Technical Deep Dive: DNS Spoofing Cache Poisoning

DNS Spoofing Cache Poisoning is a severe security exploit. It targets the Domain Name System (DNS). The attack introduces corrupted or forged data into a recursive DNS resolver’s cache. By injecting a false Resource Record (RR), the attacker forces the resolver to return an incorrect Internet Protocol (IP) address. This action diverts client traffic to a malicious server under the attacker’s control.

The Attack Vector: Cache Poisoning

The execution of a successful DNS Spoofing Cache Poisoning attack leverages the inherent trust and speed-optimization mechanisms of the DNS protocol:

1. Exploiting the Recursive Query: When a client initiates a DNS request, the intermediate Recursive DNS Server must query the DNS hierarchy. (For a detailed explanation of the process, see our glossary entry on The Internet’s Navigation System: Recursive DNS!)
2. Timing and Prediction: The attacker attempts to win the “race” against the legitimate Authoritative DNS Server. They flood the recursive server with forged response packets containing malicious RRs before the authentic response arrives. Critical parameters that must be accurately guessed include:
   • Query ID (TXID): A 16-bit transaction identifier in the DNS header.
   • Source Port: The UDP port used by the recursive server for its outgoing query (a weakness if randomization is absent).
3. Injection of Malicious Records: The forged response typically includes:
   • A false Answer section (e.g., an A record mapping the target domain to the attacker’s IP).
   • Crucially, malicious entries in the Authority or Additional sections. An attacker might inject malicious glue records, effectively pointing the nameservers for a wider domain zone to their controlled IP, dramatically increasing the attack’s scope and persistence.
4. Cache Acceptance: If the attacker’s packet arrives first and matches the necessary parameters (TXID, Port), the recursive server accepts and stores the malicious data, poisoning the cache.

Mitigating the Threat

Defending against DNS Spoofing Cache Poisoning requires increasing the entropy of DNS requests and, more importantly, validating the authenticity of DNS data.

• Source Port and Query ID Randomization: Modern recursive resolvers must implement high-quality pseudo-random number generators (PRNGs) to randomize both the 16-bit Query ID and the UDP source port. This makes the window of successful prediction computationally prohibitive for attackers.
• DNSSEC (DNS Security Extensions): This is the gold standard defense. DNSSEC uses public-key cryptography to digitally sign DNS records (RRSIG). The recursive resolver can then validate these signatures against the known trust anchor. If validation fails, the data is discarded, preventing cache poisoning.
• Response Rate Limiting (RRL): Implementing RRL on authoritative nameservers can mitigate reconnaissance and brute-force attempts by attackers, preventing them from probing for TXID and port pairs.
• Network Security Protocol Upgrades: Modern protocols like DNS over TLS (DoT) or DNS over HTTPS (DoH) encrypt traffic between the stub resolver and the recursive resolver, preventing on-path attacks against DNS queries. (For up-to-date best practices on securing DNS infrastructure, check out the ICANN DNS Policy guidelines.)

Last modified: November 3, 2025