
Registering a Domain:
- Work with a registrar (the list of which can be found at http://www.internic.net
Running a DNS:
- BIND