The Domain Name Service

Overview

• Domain Names
• Nameserver Concepts
  • Zone Example
  • Domain Delegation
  • Resolvers
  • Name Servers
  • How Does it Work?
  • Named Configuration FIles
  • Primary vs. Secondary Servers
• Resource Records
  • Standard Resource Records (RR)
  • Start of Authority RR
  • Nameserver RR
  • Address RR
  • Mail Exchanger RR
  • Canonical Name RR
  • Delegating Subdomains
  • The root cache

Domain Names

• Hierarchical
• Dot used as a searator
• Right-most component is the top-most domain
• Top-level domain: .edu, .gov, .mil, .com, .arpa, .org, .net + the country domains (.uk, .de, .no, .il, etc...), and the new .us hierarchy
• FQDN: eeunix.ee.usm.maine.edu vs. relative name eeunix (relative to a default domain, in this case ee.usm.maine.edu)

Nameserver Concepts

• Zone
  • A zone is part of the name space ( such as ee.usm.maine.edu or bbn.com delegated to a single server: If a nameserver is listed at the internic (or a higher level nameserver as authoritative for parot of the name space, and it has full data on that part of the name space then it is authoritative for that zone.
  Domain
  • A domain is also part of the name space, but it may covers several zones. (maine.maine.edu is a domain that covers both the usm.maine.edu and the caps.maine.edu zones)

Zone Example

Domain Delegation

• Top two levels delegated from the NIC
• Third level and lower delegated by the site controlling the next higher domain. For Example,
  .edu - controlled by the NIC
  .maine.edu - delegated byt the NIC to named.caps.maine.edu
  .usmacs.maine.edu - delegated by named.caps.maine.edu to csir1.usmacs.maine.edu

Resolvers

• The DNS Client
• Generates the queries, for domain name information, that are sent to a name server.
• Completely seperate from the name server function.
• Configured via /etc/resolv.conf
  

  ;
  ; Bind Data
  ;
  domain ee.usm.maine.edu ; used to fully qualify non canonical DNS requests
  nameserver 130.111.130.7 ; Where are my Nameservers
  nameserver 130.111.32.11 ; and in what order to I query them
  nameserver 130.111.128.213 ; Note, these are IP addresses only.
  nameserver 128.89.2.34 ;
  

Name Servers

• The DNS Server
  • Answers DNS Queries sent by resolvers
  • Listens at UDP and TCP port 53
    • UDP for routine queries
    • TCP used for zone transfers
• Configurations
  • Caching-only: relies on other name servers for authoritative answers
  • Primary: Contains the writable authoritative copy for the zones that it is primary for
  • Secondary: Contains mirror copy of the data from a primary nameserver. No updates take place here, used to provide redundancy

How Does It Work?

Named Configuration Files

• Boot file (named.boot): general named configuration parameters
• Root cache (named.ca): cache containing root domain name server names and addresses
• Loopback entry (named.local): Used to locally resolve loopback address
• Forward mappings (hosts.db): File mapping host names to IP addresses
• Reverse mappings (hosts.rev): file mapping IP addresses to names
• All these files except for named.boot are defined in the named.boot file, and can be set locally.

Primary vs. Secondary Servers

• Primary
  • Data loaded from a file.
  • One primary server per zone.
• Secondary
  • Data transferred from a primary server.
  • Data may be stored in a file.
  • Checks every refresh period with the primary, looking for changes.
  • Might have many secondaries per zone

Resource Records.

Standard Resource Records (RR)

• SOA: Start of Authority, Marks the begining of a zone's data, defines default parameters for a zone.
• NS: Name Server, Identifies domain's name servers.
• A: Address, Converts host namess to IP addresses.
• PTR: Pointer, Converts IP Addresses to host names.
• MX: Mail Exchanger, Identifies where to send mail for a given domain name.
• CNAME: Cannonical Name, Defines a host alias.

Start of Authority RR

@	IN SOA eeunix.ee.usm.maine.edu postmaster.ee.usm.maine.edu (
				1995061403	; Serial
				300			; Refresh - 5 Minutes
				60			; Retry - 1 minute
				1209600		; Expire - 2 Weeks
				43200)		; Minimum - 12 Hours

• A SOA record starts every zone file.
• Contains the name of the Primary Domain Server
• Technical contact for domain
  • Note the '.' instead of '@'
  • Should be someone who can fix DNS problems, or a usefull mail alias.
• Serial (or version) number:
  • Must incease whenever a zone changes
  • Used by secondary servers to detect zone changes.
  • Try to avoid using version numbers like 1.3, 1.3...
  • A usefull convention is YYYYMMDD##

Nameserver RR

• NS Records specify name servers for a domain.
  

  ;
  ; Nameservers
  ;
  ee.usm.maine.edu.	IN	NS	nameb.caps.maine.edu
  					IN	NS	named.caps.maine.edu
  					IN	NS	csir1.usmacs.maine.edu
  

Address RR

• A records specify IP addresses for hosts.
• In the example a relative name was used, so the current domain (specified in named.boot) will be appended. This entry in the musenet.org domain is equivalent to:
  

  cyber.musenet.org.	IN	A	128.89.6.189

  
  

  ;
  ; Host Information
  ;
  
  cyber			IN	A	128.89.6.189
  

Mail Exchanger RR

• MX records specify hosts which will accept mail for the specified domain object.
• Lowest preference value is tried first, equally weighted entries should be tried randomly.
  

  ;
  ; Mail Exchanger Information
  ;
  
  eeunix			IN	A	130.111.148.45
  			IN	MX	0	eeunix
  			IN	MX	1	maine.maine.edu.
  

Canonical Name RR

• CNAME: The specified object is defined as an alias for the host showed in the data field.
• The example defines "ftp.musenet.org" as a CNAME for "cyberion.musenet.org"
• If you define an object as a CNAME you can't specify any other RR's for that object.
  

  ;
  ; CNAME's
  ;
  
  cyberion		IN	A	128.89.2.139
  			IN	MX	0	cyber.musenet.org.
  ftp			IN	CNAME	cyberion
  

Delegating Subdomains

• Only list domain to be delegated, and the name servers for the domain, and perhaps A records for the nameservers. The nameservers of the delegated domain will provide all other information.
  

  ;
  ; Delegate the usm.musenet.org domain 
  ;
  
  usm			IN	NS	named.usm.musenet.org.
  

The root cache

• Every nameserver contacts a root server upon startup.
• If you restart named when you are "off the network", things will NOT work properly