[ T.E ] CN16 - DNS Lookup

Problem Statement

Write a program for DNS lookup. Given an IP address input, it should return the URL and vice versa.

What is DNS?

• DNS stands for Domain Name System. DNS is the system that translates the human-friendly domain name into the actual IP address of the server where the website is hosted.

• It comes under the Application Layer.

• DNS is a process of translating a human-readable domain name into a computer-readable IP address, allowing us to navigate the internet using easy-to-remember names instead of numerical addresses.

How DNS works?

DNS Query -

• A user types a website address [ name ] into the web browser.

DNS Query Initiation -

• The computer needs to find the IP address associated with the name, so it sends the DNS Query to a DNS server

DNS Server Lookup -

• A DNS Server is like a big directory that knows the IP addresses associated with various domain names. It looks up the IP address for the name provided.

IP Address Retrieval -

• The DNS Server finds and sends the associated IP address back to the computer.

Accessing the Website -

• With the IP address computer can connect to the web server.

CODE

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <arpa/inet.h>

int main() {
    struct hostent *host_info;
    struct in_addr *address;

    char input[50];
    int choice;

    // Asking the user for their choice
    printf("Choose an option:\n");
    printf("1. Enter a domain name\n");
    printf("2. Enter an IP address\n");
    printf("Enter your choice (1 or 2): ");
    scanf("%d", &choice);

    // Checking the user's choice
    if (choice == 1) {
        // User wants to input a domain name
        printf("Enter the domain name: ");
        scanf("%s", input);

        printf("Will do a DNS query on: %s\n", input);
        host_info = gethostbyname(input);

        if (host_info != NULL) {
            address = (struct in_addr *)(host_info->h_addr);
            printf("%s has address %s\n", input, inet_ntoa(*address));
        } else {
            herror("Error getting host information");
        }
    } else if (choice == 2) {
        // User wants to input an IP address
        printf("Enter the IP address: ");
        scanf("%s", input);

        struct in_addr addr;
        inet_pton(AF_INET, input, &addr);

        host_info = gethostbyaddr(&addr, sizeof(struct in_addr), AF_INET);

        if (host_info != NULL) {
            printf("%s has domain name %s\n", input, host_info->h_name);
        } else {
            herror("Error getting host information");
        }
    } else {
        printf("Invalid choice. Please enter 1 or 2.\n");
    }

    return 0;
}

/*
    Commands to run the code on Linux - 
        gcc dns.c 
        ./a.out 

*/

Code with line-by-line explanation

// Header Files 

// Used for socket programming, providing socket-related functions [ socket(), bind(), listen() ]
#include<sys/socket.h>

// Contains structures and constants for internet address families, particulary for IPv4 and IPv6 
#include<netinet/in.h>

// Includes functions and structures for DNS, allowing hostname to IP add convo 
#include<netdb.h>

// Provides functions for manipulating IP add, such as converting binary IP add to human-readable strings and vice versa 
#include<arpa/inet.h>

// Standard Input-Output functions 
#include<stdio.h>

// String Manipulation
#include<string.h>

// Standard library functions, used for memory allocation and deallocation 
#include<stdlib.h>

int main(){


    // - hostent structure is used to store info about a host, including its name and associated IP addresses 
    struct hostent *host_info;


    // - in_addr structure represents an IPv4 address 
    struct in_addr *address;
     // The above 2 pointers are used for storing info obtained from a DNS query, where 
     // host_info would contain info about a host 
     // address would store its IP address


    // To store domainName or IP address provided by the user 
    char input[50];
    int choice;

    // Asking the user for their choice 
    printf("Choose an Option : \n");
    printf("1. Enter a Domain Name \n");
    printf("2. Enter an IP address \n");

    // Getting the users input 
    scanf("%d",&choice);

    // Acting according to the users choice 
    if(choice == 1){
        printf("Enter the Domain Name : ");
        scanf("%s",input);

        printf("Will do a DNS query on : %s \n",input);

        /*
            - gethostbyname() : Is used to perform a DNS query* and retrieve info about the host [ whos websiteName was provided by the user ]
            - The retrieve info of the host is stored in the hostent structure and host_info is the pointer to that hostent structure 
        */
        host_info = gethostbyname(input);

        if(host_info != NULL){

            /*
                - Performs a typecast operation 
                - host_info-h_addr contains the pointer to the first IP address returned by the gethostbyname function 
                - Tht pointer is of type struct hostent, which is typecasted into struct in_addr and is stored in the variable address 
            */
            address = (struct in_addr *)(host_info->h_addr);

            /*
                - Prints the result of DNS query 
                - inet_ntoa() function converts the IP address from its binary representation [ stored in address ] to a human-readable string [ eg. 192.168.10.1 ]
            */
            printf("%s has address %s \n",input,inet_ntoa(*address));

        }
        else{

            // Used to print an error message
            herror("Error getting the host information ");
        }
    }
    else if(choice == 2){

        printf("Enter the IP Address : ")

        // Getting the IP address from the user in string format 
        scanf("%s",input);

        // structure that represents [ can store ] IPv4 Address 
        struct in_addr addr;


        /*
            - inet_pton() 
                - Used to convert the IP address in string format to its numeric format and store it in a specified memory location
                - AF_INET 
                    - This specifies that we are working with IPv4 addresses. It stands for Address Family - Internet Protocol version 4.
                - input : contains the IP address in string format 
                - addr : the numeric form of IP address will be stored in addr 
        */
        inet_pton(AF_INET, input, &addr);


        /*
            - gethostbyaddr : 
                - Function used to retrieve host information associated with the provided IPv4 address.
                - addr 
                    - A pointer to the binary representation of the address.
                - sizeof(struct in_addr)
                    - The length of the address.
                - AF_INET 
                    - The address family type, AF_INET for IPv4.
        */
        host_info = gethostbyaddr(&addr, sizeof(struct in_addr), AF_INET);

        if(host_info != NULL){
            // Prints the respective domain name 
            printf("%s has domain name %s \n",input,host_info->h_name);
        }
        else{
            herror("Error getting host information ");
        }

    }
    else{
        printf("Invalid Choice !!");
    }

    return 0;
}

Output

Using Domain Name :

Using IP address :