CS 480 M01 Lecture Notes 18 - Nov 7, 2022

Future Topics / Votes

Chapter 16: Domain Name Service (DNS)

DNS Defines :

• hierarchical namespace for hosts and IP addresses
• host table implemented as a distributed database
• "resolver" - lib routines to query it

• improved routing for e-mail and sender authentication
• mechanism to find services on the network
• protocol for exchanging naming info

distributed db - each site maintains data for its own hosts and cooperates with others to help with look ups
queries (query consists of name and query type) and responses (set of resource records)

DNS service provider commercial providers or own DNS server (lab)

configure your systems to look up names in DNS - DNS clients
resolv.conf - client resolver
nsswitch.conf - who do I ask for a name?

• Organized into a tree that contains both forward mappings and reverse mappings
• Tree of "domains" loosely managed by single entity
• the root is "." ( dot )
• top level domains
  8 traditional top level generic domains in 2 branches: forward (name -> IP address) and reverse (IP address -> name) mapping
    7 new top level generic domains approved in 2001 by ICANN (many more recently)
  plus country code domains
• domain names are case insensitive ( X internationalization & DNSSEC)
• fully qualified name is terminated by dot ( cs.nmsu.edu. )
  no "dot" means the address is relative
• more names for one host is common (www, ftp, ...)
• 
  
  
  
• 
  
• Root Zone Database (IANA)
• Registering a second-level domain name - through authority of the domain
( ICANN (interNIC) Accredited Registrars, ...)
name (not taken), tech contact, admin contact, 2 or more servers
• Creating own subdomains
  name (unique in local context), 2 or more servers, coordinate with parent domain

• Each host using DNS is either a client or both client and server.
• Sites maintain their own resource records about their hosts (distributed DB) - data that flows around the DNS and gets cached
• delegation
  Each zone delegates authority over its subdomains to other servers.
  Root servers in config files or built-in, root servers know servers for top-level domains,...
• DNS query process
  
• caching and efficiency
  caching increases the efficiency of lookups and is almost free - mappings change infrequently
  answer saved for a period of time (TTL) specified by the owner of the data - 1 hour to 1 day normally (load balanced service may have TTL just seconds or 1 minute)
  + negative caching:
  • no host or domain - 1-3 hours
  • type of data requested doesn't exist for the host - 1-3 hours
  • server not responding - 5 min
  • server unreachable - 5 min

• answers the queries
• if it doesn't know -> asks other servers and caches it
• performs "zone transfers" - copy data among servers
• deals with zones (domain minus subdomains)
• operates in several different modes. Categorizations vary according to
  • the name server’s source of data (authoritative, caching, master, slave),
  • the type of data saved (stub),
  • the query path (forwarder),
  • the completeness of answers handed out (recursive, nonrecursive), and
  • the visibility of the server (distribution)
  
• single server may play different roles with respect to different zones

• IPv6 support (32 -> 128 bits address)
• DNSSEC (authentication using public key cryptography)
• Dynamic update (for sites with DHCP)
• Async notifications of zone changes
• ...

• set of text (zone) files on the domain's master name server
• Two types of records
  • parser commands: $ORIGIN, $INCLUDE, $TTL, .. - make zone files easier to read and maintain, they
    • either influence the way the parser interprets subsequent records,
    • or expand into multiple DNS records
  • resource records: [name] [ttl] [class] type data
    name identifies the entity (host, domain), it can be relative or absolute
• Common record types:
  

  SOA - Start of Authority (master=primary name server, tech contact, various timeout values)

  @               IN SOA          test.cs480.cs.nmsu.edu.        root.test.cs480.cs.nmsu.edu. (
                                  2009101101      ; serial
                                  3h              ; refresh
                                  1h              ; retry
                                  1w              ; expiry
                                  1d )            ; minimum
  
  

  ( @ - is a short hand for the current zone)
  
  NS - Name Server

  cs480.cs.nmsu.edu.      IN NS           test.cs480.cs.nmsu.edu.
  

  or

        IN NS           test.cs480.cs.nmsu.edu.
  

  
  A - IPv4 Address ( Name -> Address)

  lbc             IN A            192.168.8.1
  

  
  PTR - Pointer ( Address -> Name )

  1               IN PTR          lbc.cs480.cs.nmsu.edu.
  
  
  

• Components:
  • named - the BIND name server daemon,
  • resolver library (queries DNS on behalf of users) - gethostbyname family of library routines
  • command line interfaces: nslookup, dig, host and drill, delv
  • rndc - pgm to remotely control named