5. DNSSEC

DNS Security Extensions (DNSSEC) provide reliable protection from cache poisoning attacks. At the same time these extensions also provide other benefits: they limit the impact of random subdomain attacks on resolver caches and authoritative servers, and provide the foundation for modern applications like authenticated and private e-mail transfer.

To achieve this goal, DNSSEC adds digital signatures to DNS records in authoritative DNS zones, and DNS resolvers verify the validity of the signatures on the received records. If the signatures match the received data, the resolver can be sure that the data was not modified in transit.

Note

DNSSEC and transport-level encryption are complementary! Unlike typical transport-level encryption like DNS-over-TLS, DNS-over-HTTPS, or VPN, DNSSEC makes DNS records verifiable at all points of the DNS resolution chain.

This section focuses on ways to deploy DNSSEC using BIND. For a more in-depth discussion of DNSSEC principles (e.g. How Does DNSSEC Change DNS Lookup?) please see DNSSEC Guide.

5.1. Zone Signing

BIND offers several ways to generate signatures and maintain their validity during the lifetime of a DNS zone:

• Fully Automated (Key and Signing Policy) - strongly recommended

• Manual Key Management - only for special needs

• Manual Signing - discouraged, use only for debugging

5.1.1. Zone keys

Regardless of the zone-signing method in use, cryptographic keys are stored in files named like Kdnssec.example.+013+12345.key and Kdnssec.example.+013+12345.private. The private key (in the .private file) is used to generate signatures, and the public key (in the .key file) is used for signature verification. Additionally, the Fully Automated (Key and Signing Policy) method creates a third file, Kdnssec.example+013+12345.state, which is used to track DNSSEC key timings and to perform key rollovers safely.

These filenames contain:

• the key name, which always matches the zone name (dnssec.example.),

• the algorithm number (013 is ECDSAP256SHA256, 008 is RSASHA256, etc.),

• and the key tag, i.e. a non-unique key identifier (12345 in this case).

Warning

Private keys are required for full disaster recovery. Back up key files in a safe location and protect them from unauthorized access. Anyone with access to the private key can create fake but seemingly valid DNS data.

5.1.2. Fully Automated (Key and Signing Policy)

Key and Signing Policy (KASP) is a method of configuration that describes how to maintain DNSSEC signing keys and how to sign the zone.

This is the recommended, fully automated way to sign and maintain DNS zones. For most use cases users can simply use the built-in default policy, which applies up-to-date DNSSEC practices:

  zone "dnssec.example" {
      type primary;
      file "dnssec.example.db";
      dnssec-policy default;
      inline-signing yes;
  };

The dnssec-policy statement requires dynamic DNS to be set up, or inline-signing to be enabled. In the example above we use the latter.

This is sufficient to create the necessary signing keys, and generate DNSKEY, RRSIG, and NSEC records for the zone. BIND also takes care of any DNSSEC maintenance for this zone, including replacing signatures that are about to expire and managing Key Rollovers.

Note

dnssec-policy needs write access to the zone. Please see dnssec-policy Statement Definition and Usage for more details about implications for zone storage.

The default policy creates one key that is used to sign the complete zone, and uses NSEC to enable authenticated denial of existence (a secure way to tell which records do not exist in a zone). This policy is recommended and typically does not need to be changed.

If needed, a custom policy can be defined by adding a dnssec-policy statement into the configuration:

dnssec-policy "custom" {
    dnskey-ttl 600;
    keys {
        ksk lifetime P1Y algorithm ecdsap384sha384;
        zsk lifetime 60d algorithm ecdsap384sha384;
    };
    nsec3param iterations 0 optout no salt-length 0;
};

This custom policy, for example:

• uses a very short DNSKEY TTL (600 seconds),

• uses two keys to sign the zone: a Key Signing Key (KSK) to sign the key related RRsets (DNSKEY, CDS, and CDNSKEY), and a Zone Signing Key (ZSK) to sign the rest of the zone. The KSK is automatically rotated after one year and the ZSK after 60 days.

Also:

• The configured keys have a lifetime set and use the ECDSAP384SHA384 algorithm.

• The last line instructs BIND to generate NSEC3 records for Proof of Non-Existence, using zero extra iterations and no salt. NSEC3 opt-out is disabled, meaning insecure delegations also get an NSEC3 record.

For more information about KASP configuration see dnssec-policy Statement Grammar.

The Advanced Discussions section in the DNSSEC Guide discusses the various policy settings and may be useful for determining values for specific needs.

5.1.2.1. Key Rollover

When using a dnssec-policy, a key lifetime can be set to trigger key rollovers. ZSK rollovers are fully automatic, but for KSK and CSK rollovers a DS record needs to be submitted to the parent. See Secure Delegation for possible ways to do so.

Once the DS is in the parent (and the DS of the predecessor key is withdrawn), BIND needs to be told that this event has happened. This can be done automatically by configuring parental agents:

  zone "dnssec.example" {
      type primary;
      file "dnssec.example.db";
      dnssec-policy default;
      inline-signing yes;
      parental-agents { 192.0.2.1; };
  };

Here one server, 192.0.2.1, is configured for BIND to send DS queries to, to check the DS RRset for dnssec-example during key rollovers. This needs to be a trusted server, because BIND does not validate the response.

If setting up a parental agent is undesirable, it is also possible to tell BIND that the DS is published in the parent with: rndc dnssec -checkds -key 12345 published dnssec.example. <rndc dnssec>. and the DS for the predecessor key has been removed with: rndc dnssec -checkds -key 54321 withdrawn dnssec.example. <rndc dnssec>. where 12345 and 54321 are the key tags of the successor and predecessor key, respectively.

To roll a key sooner than scheduled, or to roll a key that has an unlimited lifetime, use: rndc dnssec -rollover -key 12345 dnssec.example. <rndc dnssec>.

To revert a signed zone back to an insecure zone, change the zone configuration to use the built-in “insecure” policy. Detailed instructions are described in Reverting to Unsigned.

5.1.3. Manual Key Management

Warning

The method described here allows full control over the keys used to sign the zone. This is required only for very special cases and is generally discouraged. Under normal circumstances, please use Fully Automated (Key and Signing Policy).

5.1.3.1. Multi-Signer Model

Dynamic zones provide the ability to sign a zone by multiple providers, meaning each provider signs and serves the same zone independently. Such a setup requires some coordination between providers when it comes to key rollovers, and may be better suited to be configured with auto-dnssec allow;. This permits keys to be updated and the zone to be re-signed only if the user issues the command rndc sign zonename <rndc sign>.

A zone can also be configured with auto-dnssec maintain, which automatically adjusts the zone’s DNSSEC keys on a schedule according to the key timing metadata. However, keys still need to be generated separately, for example with dnssec-keygen.

Of course, dynamic zones can also use dnssec-policy to fully automate DNSSEC maintenance. The next sections assume that more key management control is needed, and describe how to use dynamic DNS update to perform various DNSSEC operations.

5.1.3.2. Enabling DNSSEC Manually

As an alternative to fully automated zone signing using dnssec-policy, a zone can be changed from insecure to secure using a dynamic DNS update. named must be configured so that it can see the K* files which contain the public and private parts of the zone keys that are used to sign the zone. Key files should be placed in the key-directory, as specified in named.conf:

zone update.example {
    type primary;
    update-policy local;
    auto-dnssec allow;
    file "dynamic/update.example.db";
    key-directory "keys/update.example/";
};

If there are both a KSK and a ZSK available (or a CSK), this configuration causes the zone to be signed. An NSEC chain is generated as part of the initial signing process.

In any secure zone which supports dynamic updates, named periodically re-signs RRsets which have not been re-signed as a result of some update action. The signature lifetimes are adjusted to spread the re-sign load over time rather than all at once.

5.1.3.3. Publishing DNSKEY Records

To insert the keys via dynamic update:

% nsupdate
> ttl 3600
> update add update.example DNSKEY 256 3 7 AwEAAZn17pUF0KpbPA2c7Gz76Vb18v0teKT3EyAGfBfL8eQ8al35zz3Y I1m/SAQBxIqMfLtIwqWPdgthsu36azGQAX8=
> update add update.example DNSKEY 257 3 7 AwEAAd/7odU/64o2LGsifbLtQmtO8dFDtTAZXSX2+X3e/UNlq9IHq3Y0 XtC0Iuawl/qkaKVxXe2lo8Ct+dM6UehyCqk=
> send

In order to sign with these keys, the corresponding key files should also be placed in the key-directory.

5.1.3.4. NSEC3

To sign using NSEC3 instead of NSEC, add an NSEC3PARAM record to the initial update request. The OPTOUT bit in the NSEC3 chain can be set in the flags field of the NSEC3PARAM record.

% nsupdate
> ttl 3600
> update add update.example DNSKEY 256 3 7 AwEAAZn17pUF0KpbPA2c7Gz76Vb18v0teKT3EyAGfBfL8eQ8al35zz3Y I1m/SAQBxIqMfLtIwqWPdgthsu36azGQAX8=
> update add update.example DNSKEY 257 3 7 AwEAAd/7odU/64o2LGsifbLtQmtO8dFDtTAZXSX2+X3e/UNlq9IHq3Y0 XtC0Iuawl/qkaKVxXe2lo8Ct+dM6UehyCqk=
> update add update.example NSEC3PARAM 1 0 0 -
> send

Note that the NSEC3PARAM record does not show up until named has had a chance to build/remove the relevant chain. A private type record is created to record the state of the operation (see below for more details), and is removed once the operation completes.

The NSEC3 chain is generated and the NSEC3PARAM record is added before the NSEC chain is destroyed.

While the initial signing and NSEC/NSEC3 chain generation are occurring, other updates are possible as well.

A new NSEC3PARAM record can be added via dynamic update. When the new NSEC3 chain has been generated, the NSEC3PARAM flag field is set to zero. At that point, the old NSEC3PARAM record can be removed. The old chain is removed after the update request completes.

named only supports creating new NSEC3 chains where all the NSEC3 records in the zone have the same OPTOUT state. named supports updates to zones where the NSEC3 records in the chain have mixed OPTOUT state. named does not support changing the OPTOUT state of an individual NSEC3 record; if the OPTOUT state of an individual NSEC3 needs to be changed, the entire chain must be changed.

To switch back to NSEC, use nsupdate to remove any NSEC3PARAM records. The NSEC chain is generated before the NSEC3 chain is removed.

5.1.3.5. DNSKEY Rollovers

To perform key rollovers via a dynamic update, the K* files for the new keys must be added so that named can find them. The new DNSKEY RRs can then be added via dynamic update. When the zones are being signed, they are signed with the new key set; when the signing is complete, the private type records are updated so that the last octet is non-zero.

If this is for a KSK, the parent and any trust anchor repositories of the new KSK must be informed.

The maximum TTL in the zone must expire before removing the old DNSKEY. If it is a KSK that is being updated, the DS RRset in the parent must also be updated and its TTL allowed to expire. This ensures that all clients are able to verify at least one signature when the old DNSKEY is removed.

The old DNSKEY can be removed via UPDATE, taking care to specify the correct key. named cleans out any signatures generated by the old key after the update completes.

5.1.3.6. Going Insecure

To convert a signed zone to unsigned using dynamic DNS, delete all the DNSKEY records from the zone apex using nsupdate. All signatures, NSEC or NSEC3 chains, and associated NSEC3PARAM records are removed automatically when the zone is supposed to be re-signed.

This requires the dnssec-secure-to-insecure option to be set to yes in named.conf.

In addition, if the auto-dnssec maintain or a dnssec-policy is used, it should be removed or changed to allow instead; otherwise it will re-sign.

5.1.4. Manual Signing

There are several tools available to manually sign a zone.

Warning

Please note manual procedures are available mainly for backwards compatibility and should be used only by expert users with specific needs.

To set up a DNSSEC secure zone manually, a series of steps must be followed. Please see chapter Manual Signing in the DNSSEC Guide for more information.

5.1.5. Monitoring with Private Type Records

The state of the signing process is signaled by private type records (with a default type value of 65534). When signing is complete, those records with a non-zero initial octet have a non-zero value for the final octet.

If the first octet of a private type record is non-zero, the record indicates either that the zone needs to be signed with the key matching the record, or that all signatures that match the record should be removed. Here are the meanings of the different values of the first octet:

• algorithm (octet 1)

• key ID in network order (octet 2 and 3)

• removal flag (octet 4)

• complete flag (octet 5)

Only records flagged as “complete” can be removed via dynamic update; attempts to remove other private type records are silently ignored.

If the first octet is zero (this is a reserved algorithm number that should never appear in a DNSKEY record), the record indicates that changes to the NSEC3 chains are in progress. The rest of the record contains an NSEC3PARAM record, while the flag field tells what operation to perform based on the flag bits:

0x01 OPTOUT

0x80 CREATE

0x40 REMOVE

0x20 NONSEC

5.2. Secure Delegation

Once a zone is signed on the authoritative servers, the last remaining step is to establish chain of trust [1] between the parent zone (example.) and the local zone (dnssec.example.).

Generally the procedure is:

• Wait for stale data to expire from caches. The amount of time required is equal to the maximum TTL value used in the zone before signing. This step ensures that unsigned data expire from caches and resolvers do not get confused by missing signatures.

• Insert/update DS records in the parent zone (dnssec.example. DS record).

There are multiple ways to update DS records in the parent zone. Refer to the documentation for the parent zone to find out which options are applicable to a given case zone. Generally the options are, from most- to least-recommended:

• Automatically update the DS record in the parent zone using CDS/CDNSKEY records automatically generated by BIND. This requires support for RFC 7344 in either parent zone, registry, or registrar. In that case, configure BIND to monitor DS records in the parent zone and everything will happen automatically at the right time.

• Query the zone for automatically generated CDS or CDNSKEY records using dig, and then insert these records into the parent zone using the method specified by the parent zone (web form, e-mail, API, …).

• Generate DS records manually using the dnssec-dsfromkey utility on zone keys, and then insert them into the parent zone.

[1]

For further details on how the chain of trust is used in practice, see The 12-Step DNSSEC Validation Process (Simplified) in the DNSSEC Guide.

5.3. DNSSEC Validation

The BIND resolver validates answers from authoritative servers by default. This behavior is controlled by the configuration statement dnssec-validation.

By default a trust anchor for the DNS root zone is used. This trust anchor is provided as part of BIND and is kept up-to-date using Dynamic Trust Anchor Management.

Note

DNSSEC validation works “out of the box” and does not require additional configuration. Additional configuration options are intended only for special cases.

To validate answers, the resolver needs at least one trusted starting point, a “trust anchor.” Essentially, trust anchors are copies of DNSKEY RRs for zones that are used to form the first link in the cryptographic chain of trust. Alternative trust anchors can be specified using trust-anchors Statement Grammar, but this setup is very unusual and is recommended only for expert use. For more information, see Trust Anchors in the DNSSEC Guide.

The BIND authoritative server does not verify signatures on load, so zone keys for authoritative zones do not need to be specified in the configuration file.

5.3.1. Validation Failures

When DNSSEC validation is configured, the resolver rejects any answers from signed, secure zones which fail to validate, and returns SERVFAIL to the client.

Responses may fail to validate for any of several reasons, including missing, expired, or invalid signatures; a key which does not match the DS RRset in the parent zone; or an insecure response from a zone which, according to its parent, should have been secure.

For more information see Basic DNSSEC Troubleshooting.

5.3.2. Coexistence With Unsigned (Insecure) Zones

Zones not protected by DNSSEC are called “insecure,” and these zones seamlessly coexist with signed zones.

When the validator receives a response from an unsigned zone that has a signed parent, it must confirm with the parent that the zone was intentionally left unsigned. It does this by verifying, via signed and validated NSEC/NSEC3 records, that the parent zone contains no DS records for the child.

If the validator can prove that the zone is insecure, then the response is accepted. However, if it cannot, the validator must assume an insecure response to be a forgery; it rejects the response and logs an error.

The logged error reads “insecurity proof failed” and “got insecure response; parent indicates it should be secure.”