Chapter 11 - DNS

alternative to hard-coding entries into /etc/hosts

• DNS Structure
  top-level domains (maintained by Network Information Center (NIC): ARPA (net), COM, EDU, GOV, MIL, ORG
  second-level domain names must be registered

• The Name Server
  • each DNS name server manages a zone (distinct area of the network) [should have a backup name server]
  • communications between servers within a zone is by zone transfer protocol
  • messages between servers outside the same zone is by UDP except for the database updates which is by TCP
  • a request to resolve a symbolic name is sent from the application to a resolver process which sends the query to the name server (if the name server knows the network address, it returns it,otherwise it sends the query to a another name server
• Resource Records
  

  Resource Record format

  DomainName (variable length)

  Type of resource record(16 bit)

  Class(16 bit)

  TTL(32 bit)

  Data Length (16 bit)

  Data (variable length)

  Resource Record types and data contents

  Nbr	Code	Description	Data Fields
  1	A	Network Address	net addr
  2	NS	Authoritative Name Server	domain name of host
  5	CNAME	Canonical alias name	alias for the machine
  6	SOA	Start of zone authority	admin fields
  8	MG	Mailbox member	domain name of mailbox
  11	WKS	Well-known service	address;protocol;bitmap to identify ports
  12	PTR	Pointer to a domain name	PTRDNAME
  13	HINFO	host information	CPU; OS
  14	MINFO	mailbox info	RMAILBX(mailbox responsible for mailing lists) EMAILBOX(mailbox for error messages)
  15	MX	Mail exchange	PREFERENCE (precedence for delivery) EXCHANGE(domain name of the mail exchange host)

  ex.
  TPCI_SCO_4    IN    A    143.23.25.7
  TPCI_SCO.TPCI.COM    IN    WKS    143.23.1.34.
                  FTP TCP SMTP TELNET

• IN-ADDR-ARPA

  frodo	IN	A	192.132.89.65
  jaring	IN	A	192.132.89.167
  route	IN	A	192.132.89.1
  	IN	A	204.134.108.1
  merlin	IN	A	192.132.89.3

  
  

  • file indexed on name, so name-to-address mapping easy
  • address-to-name requires IN-ADDR-ARPA

    65.89.132.192.IN-ADDR-ARPA.	PTR	frodo.nmhu.edu
    167.89.132.192.IN-ADDR-ARPA.	PTR	jaring.nmhu.edu
    89.132.192.IN-ADDR-ARPA.	PTR	nmhu.edu

• Messages
  

  DNS Message Format

  Header

  Question

  Answer

  Authority (name of other name servers that might have info)

  Additional info

  
  
• The Name Resolver
  application sends a query to the name resolver which attempts to resolve address from its cache; if not available, it passes the query to a name server (uses both UDP and TCP for queries)
  Berkeley Internet Name Domain (BIND) server provides cache and iterative query capabilities
• Configuring a UNIX DNS Server
  
  1. named.hosts - defines domain with hostname-to-IP mappings
  2. named.rev - uses IN-ADDR-ARPA for IP-to-hostname mappings
  3. named.local - used to resolve the loopback driver
  4. named.ca - lists root domain servers
  5. named.boot - used to set file and database locations
  • Entering the Resource Records

    ; named.hosts files
    ; Start of Autority RR
    nmhu.edu hitech.merlin.nmhu.edu ( 2 ; Serial Number 7200 ; Refresh (2 hrs) 3600 ; Retry (1 hr) 151200 ; Expire (1 week) 8640 ); min TTL	IN	SOA	frodo.nmhu.edu
    ;
    ; Name Service RRs
    nmhu.edu	IN	NS	frodo.nmhu.edu
    ;
    ; Address RRs
    jaring	IN	A	192.132.89.167
    tricera	IN	A	192.132.89.173

  • Completing the DNS Files

    ; named.rev files
    ; Start of Autority RR
    89.132.192.in-addr-arpa hitech.merlin.nmhu.edu ( 2 ; Serial Number 7200 ; Refresh (2 hrs) 3600 ; Retry (1 hr) 151200 ; Expire (1 week) 8640 ); min TTL	IN	SOA	frodo.nmhu.edu
    ;
    ; Name Service RRs
    89.132.192.in-addr-arpa	IN	NS	frodo.nmhu.edu
    ;
    ; Address RRs
    167.89.132.192.in-addr-arpa	IN	PTR	jaring

    ; named.local files
    ; Start of Autority RR
    0.0.127.in-addr-arpa hitech.merlin.nmhu.edu ( 2 ; Serial Number 7200 ; Refresh (2 hrs) 3600 ; Retry (1 hr) 151200 ; Expire (1 week) 8640 ); min TTL	IN	SOA	frodo.nmhu.edu
    ;
    ; Name Service RRs
    0.0.127.in-addr-arpa	IN	NS	frodo.nmhu.edu
    ;
    ; Address RRs
    1.0.0.127.in-addr-arpa	IN	PTR	localhist

    ; named.ca files
    ;
    ; servers for the root domain
    ;
    99999999	IN	NS	ns.nic.ddn.mil
    99999999	IN	NS	ns.nasa.gov
    99999999	IN	NS	ns.internic.net
    ;
    ; servers by address
    ;
    ns.nic.ddn.mil 99999999	IN	A	192.112.36.4
    ns.nasa.gov 99999999	IN	A	192.52.192.10
    ns.internic.net 99999999	IN	A	198.41.0.4

    ; named.boot
    directory	/usr/lib/named
    primary	nmhu.edu	named.hosts
    primary	89.132.192.in-addr-arpa	named.rev
    primary	0.0.127.in-addr-arpa	named.local
    cache	.	named.ca

  • Starting the DNS Daemons
    rc startup script should include:
    # Run DNS server if named.boot exists
    if [ -f /etc/inet/named.boot -a -x /usr/sbin/in.named ] then
        /usr/sbin/in.named
    fi

  • Configuring a Client: modify the /etc/resolv.conf file:
    domain nmhu.edu
    nameserver 192.132.89.65

• BOOTP Messages