>
 
  
 
Netprog 
 2001   DNS  
 
1  
 
 
Address 
 Conversion Functions and 
 The Domain Name System  
 
RFC 1034
 
RFC 1035
 
 
 
  
 
Netprog 
 2001   DNS  
 
2  
 
 
Hostnames 
 
 
   IP Addresses 
   are great for computers
   
     IP address 
     includes information used for routing.
     
   IP addresses 
   are tough for humans to remember.
   IP addresses 
   are impossible to guess.
   
     ever guessed 
     at the name of a WWW site?
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
3  
 
 
The Domain 
 Name System 
 
   The domain 
   name system is usually used to translate a host name into an IP 
   address .
 
  
 
   Domain names 
   comprise a hierarchy so that names are unique, yet easy to remember.
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
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DNS Hierarchy 
 
 
edu 
 
 
com 
 
 
org 
 
 
jp 
 
 
rpi 
 
 
albany
 
 
 
  
 
Netprog 
 2001   DNS  
 
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Host name 
 structure 
 
   Each host 
   name is made up of a sequence of labels separated by periods.
   
     Each label 
     can be up to 63 characters
     The total 
     name can be at most 255 characters.
     
   Examples:
   
     whitehouse.gov
     barney.the.purple.dinosaur.com
     monica.cs.rpi.edu  
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
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Domain 
 Name 
 
   The domain 
   name for a host is the sequence of labels that lead from the host (leaf 
   node in the naming tree) to the top of the worldwide naming tree.
 
  
 
   A domain is 
   a subtree of the worldwide naming tree.
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
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Top level 
 domains 
 
   edu, 
   gov, com, net, org, mil, 
   00/font>
   Countries 
   each have a top level domain (2 letter domain name).
   New top level 
   domains proposed.
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
8  
 
 
DNS Organization 
 
 
   Distributed 
   Database
   
     The organization 
     that owns a domain name is responsible for running  a DNS server 
     that can provide the mapping between hostnames within the domain to 
     IP addresses.
     So - some 
     machine run by RPI is responsible for everything within the rpi.edu 
     domain.
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
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rpi.edu
 
DNS DB 
 
 
rpi.edu
 
DNS DB 
 
 
DNS Distributed 
 Database 
 
   There is one 
   primary server for a domain, and typically a number of secondary servers 
   containing replicated databases.
 
   
 
rpi.edu
 
DNS DB 
 
 
Authoritative 
 
 
rpi.edu
 
DNS DB 
 
 
Replicas 
 
 
rpi.edu DNS server
 
 
 
  
 
Netprog 
 2001   DNS  
 
10  
 
 
DNS Clients 
 
 
   A DNS client 
   is called a resolver.
 
  
 
   A call to gethostbyname()is handled by a resolver (typically 
   part of the client).
 
  
 
   Most Unix 
   workstations have the file /etc/resolv.conf that contains the local 
   domain and the addresses of DNS servers for that domain.
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
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/etc/resolv.conf 
 
 
domain rpi.edu
 
128.113.1.5
 
128.113.1.3
 
 
 
  
 
Netprog 
 2001   DNS  
 
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nslookup 
 
 
   nslookup is 
   an interactive resolver that allows the user to communicate directly 
   with a DNS server.
 
  
 
   nslookup is 
   usually available on Unix workstations.
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
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DNS Servers 
 
 
   Servers handle 
   requests for their domain directly.
   Servers handle 
   requests for other domains by contacting remote DNS server(s).
   Servers cache 
   external mappings.
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
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Server 
 - Server Communication 
 
   If a server 
   is asked to provide the mapping for a host outside it00 domain (and 
   the mapping is not in the server cache):
   
     The server 
     finds a nameserver for the target domain.
     The server 
     asks the nameserver to provide the host name to IP translation.
     
   To find the 
   right nameserver, use DNS!
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
15  
 
 
DNS Data 
 
 
   DNS databases 
   contain more than just hostname-to-address records:
   
     Name server 
     records  NS
     Hostname aliases 
       CNAME
     Mail Exchangers   MX
     Host Information   HINFO
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
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The Root 
 DNS Server 
 
   The root server 
   needs to know the address of 1st (and many 2nd) level domain nameservers.
 
  
 
edu 
 
 
com 
 
 
org 
 
 
jp 
 
 
albany 
 
 
rpi
 
 
 
  
 
Netprog 
 2001   DNS  
 
17  
 
 
Server 
 Operation  
 
   If a server 
   has no clue about where to find the address for a hostname, ask the 
   root server.
   The root server 
   will tell you what nameserver to contact.
   A request 
   may get forwarded a few times.
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
18  
 
 
DNS Message 
 Format 
 
HEADER
 
QUERIES
 
Response 
 RESOURCE RECORDS
 
Response 
 AUTHORITY RECORDS
 
Response 
 ADDITIONAL INFORMATION
 
 
 
  
 
Netprog 
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DNS Message 
 Header 
 
   query identifier
   flags
   # of questions
   # of RRs
   # of authority 
   RRs
   # of additional 
   RRs
 
  
 
16 bit fields 
 
 
Response 
 
 
}
 
 
 
  
 
Netprog 
 2001   DNS  
 
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Message 
 Flags 
 
   QR: Query=0, 
   Response=1
   AA: Authoritative 
   Answer
   TC: response 
   truncated (> 512 bytes)
   RD: recursion 
   desired
   RA: recursion 
   available
   rcode: return 
   code
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
21  
 
 
Recursion 
 
 
   A request 
   can indicate that recursion is desired - this tells the server to find 
   out the answer (possibly by contacting other servers).
   If recursion 
   is not requested - the response may be a list of other name servers 
   to contact.
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
22  
 
 
Question 
 Format 
 
   Name: domain 
   name (or IP address)
 
  
 
   Query type 
   (A, NS, MX, 00
 
  
 
   Query class 
   (1 for IP)
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
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Response 
 Resource Record 
 
   Domain Name
   Response type 
   Class (IP)
   Time to live 
   (in seconds) 
   Length of 
   resource data 
   Resource data
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
24  
 
 
UDP & 
 TCP 
 
   Both UDP and 
   TCP are used:
   
     TCP for 
     transfers of entire database to secondary servers (replication).
     UDP for lookups
     If more than 
     512 bytes in response - requestor resubmits request using TCP.
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
25  
 
 
Lots more 
 
 
   This is not 
   a complete description ! 
   If interested 
   - look at:
   
     RFC 1034: 
     DNS  concepts and facilities.
     RFC 1035: 
     DNS implementation and protocol specification.
     play with 
     nslookup.
     Look at code 
     for BIND (DNS server code).
 
 
 
 
  
 
Netprog 
 2001   DNS  
 
26  
 
 
Name to 
 Address Conversion 
 
   The InetAddress class provides Name to address 
   conversion.
   Sample code 
   is on the web:
   
     Myaddress 
     00prints out local IP address
     Lookup 00
     prints out all the IP addresses for a hostname.