DNS poisoning, often synonymous with DNS cache poisoning, is a pivotal concept within the "Attacks and Exploits" domain of the CompTIA PenTest+ certification. Fundamentally, this attack compromises the integrity of the Domain Name System (DNS) resolution process to redirect legitimate user traffic …DNS poisoning, often synonymous with DNS cache poisoning, is a pivotal concept within the "Attacks and Exploits" domain of the CompTIA PenTest+ certification. Fundamentally, this attack compromises the integrity of the Domain Name System (DNS) resolution process to redirect legitimate user traffic to malicious destinations controlled by the attacker.
The mechanism exploits the lack of authentication in traditional DNS queries. When a recursive DNS server receives a client request for a domain (e.g., example.com) and does not possess the record in its cache, it forwards the query to an upstream authoritative server. In a poisoning attack, the threat actor floods the recursive server with forged responses containing a malicious IP address. If the attacker's spoofed response matches the transaction ID and arrives before the legitimate response from the authoritative server, the DNS server accepts the fake record. This corrupted mapping is then stored (cached) for a specific Time-to-Live (TTL).
The impact of a successful poisoning attack is severe. Users attempting to access a legitimate site are silently routed to the attacker's server. This facilitates Man-in-the-Middle (MitM) attacks, allowing the penetration tester (or adversary) to harvest credentials via phishing pages that perfectly mimic the original site or deliver malware via drive-by downloads.
In the context of PenTest+, testers utilize tools like Ettercap, Bettercap, or Responder to demonstrate these vulnerabilities on local networks. Testers also examine local DNS poisoning, which involves modifying a compromised machine's `hosts` file. To remediate these vulnerabilities, the CompTIA curriculum emphasizes the implementation of DNSSEC (DNS Security Extensions), which adds cryptographic signatures to DNS records to verify their authenticity and integrity, alongside patching DNS software to prevent port prediction.
DNS Poisoning Attacks: A Comprehensive Guide for CompTIA PenTest+
What is DNS Poisoning? DNS Poisoning (also known as DNS Cache Poisoning or DNS Spoofing) is a deceptive cyber attack where corrupt Domain Name System (DNS) data is introduced into the cache of a DNS resolver. This causes the name server to return an incorrect IP address, diverting traffic from a legitimate destination (like a bank's website) to a malicious one controlled by the attacker.
Why is it Important? For a penetration tester, this attack vector is critical because it undermines the foundational trust of internet navigation. If an attacker controls DNS resolution, they can perform Man-in-the-Middle (MitM) attacks, harvest credentials via cloned websites, or distribute malware masked as legitimate software updates. It effectively bypasses user vigilance, as the user may type the correct URL but still arrive at a malicious server.
How it Works DNS servers cache translation results (Domain Name to IP) for a specific Time-To-Live (TTL) to improve efficiency. The attack exploits this mechanism: 1. The Query: A resolver receives a query for a domain (e.g., example.com). 2. The Race Condition: If the resolver does not have the IP cached, it asks an authoritative server. The attacker floods the resolver with fake responses claiming to be the authoritative server. 3. The Exploitation: If the attacker matches the Transaction ID and UDP port before the legitimate server responds, the resolver accepts the fake data. 4. The Poisoning: The resolver stores the malicious IP in its cache. Anyone using this resolver is now redirected to the attacker's IP until the TTL expires.
Exam Tips: Answering Questions on DNS Poisoning Attacks
1. Identify the Indicators: In scenario-based questions, look for users complaining about certificate errors (SSL/TLS warnings) or strange login pages despite typing the correct URL. This is a hallmark of DNS poisoning because the attacker's server rarely possesses a valid certificate for the spoofed domain.
2. Distinguish the Scope: Local Poisoning: If the question mentions editing the /etc/hosts (Linux) or C:\Windows\System32\drivers\etc\hosts (Windows) file, this is local DNS poisoning affecting a single machine. Network Poisoning: If the question involves ARP Spoofing leading to traffic redirection, the attacker is likely poisoning the victim's DNS cache by intercepting the request.
3. Know the Tools: Be prepared to identify tools associated with these attacks, such as Ettercap, Bettercap, and Responder (specifically for LLMNR/NBT-NS poisoning).
4. The Solution is DNSSEC: If a question asks how to prevent or mitigate DNS poisoning, the correct answer is almost always DNSSEC (Domain Name System Security Extensions). DNSSEC uses cryptographic digital signatures to validate that DNS records are authentic and unmodified.
5. Pharming: Recognize the term Pharming. While Phishing uses email lures, Pharming relies on DNS poisoning to redirect users without their knowledge.