How does DNS work?

• explain how DNS works briefly when user types www.google.com on the browser. the user's computer is connected to local network. there is a local dns server in the local network.

When a user on a local network types "www.google.com" in a web browser, and there is a local DNS server in the network, here's how DNS resolution works:

1. User's Request: The user's computer sends a DNS query to the local DNS server, requesting the IP address for "www.google.com."

2. Local DNS Server Check: The local DNS server checks its cache to see if it already knows the IP address for "www.google.com." If the information is cached and valid, it responds to the request immediately. If not, it proceeds with the resolution.

3. Local DNS Server Query: The local DNS server becomes a DNS client and queries a series of DNS servers in a recursive process. Here's a simplified version of the steps it follows:

   a. Root DNS Server: The local DNS server starts by querying a root DNS server, asking for information about the root domain (".").

   b. Top-Level Domain (TLD) DNS Server: The root DNS server responds with a referral to the TLD DNS server responsible for the "com" domain.

   c. Authoritative DNS Server for "google.com": The TLD DNS server provides a referral to the authoritative DNS server for "google.com."

   d. Authoritative DNS Server for "www.google.com": The authoritative DNS server for "google.com" finally provides the IP address for "www.google.com."

4. Response to the Local DNS Server: The local DNS server receives the IP address for "www.google.com" from the authoritative DNS server.

5. Response to User's Computer: The local DNS server now has the IP address and responds to the user's computer with the IP address.

6. Browser Connects to the Website: With the IP address, the user's web browser establishes a connection to the web server associated with "www.google.com." This allows the user to access the Google website.

The local DNS server plays a crucial role in the network by efficiently managing DNS queries and caching resolved domain information. It helps reduce the load on external DNS servers and speeds up DNS resolution for devices on the local network

• How the DNS works

How Does DNS Works : Resolving DNS

Components of DNS
-domain name space
-zones
-name servers(zone databases)

FQDN(fully qualified domain name)
DNS use FQDN to map a hostname to an ip address

name resolution process
to locate the ip address of requested domain name

forward lookup query
to map a name to an ip address

DNS is a distributed database with a hierarchical structure
that can serve as the foundation
for name resolution in a TCP/IP network

Understanding How DNS Works in Depth

windows client first looks into dns resolver cache

ipconfig /flushdns #clears dns resolver cache 

troubleshooting, could have old IP address in its cache, dns name is resolving old IP address 

ipconfig /displaydns # shows dns resolver cache

if windows client needs to resolve a dns it does not go out to a dns server,because it has it in its local cache 

time to live #how long it can keep this record in its cache before it has to query a dns server again

to make sure IP address does not change 

hosts file # records are loaded into local dns resolver cache 

any change causes to unload local dns resolver cache and load records in the hosts file 

if it can not find dns record in local dns resolver cache

it goes to query dns servers defined TCP/IPv properties, local dns server in the local network

it first look into forward look zones to see if it is authoritative for this domain

if it is authoritative for this domain it has zone files then.

it sends back the response to the client with IP address

How the DNS works

• Domain Name System

The Domain Name System (DNS) is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities. A Domain Name Service resolves queries for these names into IP addresses for the purpose of locating computer services and devices worldwide.
http://en.wikipedia.org/wiki/Domain_Name_System

• A Record : A record stands for Address record. The ‘A’ record specifies the IP address (IPv4) of a host. Whenever DNS server get a query to resolve domain-name, it will refer the A record to answer the IP address.

AAA Record : The ‘AAA’ record specifies the IP address (IPv6) of a host.
NS Record : NS Record stands for Name Server record. NS Records maps a domain name to a list of authoritative DNS servers.
MX Record : MX Record stands for mail exchange record. MX Records maps a domain name to a list of mail exchange servers.
PTR Record : PTR record stands for Reverse lookup record or Pointer record. A PTR record maps the IP address to a specific host.
CNAME Record : CNAME Record stands for Canonical Name. CNAME record is used to create aliases that point to other names such as WWW, FTP, MAIL & subdomains to a domain name. Example : a CNAME record can associate the subdomain www.2daygeek.com with 2daygeek.com
SOA Record : SOA record stands for Start of Authority records. SOA records contain information about a DNS zone such as Primary nameserver, Hostmaster E-mail address, zone file seriel number, zone transfer interval and zone expiry details.
TXT Record : TXT Record stands for text record. A TXT record is a type of DNS record that provides text information to sources outside your domain. The text can be either human-or machine-readable and can be used for a variety of purposes.
SPF Record : SPF Record stands for Sender Policy Framework. SPF is an extension to the SMTP mail protocol which is used for e-mail authentication. SPF record used to verifies that the message came from an authorized mail server or not. SPF is designed to detect SPAM & PHISHING mail sender’s, IP address which was included in e-mail header
DKIM Record : DKIM record stands for Domain Keys Identified Mail. DKIM is an email validation system designed to detect email spoofing by providing encryption authentication to receiving mail exchangers. It will check whether the incoming mail domain is authorized by the domain’s administrators and that the email (including attachments) has not been modified during transport. A digital signature included with the message can be validated by the recipient using the signer’s public key published in the DNS.
https://www.2daygeek.com/check-find-dns-records-of-domain-in-linux-terminal/#

resolver server

root server

TLD - Top Level Domain Server

Authoritative Server

Checks Browser and Operating System cache

DNS Resolver - ISP

Check DNS Resolver cache

Root Nameservers -13 different servers

TDL Namservers

Authoritative Nameserver - zone files

DNS Recursive Resolver - ISP

3rd party DNS Resolver- Google DNS / Internal resolver

Root Name Server

Top Level Domaın Nameserver - TLD

Authoritative Nameserver

• It is also used to troubleshoot DNS related problems

network administration tool for querying the Domain Name System (DNS) to obtain domain name or IP address mapping or any other specific DNS record.
nslookup can operate on both “Interactive mode” and “Non-Interactive mode”. Interactive mode allows the user to query the DNS-Server about various host, and domains. Non-Interactive mode allows the user to query the information for a host or domain.

MX ( Mail Exchange ) record maps a domain name to a list of mail exchange servers for that domain.
The MX record tells that all the mails sent to “@redhat.com” should be routed to the Mail server in that domain.

nslookup -query=mx redhat.com

we have 2 MX records for the domain “redhat.com”. The number ( 5, 10 ), associated with the MX records tells the preference of mail server. Lower the number, higher the preference. So when a mail is sent to “@redhat.com”, first preference will be “mx1.redhat.com”, then “mx2.redhat.com


Any answer that originates from the DNS Server which has the complete zone file information available for the domain is said to be authoritative answer.


NS ( Name Server ) record maps a domain name to a list of DNS servers authoritative for that domain. It will output the name serves which are associated with the given domain.

SOA record ( start of authority ), provides the authoritative information about the domain, the e-mail address of the domain admin, the domain serial number,
nslookup -type=soa redhat.com

http://www.thegeekstuff.com/2012/07/nslookup-examples/?utm_source=tuicool

• 
  

DIG: look up DNS domain IP address information
http://www.kloth.net/services/dig.php

• DNS Checker

https://tools.keycdn.com/dig

• Common DNS Records

A: Indicates the IP address of the domain.
AAAA: IPV6 address record.
CNAME: Canonical name, used for making a domain alias.
NS: Name server, indicates which name server is authoritative for the domain.
MX: Mail exchange, a list of mail exchange servers used for the domain.
TXT: Administrator record use for domain facts and verifications.
SRV: Service, defines the TCP service the domain operates on.
PTR: Pointer record, maps an IPv4 address to CNAME.
SOA: State of authority, stores information about when domain was updated.
https://tools.keycdn.com/dig

• If you're a small business owner or WordPress blogger, you've probably been told to configure your A and CNAME records. 

Or, that time you tried to move your email, you were told to change your MX record.
Or, a cool web service asked you to set up a TXT record before it would work with you


A Records
If a web user types in jeffreifman.com, the request will be passed off to a directory which will look for a DNS record that corresponds to my root domain. By root, I mean no prefix, no www, i.e. no sub-domain, just http://jeffreifman.com. For example, the root-level A record of your domain might point to 107.164.32.96. That will tell the Internet to which IP address to send your browsing request.

Subdomain Records
You can also configure A records for a variety of sub-domains. For example, if you want www.yourwebsite.com to go to the same address, you can set up an identical A record for the sub-domain www, i.e. the root domain, and the www domain would have identical IP addresses.

Wildcard Entries
DNS records also allow wildcard entries (using an asterisk *) that make it easy to route all sub-domain traffic to one IP address. For example, if I want any city request for Flee the Jungle to be hosted by one server,

CNAME records. These are helpful in a number of ways and can be especially helpful in simplifying the management of your IP addresses and future migrations from one server to another.
CNAME Records
CNAMEs are essentially domain and sub-domain text aliases to map traffic to. For example, if you've ever set up a blog through a service such as WordPress or Tumblr, they may ask you to map your domain name to a CNAME rather than with an A record to an IP address.

When a user requests misc.jeffreifman.com in their browser, the DNS points them to domains.tumblr.com which it will then recursively look up the IP address for, which will be 66.6.44.4.
One advantage of CNAMEs is that if Tumblr ever changes the IP address of their inbound server, I might not need to ever change my CNAME record. It could stay the same and Tumblr can manage the IP change by changing the A record for domains.tumblr.com.

If I need to change hosts and change the IP address of my web server, I can change one sub-domain A record for the CNAME I used, rather than changing one hundred A records for each domain name.

Another scenario you'll use CNAMEs with is CDN services
I set up four cloud subdomains named c1, c2, c3, c4, all pointing to KeyCDN's content mirror at jr-faf.kxcdn.com.

https://code.tutsplus.com/tutorials/an-introduction-to-learning-and-using-dns-records--cms-24704

• ChatGPT

rDNS (Reverse DNS) is a type of DNS (Domain Name System) record that maps an IP address to a domain name. While regular DNS maps domain names to IP addresses, rDNS maps IP addresses to domain names.

rDNS is often used to check the legitimacy of an incoming email message. This is because spammers and scammers often use fake email addresses that claim to be from legitimate domains. By checking the rDNS record, the email server can verify if the IP address is associated with the claimed domain.

rDNS can also be used for network troubleshooting and debugging. By checking the rDNS record, you can verify if the IP address is associated with the correct domain and identify any DNS misconfiguration

To set up an rDNS record, the owner of the IP address must contact the ISP (Internet Service Provider) or hosting provider that owns the IP address block. The ISP or hosting provider will then create a PTR (Pointer) record in their DNS server that maps the IP address to a domain name. The PTR record is the rDNS record

For example, if the IP address is 192.0.2.1 and the domain name is example.com, the rDNS record would be:

1.2.0.192.in-addr.arpa. IN PTR example.com.

The in-addr.arpa domain is a special domain used for rDNS. The IP address is reversed and appended to the in-addr.arpa domain to create the domain name for the PTR record.

AD Schema and Operations Master Roles

AD Schema and Operations Master Roles
http://www.youtube.com/watch?v=hGYTLtDmPCY

what's AD?
a data store
only stored on domain controllers

master list of objects in AD is called schema
schema contains list of types of objects and types of objects' properties
schema identifies relationships between all objects and properties of all objects

where's schema stored?
in a domain controller

what domain controller is schema stored?
operations master
in order to run a domain there are some rules

schema master server holds schema
all domain controllers have read-only copy of schema

what is the scope of schema?

forest-wide scope
domain naming master
schema master
a domain controller handles these two above for a forest

domain master roles--domain level
rip master
infrastructure master--to manage membership of groups across domain controllers

Difference Between List and ArrayList ?

List is an interface and ArrayList is an implementation of the List interface.
The arraylist class has only a few methods in addition to the methods available in the List interface
http://www.javabeat.net/qna/9-difference-between-list-and-arraylist-/

What are key differences between ArrayList vs Vector in Java ?

Vectors are synchronized, ArrayLists are not.
Vectors are safe-thread as Vectors are synchronized

Both are dynamically resizable.
By default, Vector doubles the size of its array when its size is increased.
But, ArrayList increases by half of its size when its size is increased

Composite Pattern

Composite

A composite IS A group of objects in which some objects may contain others, so that some objects represent groups and others represent
individual items, or leaves.

You can bring these ideas together by defining a common type for groups and items and modeling groups as containing a collection of objects of this type.

The intent of the COMPOSITE pattern is to let clients treat individual objects and compositions of objects uniformly.




In software engineering, the composite pattern is a partitioning design pattern.
The composite pattern describes that a group of objects are to be treated in the same way as a single instance of an object

When to use
Composite can be used when clients should ignore the difference between compositions of objects and individual objects.
If programmers find that they are using multiple objects in the same way, and often have nearly identical code to handle each of them, then composite is a good choice; it is less complex in this situation to treat primitives and composites as homogeneous

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

facade pattern

The intent of the FACADE pattern is to provide an interface that makes a subsystem easy to use.
Facades often arise out of normal application development.
As you separate your code’s concerns into different classes, you may refactor (restructure) the system by extracting a class whose primary job is to provide simplified access to a subsystem.

http://members.comu.edu.tr/msahin/courses/ust_duzey_files/patterns/facade.pdf

adapter pattern

AN OBJECT IS A client if it needs to call your code.

For example, a rocket-simulation program might be designed to use rocket information that you supply, but such a simulation will have its own definition of how a rocket should behave. In such circumstances, you may find that an existing class performs the services that a client needs but has different method names. In this situation, you can apply the ADAPTER pattern.

The intent of ADAPTER is to provide the interface that a client expects while using the services of a class with a different interface.

The ADAPTER pattern lets you use an existing class to meet a client class’s needs.

When a client specifies its requirements in an interface, you can usually create a new class that implements the interface and subclasses an existing class.
This approach creates a class adapter that translates a client’s calls into calls to the existing class’s methods.




In computer programming, the adapter pattern (often referred to as the wrapper pattern or simply a wrapper) is a design pattern that translates one interface for a class into a compatible interface.

An adapter allows classes to work together that normally could not because of incompatible interfaces, by providing its interface to clients while using the original interface.

The adapter translates calls to its interface into calls to the original interface, and the amount of code necessary to do this is typically small. The adapter is also responsible for transforming data into appropriate forms

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

Categorization of Patterns

According to their intents:

• Interfaces 

ADAPTER, FACADE, COMPOSITE, BRIDGE

• Responsibility 

SINGLETON, OBSERVER, MEDIATOR, PROXY, CHAIN OF RESPONSIBILITY, FLYWEIGHT

• Construction 

BUILDER, FACTORY METHOD, ABSTRACT FACTORY,PROTOTYPE, MEMENTO

• Operations 

TEMPLATE METHOD, STATE, STRATEGY, COMMAND,INTERPRETER

• Extensions 

DECORATOR, ITERATOR, VISITOR



     Types of design patterns according to their purposes:

• – Creational: creating, initializing and configuring classes and objects
• – Structural patterns: composition of classes and objects
• – Behavioral patterns: dynamic interactions among classes and objects

• Classification and list

1. Creational patterns
2. Structural patterns
3. Behavioral patterns
4. Concurrency patterns

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

• What are the categories design patterns divided into?

Ans. There are total three categories into which design patterns divided.
Creational Pattern:Creational patterns provide ways to instantiate single objects or group of related objects.
Structural Patterns:Structural patterns provide a way to define relationship between classes and objects.
Behavioral Patterns:Behavioral patterns define ways of communication between classes and objects.

Can you give name of patterns fall in each category?
Ans. Five patterns fall under creational pattern.
Abstract Factory
Factory
Factory Method
Prototype
Singleton
Seven patterns fall under structural pattern.
Adapter
Bridge
Composite
Decorator
Façade
Flyweight
Proxy
Eleven patterns fall under behavioral pattern.
Chain of Responsibility
Command
Interpreter
Iterator
Mediator
Memento
Observer
State
Strategy
Template Method
Visitor

What are object patterns?
Ans. Object patterns deals with object relationships that can be changed at runtime.
Object patterns mostly use object composition to establish relationships between objects.

What are class patterns?
Ans. Class patterns deals with class relationships that can be changed at compile time
class patterns tend to use inheritance to establish relationships.

Can you list down different class patterns and object patterns?
Ans.
Class Patterns
Adapter
Factory Method
Interpreter
Template Method
Object Patterns
Abstract Factory
Builder
Bridge
Chain of Responsibility
Command
Composition
Decorator
Façade
Flyweight
Iterator
Mediator
Memento
Observer
Prototype
Proxy
Singleton
State
Strategy
Visitor

http://jinaldesai.net/basic-design-patterns-interview-questions/

mock exam,test,quiz

• SCJA Mini Mock Exam

http://www.ejavaguru.com/scjafreemockexam.php#a1

• SCJA Sun Certified Java Associate Mock Exams

http://scja.de/

MVC over page centric

MVC is a design pattern
It contains two models:
MVC Model 1
MVC Model 2
Struts framework implements MVC Design Pattern.
Struts can implement Model 1 and Model 2

the important features of MVC1 architecture:
(1) HTML or JSP files are used to code the presentation.
JSP files use java beans to retrieve data if required.

(2)MVC1 architecture is page-centric design all the business and processing logic means any JSP page can either present in the JSP or may be called directly from the JSP page.

(3)Data access is usually done using Custom tag or through java bean call.
Therefore we can say that in MVC1 there is tight coupling between page and model


the important features of MVC2 architecture
(1)This architecture removes the page-centric property of MVC1 architecture by separating Presentation, Control logic and Application state

(2)In MVC2 architecture there is one Controller which receive all request for the application and is responsible for taking appropriate action in response to each request.

Second one is Model which is represented by JavaBeans, business object, and database.

Third one is View or is JSP page it takes the information provided by Controller and Module and presents it to user.


in MVC-I all the view,control elements are implemented using Servlets
in MVC-II the view is implemented using JSP,and the controller is implemented using Servlets,as JSP provides better user interface than Servlets

http://forums.sureshkumar.net/java-technologies/11167-struts-difference-between-mvc1-architecture-mvc2-architecture.html

PM course pages

lecture notes
http://efe.ege.edu.tr/~sengonca/Projeyonetimi.html

videos books
http://efe.ege.edu.tr/~sengonca/




PMP Study Process
http://pmstudent.com/pmp-exam-study-advice-that-works/pmp-study-process/

Earned Value Basics
http://pmstudent.com/evm-example-part-1-finding-pv-ev-and-ac/

Project Cost Management
http://www.preparepm.com/notes/cost.html


Project Cost Management
http://www.tutorialspoint.com/pmp-exams/project_cost_management.htm

bubble sort vs quick sort

• bubble sort vs quick sort

http://www.youtube.com/watch?v=vxENKlcs2Tw
bubble sort compares every adjacent element(something next to it) and moves the biggest element forward

(if you are supposed to list numbers in descending order)
the same operation is run on the remaining part of the elements

• Bubble sort

Bubble sort  is a simple sorting algorithm that works by repeatedly stepping through the list to be sorted, comparing each pair of adjacent items and swapping them if they are in the wrong order.
The algorithm gets its name from the way smaller elements "bubble" to the top of the list. 
Because it only uses comparisons to operate on elements, it is a comparison sort. 
Although the algorithm is simple, most other algorithms are more efficient for sorting large lists
Bubble sort has worst-case and average complexity both ?(n2), where n is the number of items being sorted

There exist many sorting algorithms with substantially better worst-case or average complexity of O(n log n). 
Even other O(n2) sorting algorithms, such as insertion sort, tend to have better performance than bubble sort
Therefore, bubble sort is not a practical sorting algorithm when n is large.
Performance of bubble sort over an already-sorted list (best-case) is O(n).

 Large elements at the beginning of the list do not pose a problem, as they are quickly swapped. 
 Small elements towards the end, however, move to the beginning extremely slowly.
 This has led to these types of elements being named rabbits and turtles, respectively



 Pseudocode implementation

 procedure bubbleSort( A : list of sortable items )
   repeat     
     swapped = false
     for i = 1 to length(A) - 1 inclusive do:
       /* if this pair is out of order */
       if A[i-1] > A[i] then
         /* swap them and remember something changed */
         swap( A[i-1], A[i] )
         swapped = true
       end if
     end for
   until not swapped
end procedure

many modern algorithm textbooks avoid using the bubble sort algorithm in favor of insertion sort


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

• Code snippets

Java

public void bubbleSort(int[] arr) {
      boolean swapped = true;
      int j = 0;
      int tmp;
      while (swapped) {
            swapped = false;
            j++;
            for (int i = 0; i < arr.length - j; i++) {                                       
                  if (arr[i] > arr[i + 1]) {                          
                        tmp = arr[i];
                        arr[i] = arr[i + 1];
                        arr[i + 1] = tmp;
                        swapped = true;
                  }
            }                
      }
}
C++

void bubbleSort(int arr[], int n) {
      bool swapped = true;
      int j = 0;
      int tmp;
      while (swapped) {
            swapped = false;
            j++;
            for (int i = 0; i < n - j; i++) {
                  if (arr[i] > arr[i + 1]) {
                        tmp = arr[i];
                        arr[i] = arr[i + 1];
                        arr[i + 1] = tmp;
                        swapped = true;
                  }
            }
      }
}


http://www.algolist.net/Algorithms/Sorting/Bubble_sort