Welcome!

Java IoT Authors: Stackify Blog, Nitin Donde, Elizabeth White, Liz McMillan, Kevin Benedict

Related Topics: Java IoT

Java IoT: Blog Post

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.

@ThingsExpo Stories
"We are focused on SAP running in the clouds, to make this super easy because we believe in the tremendous value of those powerful worlds - SAP and the cloud," explained Frank Stienhans, CTO of Ocean9, Inc., in this SYS-CON.tv interview at 20th Cloud Expo, held June 6-8, 2017, at the Javits Center in New York City, NY.
"We've been engaging with a lot of customers including Panasonic, we've been involved with Cisco and now we're working with the U.S. government - the Department of Homeland Security," explained Peter Jung, Chief Product Officer at Pulzze Systems, in this SYS-CON.tv interview at @ThingsExpo, held June 6-8, 2017, at the Javits Center in New York City, NY.
Everything run by electricity will eventually be connected to the Internet. Get ahead of the Internet of Things revolution and join Akvelon expert and IoT industry leader, Sergey Grebnov, in his session at @ThingsExpo, for an educational dive into the world of managing your home, workplace and all the devices they contain with the power of machine-based AI and intelligent Bot services for a completely streamlined experience.
The financial services market is one of the most data-driven industries in the world, yet it’s bogged down by legacy CPU technologies that simply can’t keep up with the task of querying and visualizing billions of records. In his session at 20th Cloud Expo, Karthik Lalithraj, a Principal Solutions Architect at Kinetica, discussed how the advent of advanced in-database analytics on the GPU makes it possible to run sophisticated data science workloads on the same database that is housing the rich...
IoT is at the core or many Digital Transformation initiatives with the goal of re-inventing a company's business model. We all agree that collecting relevant IoT data will result in massive amounts of data needing to be stored. However, with the rapid development of IoT devices and ongoing business model transformation, we are not able to predict the volume and growth of IoT data. And with the lack of IoT history, traditional methods of IT and infrastructure planning based on the past do not app...
With tough new regulations coming to Europe on data privacy in May 2018, Calligo will explain why in reality the effect is global and transforms how you consider critical data. EU GDPR fundamentally rewrites the rules for cloud, Big Data and IoT. In his session at 21st Cloud Expo, Adam Ryan, Vice President and General Manager EMEA at Calligo, will examine the regulations and provide insight on how it affects technology, challenges the established rules and will usher in new levels of diligence...
DX World EXPO, LLC., a Lighthouse Point, Florida-based startup trade show producer and the creator of "DXWorldEXPO® - Digital Transformation Conference & Expo" has announced its executive management team. The team is headed by Levent Selamoglu, who has been named CEO. "Now is the time for a truly global DX event, to bring together the leading minds from the technology world in a conversation about Digital Transformation," he said in making the announcement.
In the enterprise today, connected IoT devices are everywhere – both inside and outside corporate environments. The need to identify, manage, control and secure a quickly growing web of connections and outside devices is making the already challenging task of security even more important, and onerous. In his session at @ThingsExpo, Rich Boyer, CISO and Chief Architect for Security at NTT i3, discussed new ways of thinking and the approaches needed to address the emerging challenges of security i...
Internet of @ThingsExpo, taking place October 31 - November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with 21st Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world. The Internet of Things (IoT) is the most profound change in personal and enterprise IT since the creation of the Worldwide Web more than 20 years ago. All major researchers estimate there will be tens of billions devic...
What sort of WebRTC based applications can we expect to see over the next year and beyond? One way to predict development trends is to see what sorts of applications startups are building. In his session at @ThingsExpo, Arin Sime, founder of WebRTC.ventures, discussed the current and likely future trends in WebRTC application development based on real requests for custom applications from real customers, as well as other public sources of information.
"The Striim platform is a full end-to-end streaming integration and analytics platform that is middleware that covers a lot of different use cases," explained Steve Wilkes, Founder and CTO at Striim, in this SYS-CON.tv interview at 20th Cloud Expo, held June 6-8, 2017, at the Javits Center in New York City, NY.
SYS-CON Events announced today that Calligo, an innovative cloud service provider offering mid-sized companies the highest levels of data privacy and security, has been named "Bronze Sponsor" of SYS-CON's 21st International Cloud Expo ®, which will take place on Oct 31 - Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Calligo offers unparalleled application performance guarantees, commercial flexibility and a personalised support service from its globally located cloud plat...
SYS-CON Events announced today that Massive Networks will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Massive Networks mission is simple. To help your business operate seamlessly with fast, reliable, and secure internet and network solutions. Improve your customer's experience with outstanding connections to your cloud.
SYS-CON Events announced today that DXWorldExpo has been named “Global Sponsor” of SYS-CON's 21st International Cloud Expo, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Digital Transformation is the key issue driving the global enterprise IT business. Digital Transformation is most prominent among Global 2000 enterprises and government institutions.
SYS-CON Events announced today that Datera, that offers a radically new data management architecture, has been named "Exhibitor" of SYS-CON's 21st International Cloud Expo ®, which will take place on Oct 31 - Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Datera is transforming the traditional datacenter model through modern cloud simplicity. The technology industry is at another major inflection point. The rise of mobile, the Internet of Things, data storage and Big...
While the focus and objectives of IoT initiatives are many and diverse, they all share a few common attributes, and one of those is the network. Commonly, that network includes the Internet, over which there isn't any real control for performance and availability. Or is there? The current state of the art for Big Data analytics, as applied to network telemetry, offers new opportunities for improving and assuring operational integrity. In his session at @ThingsExpo, Jim Frey, Vice President of S...
"DX encompasses the continuing technology revolution, and is addressing society's most important issues throughout the entire $78 trillion 21st-century global economy," said Roger Strukhoff, Conference Chair. "DX World Expo has organized these issues along 10 tracks with more than 150 of the world's top speakers coming to Istanbul to help change the world."
"We provide IoT solutions. We provide the most compatible solutions for many applications. Our solutions are industry agnostic and also protocol agnostic," explained Richard Han, Head of Sales and Marketing and Engineering at Systena America, in this SYS-CON.tv interview at @ThingsExpo, held June 6-8, 2017, at the Javits Center in New York City, NY.
"MobiDev is a Ukraine-based software development company. We do mobile development, and we're specialists in that. But we do full stack software development for entrepreneurs, for emerging companies, and for enterprise ventures," explained Alan Winters, U.S. Head of Business Development at MobiDev, in this SYS-CON.tv interview at 20th Cloud Expo, held June 6-8, 2017, at the Javits Center in New York City, NY.
SYS-CON Events announced today that DXWorldExpo has been named “Global Sponsor” of SYS-CON's 21st International Cloud Expo, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Digital Transformation is the key issue driving the global enterprise IT business. Digital Transformation is most prominent among Global 2000 enterprises and government institutions.