How to Implement Forward DNS Look Up Cache?

3.6

We have discussed implementation of Reverse DNS Look Up Cache. Forward DNS look up is getting IP address for a given domain name typed in the web browser.

The cache should do the following operations :
1. Add a mapping from URL to IP address
2. Find IP address for a given URL.

There are a few changes from reverse DNS look up cache that we need to incorporate.
1. Instead of [0-9] and (.) dot we need to take care of [A-Z], [a-z] and (.) dot. As most of the domain name contains only lowercase characters we can assume that there will be [a-z] and (.) 27 children for each trie node.

2. When we type www.google.in and google.in the browser takes us to the same page. So, we need to add a domain name into trie for the words after www(.). Similarly while searching for a domain name corresponding IP address remove the www(.) if the user has provided it.

This is left as an exercise and for simplicity we have taken care of www. also.

One solution is to use Hashing. In this post, a Trie based solution is discussed. One advantage of Trie based solutions is, worst case upper bound is O(1) for Trie, for hashing, the best possible average case time complexity is O(1). Also, with Trie we can implement prefix search (finding all IPs for a common prefix of URLs). The general disadvantage of Trie is large amount of memory requirement.
The idea is to store URLs in Trie nodes and store the corresponding IP address in last or leaf node.

Following is C style implementation in C++.

// C based program to implement reverse DNS lookup
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

// There are atmost 27 different chars in a valid URL
// assuming URL consists [a-z] and (.)
#define CHARS 27

// Maximum length of a valid URL
#define MAX 100

// A utility function to find index of child for a given character 'c'
int getIndex(char c)
{
    return (c == '.') ? 26 : (c - 'a');
}

// A utility function to find character for a given child index.
char getCharFromIndex(int i)
{
    return (i == 26) ? '.' : ('a' + i);
}

// Trie Node.
struct trieNode
{
    bool isLeaf;
    char *ipAdd;
    struct trieNode *child[CHARS];
};

// Function to create a new trie node.
struct trieNode *newTrieNode(void)
{
    struct trieNode *newNode = new trieNode;
    newNode->isLeaf = false;
    newNode->ipAdd = NULL;
    for (int i = 0; i<CHARS; i++)
        newNode->child[i] = NULL;
    return newNode;
}

// This method inserts a URL and corresponding IP address
// in the trie. The last node in Trie contains the ip address.
void insert(struct trieNode *root, char *URL, char *ipAdd)
{
    // Length of the URL
    int len = strlen(URL);
    struct trieNode *pCrawl = root;

    // Traversing over the length of the URL.
    for (int level = 0; level<len; level++)
    {
        // Get index of child node from current character
        // in URL[]  Index must be from 0 to 26 where
        // 0 to 25 is used for alphabets and 26 for dot
        int index = getIndex(URL[level]);

        // Create a new child if not exist already
        if (!pCrawl->child[index])
            pCrawl->child[index] = newTrieNode();

        // Move to the child
        pCrawl = pCrawl->child[index];
    }

    //Below needs to be carried out for the last node.
    //Save the corresponding ip address of the URL in the
    //last node of trie.
    pCrawl->isLeaf = true;
    pCrawl->ipAdd = new char[strlen(ipAdd) + 1];
    strcpy(pCrawl->ipAdd, ipAdd);
}

// This function returns IP address if given URL is
// present in DNS cache. Else returns NULL
char  *searchDNSCache(struct trieNode *root, char *URL)
{
    // Root node of trie.
    struct trieNode *pCrawl = root;
    int  len = strlen(URL);

    // Traversal over the length of URL.
    for (int level = 0; level<len; level++)
    {
        int index = getIndex(URL[level]);
        if (!pCrawl->child[index])
            return NULL;
        pCrawl = pCrawl->child[index];
    }

    // If we find the last node for a given ip address,
    // print the ip address.
    if (pCrawl != NULL && pCrawl->isLeaf)
        return pCrawl->ipAdd;

    return NULL;
}

// Driver function.
int main()
{
    char URL[][50] = { "www.samsung.com", "www.samsung.net",
                       "www.google.in"
                     };
    char ipAdd[][MAX] = { "107.108.11.123", "107.109.123.255",
                          "74.125.200.106"
                        };
    int n = sizeof(URL) / sizeof(URL[0]);
    struct trieNode *root = newTrieNode();

    // Inserts all the domain name and their corresponding
    // ip address
    for (int i = 0; i<n; i++)
        insert(root, URL[i], ipAdd[i]);

    // If forward DNS look up succeeds print the url along
    // with the resolved ip address.
    char url[] = "www.samsung.com";
    char *res_ip = searchDNSCache(root, url);
    if (res_ip != NULL)
        printf("Forward DNS look up resolved in cache:\n%s --> %s",
               url, res_ip);
    else
        printf("Forward DNS look up not resolved in cache ");

    return 0;
}

Output:

Forward DNS look up resolved in cache:
www.samsung.com --> 107.108.11.123

This article is contributed by Kumar Gautam. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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