In addition to verifying the integrity of your zone data, the DNSSEC chain of trust can also be used to associate other information with your domain, such as PGP keys, to help improve the security of applications.
What is DNSSEC?
DNSSEC (Domain Name System Security Extensions) adds resource records and message header bits which can be used to verify that the requested data matches what the zone administrator put in the zone and has not been altered in transit. DNSSEC doesn’t provide a secure tunnel; it doesn’t encrypt or hide DNS data. It was designed with backwards compatibility in mind. The original standard DNS protocol continues to work the same.
The resource record types are: RRSIG (for digital signature), DNSKEY (the public key), DS (Delegation Signer), and NSEC (pointer to next secure record). The message header bits are: AD (for authenticated data) and CD (checking disabled). A DNSSEC validating resolver uses these records and public key (asymmetric) cryptography to prove the integrity of the DNS data. A private key (specific to a zone) is used to encrypt a hash of a set of resource records; this is the digital signature stored in a RRSIG record.
The corresponding public key is stored in the DNSKEY resource record. The validating resolver uses that DNSKEY to decrypt the RRSIG and then compares the result with the hash of the corresponding resource record set to verify it has not changed. A hash of the public DNSKEY is stored in a DS record. This is stored in the parent zone. The validating resolver retrieves from the parent the DS record and its corresponding signature (RRSIG) and public key (DNSKEY); a hash of that public key is available from its parent. This becomes a chain of trust, also called an authentication chain. The validating resolver is configured with a trust anchor; this is the starting point which refers to a signed zone. The trust anchor is a DNSKEY or DS record and should be securely retrieved from a trusted source (not using DNS).
All the names in the zone have corresponding NSEC records listed in order, creating a chain of all the signed record sets. (Corresponding RRSIG records are also created to verify the NSEC data.) Because there is no gap, NSEC records are used to provide proof of non-existence of an resource record or to authenticate negative replies.
Preparing for DNSSEC
Use resolvers that are DNSSEC-capable and configured to do the validation. All versions of BIND 9 are DNSSEC-capable. Make sure network devices don’t lose or stop EDNS0 (Extension Mechanisms for DNS) or squash DNSSEC-related traffic. DNSSEC requires EDNS0 to support the larger DNS message sizes and for the DNSSEC OK (DO) EDNS header bit.
DNSSEC will increase DNS traffic with more requests and larger responses. For high-volume DNS traffic, prepare for increased bandwidth needs. DNSSEC is more sensitive to time issues (i.e. system clocks being really out of sync) than plain DNS; make sure your system clocks are reasonably accurate.
If hosting DNSSEC-signed zones, make sure your secondaries also support and have DNSSEC enabled.
- 10-part 2020 webinar series on implementing DNSSEC with BIND, covering theory, configuration, operation and troubleshooting. On Youtube.
- BIND 9 DNSSEC Guide, now an appendix to the BIND Administrative Reference Guide. This is a long-format Guide that explains a lot of the background and ‘why’ of DNSSEC, as well as ‘how’. Updated to include the KASP (Key and Signing Policy) tool.
- The BIND 9 Administrator Reference Manual (ARM) on ReadTheDocs. Choose the version of BIND 9 that you are running or plan to run, and you can read about the DNSSEC Key Manager utility, dnssec-keygen, dnssec-signzone, and other DNSSEC features built into BIND 9.
- State of DNSSEC Deployment 2016 (report from the Internet Society)
- DELV, a BIND tool that checks the DNSSEC validation chain
- ISOC 360 DNSSEC page
Older, possibly still useful references
- “Deploying DNSSEC with BIND 9.7” given by Alan Clegg at NANOG50
- DNSSEC goes Mainstream: Deployment Incentives, Experience, Questions, a presentation by Suzanne Woolf at NANOG Jun 2009.