Domain Name System Security Extensions (DNSSEC) were developed to address critical security vulnerabilities in the original DNS protocol. DNS, by design, does not validate the authenticity of responses, leaving it susceptible to attacks such as cache poisoning and man-in-the-middle exploits. DNSSEC aims to provide data integrity and origin authentication for DNS responses, ensuring that when you query a domain name, the answer you receive is both untampered and genuinely from the authoritative source.

The primary goal of DNSSEC is to prevent attackers from injecting false DNS data into the resolution process. It does this by using digital signatures based on public-key cryptography. When a DNS zone is secured with DNSSEC, each set of DNS records is accompanied by a digital signature. DNS resolvers that support DNSSEC can then verify these signatures, confirming that the data has not been altered and is indeed from the expected zone. This mechanism adds a critical layer of authenticity to DNS, reducing the risk of receiving forged or manipulated DNS responses.

To understand how DNSSEC signatures are verified, consider the following high-level pseudo code that outlines the steps a DNS resolver might take during the resolution process:

1. Receive DNS response with attached signature (RRSIG record);
2. Obtain the public key (DNSKEY record) for the zone;
3. Hash the DNS data in the response;
4. Use the public key to verify the digital signature against the hash;
5. If verification succeeds, accept the DNS response as authentic; otherwise, treat it as invalid.

This process ensures that only DNS data signed by the legitimate zone owner is accepted, mitigating the risk of forged DNS responses.

DNSSEC relies on a robust system of key management and a hierarchical model of trust known as the chain of trust. At the core of DNSSEC key management are two types of keys: the Zone Signing Key (ZSK) and the Key Signing Key (KSK).

• The ZSK is used to sign the zone's resource records;
• The KSK signs the DNSKEY record set, which contains both the ZSK and KSK public keys.

Trust in DNSSEC is established through trust anchors, which are known, trusted public keys configured in DNS resolvers. The most common trust anchor is the root DNSSEC key, which is securely distributed to all validating resolvers.

When you receive a DNSSEC-protected response, validation begins by checking the signature against the public key in the DNSKEY record. The resolver then verifies that this key has itself been signed by a parent zone, continuing up the DNS hierarchy. This forms the chain of trust:

• Each zone's key is authenticated by its parent's signature;
• This process continues all the way up to the root, whose key is the ultimate trust anchor.

If any link in this chain is broken—such as a missing or invalid signature—the resolver will not trust the response. Secure key management is crucial:

• Keys must be rotated regularly;
• Keys must be protected against compromise.

A stolen key could allow attackers to forge valid-looking signatures, undermining the entire security model.

Here is a simplified pseudo code example that demonstrates how a DNS resolver might validate a DNSSEC-signed response:

1. Start with the DNS response and its RRSIG and DNSKEY records;
2. Check if the DNSKEY is trusted, either directly (trust anchor) or via a valid signature from its parent zone;
3. Use the DNSKEY to verify the RRSIG signature over the DNS data;
4. If the signature is valid, proceed; if not, reject the response;
5. If the zone is not the root, repeat the process with the parent zone's DNSKEY and DS (Delegation Signer) records, working up the chain of trust until reaching a trust anchor;
6. Accept the response only if every link in the chain is validated.

This recursive validation process ensures that DNS data cannot be spoofed without access to the appropriate private keys at each level of the DNS hierarchy.