The Domain Name System (DNS)
An Introduction
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Prof. David Bernstein
James Madison University
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Computer Science Department
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bernstdh@jmu.edu
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Overview
- Purpose:
- Maps human-readable machine names to IP addresses
- Specification:
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
- Header (12 octets):
- ID
- Flags
- Number of Questions
- Number of Answer Resource Records (RRs)
- Number of Authority RRs
- Number of Additional RRs
- Questions
- Answer Resource Records
- Authority Resource Records
- Additional Resource Records
Format of Resource Records (RRs)
- Name (variable):
- Type (2 octets):
- Type of the record (e.g., " A" for IPv4 lookup)
- Class (2 octets)
- Usually "IN" (for internet)
- TTL (4 octets):
- RDLENGTH (2 octets):
- Length of the RDATA field
- RDATA (variable)
- Additional data (e.g., the IP address for "A" records)
Interacting with DNS
- Performing a Lookup from the Command Line:
- nslookup [-option=value]... host
- Registering a Domain:
- Running a DNS:
There's Always More to Learn
