Address Conversion Functions and The Domain Name System

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Address Conversion Functions and  The Domain Name System  

RFC 1034

RFC 1035

 

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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?  

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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.  

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DNS Hierarchy 

edu 

com 

org 

jp 

rpi 

albany

 

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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    

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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.  

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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.  

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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.  

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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

 

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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.  

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/etc/resolv.conf 

domain rpi.edu

128.113.1.5

128.113.1.3

 

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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.  

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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.  

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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!  

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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  

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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

 

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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.  

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DNS Message Format 

HEADER

QUERIES

Response RESOURCE RECORDS

Response AUTHORITY RECORDS

Response ADDITIONAL INFORMATION

 

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DNS Message Header 

query identifier flags # of questions # of RRs # of authority RRs # of additional RRs  

16 bit fields 

Response 

}

 

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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  

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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.  

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Question Format 

Name: domain name (or IP address)   Query type (A, NS, MX, 00   Query class (1 for IP)  

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Response Resource Record 

Domain Name Response type Class (IP) Time to live (in seconds) Length of resource data Resource data  

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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.  

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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).  

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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.

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