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DNS and Java

Computers on the ARPANET used to download the hosts.txt file the central location and used it for Name Resolution

This article discusses DNS and HostName resolution in Java. The following are the topics that will be briefed in this article:

    • Hostname Resolution
    • What is DNS
    • Methods of Hostname Resolution
    • Hostname Resolution on Windows/Linux
    • DNS Client Resolver Cache
    • Viewing and Flushing DNS Cache
    • Hostname resolution in Java.
    • Debugging Approach

Hostname Resolution.
Determining the IP Address of the machine from the Hostname is termed as Hostname Resolution. Every computer (host) has a name. The Hostname should correspond to an IP address mapping stored in local Hosts file or in a database on a DNS Server.

Translating (resolving) machine (and domain) names into the numbers actually used on the Internet is the business of machines that offer the Domain Name Service.

What is DNS.

Background:
During the earlier days(in the age of ARPANET Project) the hosts.txt was used for Name Resolution. Computers on the ARPANET used to download the hosts.txt file the central location and used it for Name Resolution. As the number of hosts on internet increased usage of hosts.txt files became difficult and cumbersome process to maintain/update the hosts.txt file.

The replacement of hosts.txt file is the DNS.

Definition of DNS:
Domain Name System is a hierarchical naming system for computers, services or any resources connected to the internet or a private network.

It translates meaningful domain names to IP Addresses. An often-analogy to explain the Domain Name System is that it serves as phone-book for the internet by translating human-friendly computer hostname into IP Addresses.

Methods of HostName Resolution:

Local HostName

Return value of command hostname or the name configured for the computer.This name is compared with destination hostname.

Hosts File This file maps hostname to IP Addresses
DNS Server A server that maintains a database of IP Address-to-host name  mappings and has the ability query other DNS servers for mappings that it does not contain.

Hostname Resolution on Windows/Linux:
Windows

(a) Local hostname, if the destination address is same as local address then IP address for the same is returned and hostname resolution process is stopped.

(b) If (a) returns false, then "DNS Client Resolver Cache" is looked upon to check whether it consists of required hostname, if true then IP Address is returned else DNS server is queried to get the IP Address.

Linux

If your application needs to know the IP address of a particular computer. The application requiring this information asks the 'resolver' on your Linux PC to provide this information.

a) The resolver queries the local host file (/etc/hosts and/or the domain name servers it knows about (the exact behavior of the resolver is determined by /etc/host.conf) ;

b) If the answer is found in the host file, this answer is returned;

  • if a domain name server is specified, your PC queries this machine
  • The DNS machine already knows the IP number for the required name, it returns it. If it does not, it queries other name servers across the Internet to find the information. The name server than passes this information back to the requesting resolver - which gives the information to the requesting application.

DNS Client Resolver Cache
"DNS Client Resolver Cache" is a RAM based table (dynamically constructed) consisting of entries in the Hosts file and the host names that OS has previously resolved successfully through DNS. Response received from DNS queries are cached for a certain period of time and that period of time is configurable thru the parameter "Time to Live" (TTL).

Managing DNS Resolver cache: Following variables are used to maintain the DNS Resolver cache.

networkaddress.cache.ttl Specified in java. Security to indicate the caching policy for successful name lookups from the name service. The value is specified as integer to indicate the number of seconds to cache the successful lookup.
networkaddress.cache.negative.ttl Specified in java. Security to indicate the caching policy for un-successful name lookups from the name service. The value is specified as integer to indicate the number of seconds to cache the failure for un-successful lookups. A value of 0 indicates never cache. A value of -1 indicates cache forever.

Starting Java 6 the default value for "networkaddress.cache.ttl" is changed to 30 and this can be determined thru the following program:

public class Foo {

public static void main (String[] args) {

System.out.println(sun.net.InetAddressCachePolicy.get());

}

}

Viewing and Flushing DNS Cache

View the contents of "DNS Client Resolver Cache" on windows

Issue ipconfig /displaydns.from command prompt.
Flush the DNS Cache contents on windows. Issue ipconfig /flushdns from command prompt

Flush the DNS Cache contents on Linux. On Linux the nscd daemon manages the DNS cache.

restart the nscd daemon To restart the nscd daemon, use the command `/etc/init.d/nscd restart`.

Hosts File is a common way to resolve hostname to IP Address through a locally stored text file containing IP-address-to-host-name mappings. The file is resident in /etc/hosts on UNIX platform and systemroot/System32/Drivers/Etc folder on windows.

Hostname resolution in Java.

Hostname to IP Address is resolved in Java thru the

A) Use of combination of local machine configuration and

b) Network naming service such as DNS and

c) Network information service (NIS).

The inetAddress class provides methods to resolve hostname to IP Address and vice-versa also. Following are the methods that can be used for hostname resolution:

getAddress Returns the raw IP Address for this object.
getAllByName(String host) Given the name of host, an array of IP address is returned.
getByAddress(byte[] addr) Returns an InetAddress object given the raw IP address
getByAddress(String host, byte[] addr) Create an InetAddress based on the provided host name and IP address
getByName(String host) Determines the IP address of a host, given the host's name.
getCanonicalHostName() Gets the fully qualified domain name for this IP address.
getHostAddress() Returns the IP address string in textual presentation
getHostName() Gets the host name for this IP address
getLocalHost() Returns the local host.

 

Simple Program exercising few of the above methods:

import java.net.*;

class InetAddressTest

{

public static void main(String args[]) throws

UnknownHostException {

InetAddress Address = InetAddress.getLocalHost();

System.out.println(Address);

Address = InetAddress.getByName("starwave.com");

System.out.println(Address);

InetAddress SW[] = InetAddress.getAllByName("www.nba.com");

for (int i=0; i<SW.length; i++)

System.out.println(SW[i]);

System.out.println("InetAddress.getCanonicalHostName()"+Address.getCanonicalHostName());

}

}

Debugging Approach:

What process is making the DNS queries on my server:

Use Dtrace to determine the processes making the DNS requests.

Reference:

http://en.wikipedia.org/wiki/DTrace

http://www.mactech.com/articles/mactech/Vol.23/23.11/ExploringLeopardwithDTrace/index.html

http://forums.sun.com/thread.jspa?forumID=861&threadID=5418298

Further Reading:

Understanding DNS : http://www.tech-faq.com/understanding-dns.html

Java DNS Logger :http://www.genady.net/dns/

DNS Overview : http://technet.microsoft.com/en-us/library/bb727007.aspx#EHAA

Setting up Name to Address Resolution(DNS) :http://tldp.org/HOWTO/PPP-HOWTO/dns.html#AEN877

More Stories By ChandraShekar Dattatreya

Chandra Shekara Dattatreya is a DevOps guy working in a Fortune-500 company and has 10+ years' of experience debugging JVM-related issues. In the course of debugging, he has encountered multiple scenarios from various customers and provided solutions to all of them with success. In his current role, he is engaged in identifying and resolving JVM-related issues for an e-commerce company.

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