What happens if your DNS is poisoned?

"DNS poisoning" is an attack targeting the Internet's fundamental directory system—the Domain Name System. It doesn't directly damage your computer, but rather subtly alters the results of network navigation. Its harm ranges from stealing personal privacy to paralyzing large-scale network services, a scope far exceeding many people's imagination.

DNS poisoning is also technically known as DNS cache poisoning or DNS spoofing. To understand its dangers, we must first understand what DNS does. Every device on the Internet has an IP address, a string of numbers like "192.0.2.1". Humans can't remember so many numbers, so we use domain names like "example.com". DNS acts like a giant phone book, responsible for translating the domain names you enter into machine-readable IP addresses. DNS poisoning is when attackers use technical means to "poison" the DNS server or your computer, polluting this phone book and changing the phone number for services like Baidu to a fraudulent one. When your device queries a domain name, it receives an incorrect, maliciously altered IP address, thus being redirected to the wrong server.

So, what specific harms does this incorrect redirection cause? Attackers can redirect the domains of well-known banks, social networking sites, and email services to their fake websites. These fake websites are so convincing that once users are misled and enter their account numbers, passwords, and credit card information, this sensitive data falls directly into the attackers' hands. Because users access these sites through domains they trust, their vigilance is greatly reduced, leading to a very high success rate for the attack.

Secondly, there's malware distribution. Attackers can redirect the domains of software download sites and update servers to servers hosting viruses, ransomware, or Trojans. Users believe they are downloading genuine software or security updates from official channels, but in reality, they are unknowingly downloading and installing malicious programs. This attack method is more covert than sending fraudulent emails because it exploits users' trust in legitimate domains. More problematic is that malware can serve as a springboard for subsequent attacks, remaining dormant in the system for a long time.

Thirdly, DNS poisoning can be used to implement strict network censorship and content blocking. In certain network environments, operators or management agencies may use DNS poisoning to block users from accessing specific overseas websites or services. When a user attempts to access these sites, the DNS query returns an incorrect IP address or fails to connect at all, thus achieving the blocking purpose. While sometimes used for compliance management, its technical nature is no different from an attack, undermining the neutrality and integrity of internet access.

Fourth, escalating damage can lead to service interruptions and business paralysis. If attackers target the domains of large internet companies or cloud service providers, they can resolve their addresses to non-existent or extremely small IP addresses, causing a large number of users worldwide to be unable to access these services. This means direct economic losses and reputational damage for businesses reliant on online operations. Even for ordinary websites, being inaccessible for a short period will affect search engine rankings and user trust.

Finally, DNS poisoning is the perfect springboard for man-in-the-middle attacks. Attackers can not only redirect you to fake websites, but also to a server they control, located between you and the real website. This server can decrypt, view, and even tamper with all data transmitted between you and the real website, including encrypted HTTPS communication (if improperly configured), without your knowledge.

How do attackers do this? There are two main methods. One type is a local network attack: Attackers send forged DNS response packets to your computer from public Wi-Fi or a corporate intranet, arriving faster than responses from the real DNS server. Your computer, believing them, records incorrect mappings. Another type is a remote server attack: Attackers directly target ISP public DNS servers or corporate-built DNS servers, exploiting protocol vulnerabilities to inject a large number of fake domain name resolution records into their caches, thus poisoning all users using that DNS server.

To defend against this threat, we can take multiple measures. For ordinary users, the most effective method is to modify their device's DNS server settings, switching from the default DNS automatically assigned by the internet provider to a trusted public DNS service that supports DNSSEC, such as Cloudflare's 1.1.1.1 or Google's 8.8.8.8. At the same time, make it a habit to always ensure that a padlock icon (HTTPS encrypted connection) appears on the left side of the browser's address bar for websites involving logins or payments, and that the certificate authority is a legitimate organization. For website operators and corporate network administrators, it is necessary to enable DNSSEC on the server side. DNSSEC is a security extension that digitally signs DNS responses. It ensures that received DNS records do indeed come from legitimate domain administrators and have not been tampered with during transmission, thus preventing poisoning attacks at their source. In addition, regularly checking DNS records for anomalies is also essential. Below is a simple Python script example that helps you check if the A record resolution for a specified domain matches a known, correct IP address. This can be a basic method for daily monitoring:

python

import dns.resolver

import sys

def check_dns_a_record(domain, expected_ip):

"""Check if the A record resolution for the domain matches the expected IP"""

try:

resolver = dns.resolver.Resolver()

# Specify to use trusted public DNS servers, such as Cloudflare

resolver.nameservers = ['1.1.1.1']

answers = resolver.resolve(domain, 'A')

for rdata in answers:

resolved_ip = str(rdata)

print(f "The IP resolved to by the domain {domain} is: {resolved_ip}")

if resolved_ip == expected_ip:

print("[status] The resolution result is correct and matches the expected IP. ”)

return True

else:

print(f“[Warning] The resolution result is abnormal! The expected IP should be: {expected_ip}”)

return False

except dns.resolver.NXDOMAIN:

print(f“[Error] The domain name {domain} does not exist.”)

except Exception as e:

print(f“[Error] An exception occurred during the query: {e}”)

return False

if __name__ == “__main__”:

# Enter the domain name you want to monitor and its correct IP address here

my_domain = “your-important-site.com”

correct_ip = “203.0.113.1”

if not check_dns_a_record(my_domain, correct_ip):

sys.exit(1) # A non-zero return value indicates an anomaly and can be used to trigger alerts.

In short, the harm of DNS poisoning goes far beyond simply "visiting the wrong website." It undermines the foundation of the entire internet trust system, opening the door to large-scale online fraud, information theft, and service disruption. Combating this threat requires a comprehensive effort across the entire chain, from infrastructure (deploying DNSSEC), network services (using secure DNS), to personal habits (paying attention to HTTPS). In an increasingly complex network environment, understanding and preventing DNS-level risks has become an essential survival skill in the digital age.