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Configuration

Simple configuration

The following example presents a simple configuration file which can be used as a base for your Knot DNS setup:

# Example of a very simple Knot DNS configuration.

server:
    listen: 0.0.0.0@53
    listen: ::@53

zone:
  - domain: example.com
    storage: /var/lib/knot/zones/
    file: example.com.zone

log:
  - target: syslog
    any: info

Now let's walk through this configuration step by step:

  • The listen statement in the server section defines where the server will listen for incoming connections. We have defined the server to listen on all available IPv4 and IPv6 addresses, all on port 53.
  • The zone section defines the zones that the server will serve. In this case, we defined one zone named example.com which is stored in the zone file /var/lib/knot/zones/example.com.zone.
  • The log section defines the log facilities for the server. In this example, we told Knot DNS to send its log messages with the severity info or more serious to the syslog.

For detailed description of all configuration items see Configuration Reference.

Zone templates

A zone template allows a single zone configuration to be shared among several zones. The default template identifier is reserved for the default template:

template:
  - id: default
    storage: /var/lib/knot/master
    semantic-checks: on

  - id: signed
    storage: /var/lib/knot/signed
    dnssec-signing: on
    semantic-checks: on
    master: [master1, master2]

  - id: slave
    storage: /var/lib/knot/slave

zone:
  - domain: example1.com     # Uses default template

  - domain: example2.com     # Uses default template
    semantic-checks: off     # Override default settings

  - domain: example.cz
    template: signed
    master: master3          # Override masters to just master3

  - domain: example1.eu
    template: slave
    master: master1

  - domain: example2.eu
    template: slave
    master: master2

Note

Each template option can be explicitly overridden in zone-specific configuration.

Access control list (ACL)

An ACL list specifies which remotes are allowed to send the server a specific request. A remote can be a single IP address or a network subnet. Also a TSIG key can be assigned (see keymgr how to generate a TSIG key):

key:
  - id: key1
    algorithm: hmac-md5
    secret: Wg==

acl:
  - id: address_rule
    address: [2001:db8::1, 192.168.2.0/24] # Allowed IP address list
    action: [transfer, update]  # Allow zone transfers and updates

  - id: deny_rule             # Negative match rule
    address: 192.168.2.100
    action: transfer
    deny: on                  # The request is denied

  - id: key_rule
    key: key1                 # Access based just on TSIG key
    action: transfer

These rules can then be referenced from a zone acl:

zone:
  - domain: example.com
    acl: [address_rule, deny_rule, key_rule]

Slave zone

Knot DNS doesn't strictly differ between master and slave zones. The only requirement is to have a master statement set for the given zone. Also note that you need to explicitly allow incoming zone changed notifications via notify action through zone's acl list, otherwise the update will be rejected by the server. If the zone file doesn't exist it will be bootstrapped over AXFR:

remote:
  - id: master
    address: 192.168.1.1@53

acl:
  - id: notify_from_master
    address: 192.168.1.1
    action: notify

zone:
  - domain: example.com
    storage: /var/lib/knot/zones/
    # file: example.com.zone   # Default value
    master: master
    acl: notify_from_master

Note that the master option accepts a list of multiple remotes. The remotes should be listed according to their preference. The first remote has the highest preference, the other remotes are used for failover. When the server receives a zone update notification from a listed remote, that remote will be the most preferred one for the subsequent transfer.

To use TSIG for transfers and notification messages authentication, configure a TSIG key and assign the key both to the remote and the ACL rule. Notice that the remote and ACL definitions are independent:

key:
  - id: slave1_key
    algorithm: hmac-md5
    secret: Wg==

remote:
  - id: master
    address: 192.168.1.1@53
    key: slave1_key

acl:
  - id: notify_from_master
    address: 192.168.1.1
    key: slave1_key
    action: notify

Note

When transferring a lot of zones, the server may easily get into a state when all available ports are in the TIME_WAIT state, thus the transfers seize until the operating system closes the ports for good. There are several ways to work around this:

  • Allow reusing of ports in TIME_WAIT (sysctl -w net.ipv4.tcp_tw_reuse=1)
  • Shorten TIME_WAIT timeout (tcp_fin_timeout)
  • Increase available local port count

Master zone

An ACL with the transfer action must be configured to allow outgoing zone transfers. An ACL rule consists of a single address or a network subnet:

remote:
  - id: slave1
    address: 192.168.2.1@53

acl:
  - id: slave1_acl
    address: 192.168.2.1
    action: transfer

  - id: others_acl
    address: 192.168.3.0/24
    action: transfer

zone:
  - domain: example.com
    storage: /var/lib/knot/zones/
    file: example.com.zone
    notify: slave1
    acl: [slave1_acl, others_acl]

Optionally, a TSIG key can be specified:

key:
  - id: slave1_key
    algorithm: hmac-md5
    secret: Wg==

remote:
  - id: slave1
    address: 192.168.2.1@53
    key: slave1_key

acl:
  - id: slave1_acl
    address: 192.168.2.1
    key: slave1_key
    action: transfer

  - id: others_acl
    address: 192.168.3.0/24
    action: transfer

Dynamic updates

Dynamic updates for the zone are allowed via proper ACL rule with the update action. If the zone is configured as a slave and a DNS update message is accepted, the server forwards the message to its primary master. The master's response is then forwarded back to the originator.

However, if the zone is configured as a master, the update is accepted and processed:

acl:
  - id: update_acl
    address: 192.168.3.0/24
    action: update

zone:
  - domain: example.com
    file: example.com.zone
    acl: update_acl

Response rate limiting

Response rate limiting (RRL) is a method to combat DNS reflection amplification attacks. These attacks rely on the fact that source address of a UDP query can be forged, and without a worldwide deployment of BCP38, such a forgery cannot be prevented. An attacker can use a DNS server (or multiple servers) as an amplification source and can flood a victim with a large number of unsolicited DNS responses.

The RRL lowers the amplification factor of these attacks by sending some of the responses as truncated or by dropping them altogether.

You can enable RRL by setting the rate-limit option in the server section. The option controls how many responses per second are permitted for each flow. Responses exceeding this rate are limited. The option rate-limit-slip then configures how many limited responses are sent as truncated (slip) instead of being dropped.

server:
    rate-limit: 200     # Allow 200 resp/s for each flow
    rate-limit-slip: 2  # Every other response slips

Automatic DNSSEC signing

Knot DNS supports automatic DNSSEC signing for static zones. The signing can operate in two modes:

  1. Automatic key management. In this mode, the server maintains signing keys. New keys are generated according to assigned policy and are rolled automatically in a safe manner. No zone operator intervention is necessary.
  2. Manual key management. In this mode, the server maintains zone signatures only. The signatures are kept up-to-date and signing keys are rolled according to timing parameters assigned to the keys. The keys must be generated and timing parameters must be assigned by the zone operator.

The DNSSEC signing process maintains some metadata which is stored in the KASP database. This database is simply a directory in the file-system containing files in the JSON format.

Warning

Make sure to set the KASP database permissions correctly. For manual key management, the database must be readable by the server process. For automatic key management, it must be writeable. If no HSM is used, the database also contains private key material – don't set the permissions too week.

Automatic key management

For automatic key management, a signing policy has to be configured and assigned to the zone. The policy specifies how the zone is signed (i.e. signing algorithm, key size, key lifetime, signature lifetime, etc.). The policy can be configured in the policy section, or a default policy with the default parameters can be used.

A minimal zone configuration may look as follows:

zone:
  - domain: myzone.test
    dnssec-signing: on
    dnssec-policy: default

With custom signing policy, the policy section will be added:

policy:
  - id: rsa
    algorithm: RSASHA256
    ksk-size: 2048
    zsk-size: 1024

zone:
  - domain: myzone.test
    dnssec-signing: on
    dnssec-policy: rsa

After configuring the server, reload the changes:

$ knotc reload

The server will generate initial signing keys and sign the zone properly. Check the server logs to see whether everything went well.

Warning

This guide assumes that the zone myzone.test was not signed prior to enabling the automatic key management. If the zone was already signed, all existing keys must be imported using keymgr zone key import command before enabling the automatic signing. Also the algorithm in the policy must match the algorithm of all imported keys. Otherwise the zone will be resigned at all.

Manual key management

For automatic DNSSEC signing with manual key management, a signing policy with manual key management flag has to be set:

policy:
  - id: manual
    manual: on

zone:
  - domain: myzone.test
    dnssec-signing: on
    dnssec-policy: manual

To generate signing keys, use the keymgr utility. Let's use the Single-Type Signing scheme with two algorithms, which is a scheme currently not supported by the automatic key management. Run:

$ keymgr zone key generate myzone.test algorithm RSASHA256 size 1024
$ keymgr zone key generate myzone.test algorithm ECDSAP256SHA256 size 256

And reload the server. The zone will be signed.

To perform a manual rollover of a key, the timing parameters of the key need to be set. Let's roll the RSA key. Generate a new RSA key, but do not activate it yet:

$ keymgr zone key generate myzone.test algorithm RSASHA256 size 1024 active +1d

Take the key ID (or key tag) of the old RSA key and disable it the same time the new key gets activated:

$ keymgr zone key set myzone.test <old_key_id> retire +1d remove +1d

Reload the server again. The new key will be published (i.e. the DNSKEY record will be added into the zone). Do not forget to update the DS record in the parent zone to include a reference to the new RSA key. This must happen in one day (in this case) including a delay required to propagate the new DS to caches.

Note that as the +1d time specification is computed from the current time, the key replacement will not happen at once. First, a new key will be activated. A few moments later, the old key will be deactivated and removed. You can use exact time specification to make these two actions happen in one go.

Zone signing

The signing process consists of the following steps:

  1. Processing KASP database events. (e.g. performing a step of a rollover).
  2. Fixing the NSEC or NSEC3 chain.
  3. Updating the DNSKEY records. The whole DNSKEY set in zone apex is replaced by the keys from the KASP database. Note that keys added into the zone file manually will be removed. To add an extra DNSKEY record into the set, the key must be imported into the KASP database (possibly deactivated).
  4. Removing expired signatures, invalid signatures, signatures expiring in a short time, and signatures issued by an unknown key.
  5. Creating missing signatures. Unless the Single-Type Signing Scheme is used, DNSKEY records in a zone apex are signed by KSK keys and all other records are signed by ZSK keys.
  6. Updating and resigning SOA record.

The signing is initiated on the following occasions:

  • Start of the server
  • Zone reload
  • Reaching the signature refresh period
  • Received DDNS update
  • Forced zone resign via server control interface

On a forced zone resign, all signatures in the zone are dropped and recreated.

The knotc zone-status command can be used to see when the next scheduled DNSSEC resign will happen.

Limitations

The current DNSSEC implementation in Knot DNS has some limitations. Most of the limitations will be hopefully removed in the near future.

  • Automatic key management:
    • Only one DNSSEC algorithm can be used per zone.
    • Single-Type Signing scheme is not supported.
    • ZSK rollover always uses key pre-publish method (actually a feature).
    • KSK rollover is not implemented.
  • Signing:
    • Signature expiration jitter is not implemented.
    • Signature expiration skew is not implemented.
  • Utilities:
    • Legacy key import requires a private key.
    • Legacy key export is not implemented.
    • DS record export is not implemented.

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