Secure DNS works by verifying the authenticity and integrity of DNS data, preventing attackers from manipulating or poisoning the DNS resolution process. In essence, it establishes a chain of trust from the root DNS servers down to individual domain names.
Here's a breakdown of how it works, primarily focusing on DNSSEC (Domain Name System Security Extensions), the most widely adopted security mechanism for DNS:
What is DNSSEC?
DNSSEC adds cryptographic signatures to DNS records. These signatures allow resolvers to verify that the DNS data they receive is identical to what the domain owner published, and hasn't been tampered with en route.
The Chain of Trust
DNSSEC relies on a hierarchical "chain of trust." Here's how it's built:
- Root Key: The process begins with the root DNS servers, which are managed by organizations like IANA. These servers have a "root key" – a public key used to verify the signatures of the top-level domains (TLDs) like
.com,.org, and.net. - TLD Signing: Each TLD operator uses its private key to digitally sign the DNS records for the zones it manages. The corresponding public key is published as part of the TLD's DNS information and is signed by the root key.
- Domain Signing: Domain owners (e.g.,
example.com) use their own private keys to sign their DNS records. The corresponding public key (DNSKEY record) is published within the domain's DNS zone. This DNSKEY record is, in turn, signed by the domain owner's private key. A "Delegation Signer" (DS) record, containing a hash of the domain's DNSKEY, is then placed in the parent zone (e.g., in the.comzone forexample.com).
The Resolution Process with DNSSEC
When a user attempts to access www.example.com, the DNS resolution process proceeds as follows (with DNSSEC incorporated):
- Recursive Resolver Query: The user's DNS resolver (e.g., provided by their ISP or a public resolver like Google Public DNS) queries the root servers for the address of the
.comname servers. - Root Server Response: The root server responds with the address of the
.comname servers and a digital signature verifying the authenticity of that response. The resolver verifies the signature using the known root key. - TLD Query: The resolver queries the
.comname servers for the address of theexample.comname servers. - TLD Response: The
.comserver responds with the addresses ofexample.com's name servers and a digital signature. The resolver verifies this signature using the public key associated with the.comzone (which it previously obtained and verified against the root key). - Authoritative Name Server Query: The resolver queries the
example.comname servers for the IP address ofwww.example.com. - Authoritative Name Server Response: The
example.comname servers respond with the IP address ofwww.example.comand a digital signature verifying the authenticity of that response. The resolver verifies this signature using the DNSKEY record it retrieves fromexample.com, validating the DS record against the hash of the DNSKEY. - Verification and Response to User: If all signatures are valid, the resolver knows that the IP address is authentic and has not been tampered with. The resolver then provides the IP address to the user, allowing them to connect to
www.example.com.
What Happens if a Signature is Invalid?
If any signature in the chain of trust is invalid, the resolver knows that the DNS data is potentially compromised. Instead of providing the user with potentially malicious information, the resolver will return a SERVFAIL error, indicating a DNSSEC validation failure.
Other Security Measures and DNS-over-HTTPS (DoH) & DNS-over-TLS (DoT)
While DNSSEC focuses on data integrity and authentication, other measures enhance DNS security:
- DNS-over-HTTPS (DoH) and DNS-over-TLS (DoT): These protocols encrypt DNS queries between the user's device and the DNS resolver, preventing eavesdropping and tampering with DNS queries in transit. This helps to protect user privacy.
Benefits of Secure DNS
- Prevents DNS Spoofing/Cache Poisoning: Reduces the risk of attackers redirecting users to malicious websites.
- Enhances Data Integrity: Ensures that DNS data is authentic and hasn't been altered.
- Improves User Security: Protects users from phishing, malware, and other online threats.
Summary
Secure DNS, particularly through DNSSEC, uses cryptographic signatures to validate DNS data. This creates a chain of trust that protects the DNS resolution process from manipulation and ensures users are directed to the intended websites. Coupled with encryption methods like DoH and DoT, secure DNS offers a robust defense against various DNS-related attacks and enhances user privacy and security.
