How to Implement Reverse DNS Look Up Cache?
Introduction
sReverse DNS lookup is a process that allows to identify the domain name associated with a particular IP address. It is vital in various network applications, such as email servers, security systems, and website analytics. However, performing a reverse DNS lookup can be time-consuming when dealing with many requests. Implementing a reverse DNS lookup cache is recommended to overcome this difficulty and improve network capacity. In this article, we'll look at the benefits of this type of cache and provide a step-by-step guide on how to implement it effectively.
What is Reverse DNS Lookup Cache
A reverse DNS lookup cache is an element used to store and retrieve the results of reverse DNS lookups in a caching system. Reverse DNS lookup is when a DNS (Domain Name System) server is queried to obtain a hostname (domain name) from an IP address. Compared to a regular DNS lookup, where you query a domain name and receive its associated IP address, in a reverse DNS lookup, you query an IP address and receive its hostname.
Why using Reverse DNS Lookup Cache
The reverse DNS lookup cache aims to improve performance and reduce the load on DNS servers. DNS lookups can take time, and DNS servers can get overloaded with requests, especially for popular websites. By caching the results of previous reverse DNS lookups, results for the following lookups for the same IP address can be obtained as quickly as possible. It prevents the DNS server from needing to query the same IP address repeatedly, leading to faster response times and more efficient use of network resources.
Steps to implement Reverse DNS Lookup Cache
When a reverse DNS lookup is requested for an IP address, the system first checks whether that IP address is present in the cache.
If the IP address is found in the cache, the corresponding hostname is retrieved from the cache, and the lookup is completed without a new DNS query.
If the IP address is not found in the cache, the system performs a reverse DNS query to the DNS server to obtain the hostname.
After getting the hostname from the DNS server, the system stores it in the cache for future purposes.
The next time a reverse DNS lookup is requested for an IP address, the hostname can be quickly retrieved from the cache without a DNS server.
Here's a step-by-step guide to implementing a reverse DNS look-up cache
Step 1: Choose a Data Structure
Select an appropriate data structure to store the cache entries. A trie or hash map is typically used for this purpose. Tries work well for objective keys, and HashMaps provide fast lookup times.
Step 2: Define the data structure and helper functions
Create the necessary data structures to display the cache and cache entries. Define keying and aging of IP addresses and other valuable functions.
Step 3: Initialize the Cache
Initialize an empty cache data structure to store the results of the reverse DNS lookup.
Step 4: Apply the Insertion Function
Create a function to insert new entries into the cache. When the reverse DNS lookup is done, and the new entry is found, add it to the cache, where the IP address is stored as the key and its associated hostname as the value.
Step 5: Implement the Search Function
Create a function to search for entries in the cache. When a reverse DNS lookup is performed, it first checks to see if the IP address is already in the cache. If so, give the corresponding hostname return. If not, perform a DNS lookup, store the result in the cache, and return the hostname.
Step 6: Timing System
It is Optional
Alternatively, implement a time-of-life mechanism for cache entries so that stale information is not used. Set a time-to-life (TTL) for each cache entry and remove them when it expires.
Step 7: Cache Removal Process
It is Optional
If the cache has a fixed size, optionally implement a cache eviction mechanism to implement the cache eviction process. When the cache is full, of course, delete old entries if the cache is full. Popular cache removal policies include the least or least used (LRU) or the least frequently used (LFU).
Step 8: Error Handling
Add error handling to handle minor error-related issues. Decide how to handle these errors and return the appropriate response.
Step 9: Test the Cache
Test the cache with different IP addresses and hostnames. Ensure it is thoroughly and efficiently displayed in the correct framework and handles cache aging and removal (if implemented).
Step 10: Integration
It is Optional
Add reverse DNS lookup caches to your application or network environment as needed. Use the cache for reverse DNS lookups instead of querying the network servers directly.
Java implementation for Reverse DNS Look Up Cache
import java.util.HashMap;
import java.util.Map;
public class ReverseDnsLookupCache {
private static final int CHARS = 11;
private static final int MAX = 50;
// Trie Node.
private static class TrieNode {
boolean isLeaf;
String URL;
TrieNode[] child;
TrieNode() {
isLeaf = false;
URL = null;
child = new TrieNode[CHARS];
}
}
// A utility function to find index of child for a given character 'c'
private static int getIndex(char c) {
return (c == '.') ? 10 : (c - '0');
}
// A utility function to find character for a given child index.
private static char getCharFromIndex(int i) {
return (i == 10) ? '.' : (char) ('0' + i);
}
// Function to create a new trie node.
private static TrieNode newTrieNode() {
return new TrieNode();
}
// This method inserts an IP address and the corresponding
// domain name in the trie. The last node in Trie contains the URL.
private static void insert(TrieNode root, String ipAdd, String URL) {
int len = ipAdd.length();
TrieNode pCrawl = root;
// Traversing over the length of the IP address.
for (int level = 0; level < len; level++) {
int index = getIndex(ipAdd.charAt(level));
// Create a new child if not exist already
if (pCrawl.child[index] == null)
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 URL of the IP address in the
// last node of trie.
pCrawl.isLeaf = true;
pCrawl.URL = URL;
}
// This function returns URL if given IP address is present in DNS cache.
// Else returns null.
private static String searchDNSCache(TrieNode root, String ipAdd) {
int len = ipAdd.length();
TrieNode pCrawl = root;
// Traversal over the length of the IP address.
for (int level = 0; level < len; level++) {
int index = getIndex(ipAdd.charAt(level));
if (pCrawl.child[index] == null)
return null;
pCrawl = pCrawl.child[index];
}
// If we find the last node for a given IP address, return the URL.
if (pCrawl != null && pCrawl.isLeaf)
return pCrawl.URL;
return null;
}
// Driver function.
public static void main(String[] args) {
/* Change third ipAddress for validation */
String[] ipAdd = {"107.108.11.123", "107.109.123.255", "74.125.200.106"};
String[] URL = {"www.samsung.com", "www.samsung.net", "www.google.in"};
int n = ipAdd.length;
TrieNode root = newTrieNode();
// Inserts all the IP addresses and their corresponding
// domain names after IP address validation.
for (int i = 0; i < n; i++)
insert(root, ipAdd[i], URL[i]);
// If reverse DNS look up succeeds, print the domain
// name along with the DNS resolved.
String ip = "107.108.11.123";
String res_url = searchDNSCache(root, ip);
if (res_url != null)
System.out.println("Reverse DNS look up resolved in cache:\n" + ip + " --> " + res_url);
else
System.out.println("Reverse DNS look up not resolved in cache ");
}
}
Output
![How to Implement Reverse DNS Look Up Cache?](https://static.tutorialandexample.com/data-structure/how-to-implement-reverse-dns-look-up-cache1.png)
Python implementation for Reverse DNS Look Up Cache
class TrieNode:
def __init__(self):
self.is_leaf = False
self.URL = None
self.child = [None] * 11 # There are at most 11 different characters in a valid IP address
def get_index(c):
return 10 if c == '.' else int(c)
def get_char_from_index(i):
return '.' if i == 10 else str(i)
def new_trie_node():
return TrieNode()
def insert(root, ip_add, URL):
p_crawl = root
for level in range(len(ip_add)):
index = get_index(ip_add[level])
if not p_crawl.child[index]:
p_crawl.child[index] = new_trie_node()
p_crawl = p_crawl.child[index]
p_crawl.is_leaf = True
p_crawl.URL = URL
def search_dns_cache(root, ip_add):
p_crawl = root
for level in range(len(ip_add)):
index = get_index(ip_add[level])
if not p_crawl.child[index]:
return None
p_crawl = p_crawl.child[index]
if p_crawl and p_crawl.is_leaf:
return p_crawl.URL
return None
# Driver function.
if __name__ == "__main__":
# Change the third ipAddress for validation
ip_add = ["107.108.11.123", "107.109.123.255", "74.125.200.106"]
URL = ["www.samsung.com", "www.samsung.net", "www.google.in"]
n = len(ip_add)
root = new_trie_node()
# Inserts all the IP addresses and their corresponding
# domain names after IP address validation.
for i in range(n):
insert(root, ip_add[i], URL[i])
# If reverse DNS look up succeeds, print the domain
# name along with the DNS resolved.
ip = "107.108.11.123"
res_url = search_dns_cache(root, ip)
if res_url:
print(f"Reverse DNS look up resolved in cache:\n{ip} --> {res_url}")
else:
print("Reverse DNS look up not resolved in cache ")
Output
![How to Implement Reverse DNS Look Up Cache?](https://static.tutorialandexample.com/data-structure/how-to-implement-reverse-dns-look-up-cache2.png)
C++ implementation for Reverse DNS Look Up Cache
#include <iostream>
#include <unordered_map>
using namespace std;
struct TrieNode {
bool isLeaf;
string URL;
unordered_map<char, TrieNode*> child;
TrieNode() : isLeaf(false) {}
};
class ReverseDnsLookupCache {
private:
TrieNode* root;
public:
ReverseDnsLookupCache() {
root = new TrieNode();
}
void insert(const string& ipAdd, const string& URL) {
TrieNode* current = root;
for (char c : ipAdd) {
if (current->child.find(c) == current->child.end()) {
current->child[c] = new TrieNode();
}
current = current->child[c];
}
current->isLeaf = true;
current->URL = URL;
}
string search(const string& ipAdd) {
TrieNode* current = root;
for (char c : ipAdd) {
if (current->child.find(c) == current->child.end()) {
return "Not found";
}
current = current->child[c];
}
if (current->isLeaf) {
return current->URL;
}
return "Not found";
}
};
int main() {
ReverseDnsLookupCache cache;
cache.insert("107.108.11.123", "www.samsung.com");
cache.insert("107.109.123.255", "www.samsung.net");
cache.insert("74.125.200.106", "www.google.in");
string ip = "107.108.11.123";
string res_url = cache.search(ip);
if (res_url != "Not found") {
cout << "Reverse DNS look up resolved in cache:\n" << ip << " --> " << res_url;
} else {
cout << "Reverse DNS look up not resolved in cache";
}
return 0;
}
Output
![How to Implement Reverse DNS Look Up Cache?](https://static.tutorialandexample.com/data-structure/how-to-implement-reverse-dns-look-up-cache3.png)
This arrangement of the Trie-based reverse DNS lookup cache ultimately depends on its space complexity, whether the number of IP addresses stored here or the length of the IP addresses. In the worst case, where IP addresses have no shared priority, the locality time will be O(N*M), where N is the number of IP addresses, and M is the length of the IP addresses.
Time complexity
Insertion: The time complexity of storing an IP address and its associated domain name in a trie is O(M), where M is the length of the IP address.
Lookup: The time taken to look up an IP address is also O(M), where M is the length of the IP address. In the search operation, TRAI is observed based on the digits of the IP address.
Advantage of Reverse DNS Lookup Cache
Performance improvement: One of the most important benefits of the reverse DNS lookup cache is its performance improvement. Because of the cache, there is no need to repeat the results of DNS lookups for already cached IP addresses, reducing response times and data usage.
Reduced disk and network traffic: Due to the reverse DNS lookup cache, there is a reduction in disk and network traffic, depending on the caching method. Repeating entries already in the cache is unnecessary, reducing overall disk and network load.
Reduce load on DNS servers: The reverse DNS lookup cache reduces the time it takes to receive lookup requests for the same IP address multiple times. Relookups do not need to be done with entries already in the cache; this helps DNS servers handle more requests.
Reduce network latency: The reverse DNS lookup cache can improve performance and reduce latency. For this, the user does not have to do a lookup for an already cached IP address, which decreases the response time to the user's request.
Security enhancement: With the cache, you can also enhance the security of the DNS server for lookup requests coming from unauthorized and unrelated IP addresses. Only specific entries are stored in the cache, making unauthorized or dangerous requests impossible.
Over usage of network usage: There is over usage due to reverse DNS lookup cache. Unauthorized or dangerous IP addresses don't need to make requests to DNS servers, which doesn't overuse network resources.
Note: Reverse DNS, also called reverse DNS lookup, is a process in which an IP address is converted to its corresponding hostname, which helps identify the source of network activity and improve network security.
Conclusion
In Conclusion, The reverse DNS lookup cache is an effective technique that helps derive hostnames for IP addresses. When a reverse DNS lookup request occurs, the system first checks for the presence of the IP address in the cache. If the address is found in the cache, the reverse DNS lookup is completed without a new DNS query by retrieving the hostname from the cache. If the address is not found in the cache, the system queries the DNS server for a lookup and obtains the hostname. The hostname is stored in the cache for subsequent queries for the purpose. In this way, reverse DNS lookup improves cache performance, reduces network load, and reduces load for DNS servers. It is an essential technology that improves network performance and security.