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)

The Access control list is a list of rules specifying remotes which are allowed to send certain types of requests to the server. Remotes can be specified by a single IP address or a network subnet. A TSIG key can also be assigned (see keymgr on how to generate a TSIG key).

Without any ACL rules, all the actions are denied for the zone. Each ACL rule can allow one or more actions for a given address/subnet/TSIG, or deny them.

If there are multiple ACL rules for a single zone, they are applied in the order of appearance in the acl configuration item of a zone or a template. The first one to match the given remote is applied, the rest is ignored.

See the following examples and ACL section.

acl:
  - id: address_rule
    address: [2001:db8::1, 192.168.2.0/24]
    action: transfer

  - id: deny_rule
    address: 192.168.2.100
    action: transfer
    deny: on

zone:
  - domain: acl1.example.com.
    acl: [deny_rule, address_rule] # deny_rule first here to take precendence
key:
  - id: key1                  # The real TSIG key name
    algorithm: hmac-md5
    secret: Wg==

acl:
  - id: deny_all
    address: 192.168.3.0/24
    deny: on # no action specified and deny on implies denial of all actions

  - id: key_rule
    key: key1                 # Access based just on TSIG key
    action: [transfer, notify]

zone:
  - domain: acl2.example.com
    acl: [deny_all, key_rule]

Note

If more conditions (address ranges and/or a key) are given in a single ACL rule, all of them have to be satisfied for the rule to match.

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

Note that a slave zone may serve as a master zone at the same time:

remote:
  - id: master
    address: 192.168.1.1@53
  - id: slave1
    address: 192.168.2.1@53

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

  - 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
    master: master
    notify: slave1
    acl: [notify_from_master, slave1_acl, others_acl]

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

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 backed by LMDB.

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 weak.

Automatic ZSK management

For automatic ZSK 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.). If no policy is specified or the default one is assigned, the default signing parameters are used.

A minimal zone configuration may look as follows:

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

With a 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 import-bind 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 re-signed at all.

Automatic KSK management

For automatic KSK management, first configure ZSK management like above, and use additional options in policy section, mostly specifying desired (finite) lifetime for KSK:

remote:
  - id: test_zone_server
    address: 192.168.12.1@53

submission:
  - id: test_zone_sbm
    parent: [test_zone_server]

policy:
  - id: rsa
    algorithm: RSASHA256
    ksk-size: 2048
    zsk-size: 1024
    zsk-lifetime: 30d
    ksk-lifetime: 365d
    ksk-submission: test_zone_sbm

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

After the initially-generated KSK reaches its lifetime, new KSK is published and after convenience delay the submission is started. The server publishes CDS and CDNSKEY records and the user shall propagate them to the parent. The server periodically checks for DS at the master and when positive, finishes the rollover.

To share KSKs among zones, set the ksk-shared policy parameter. It is strongly discouraged to change the policy id afterwards! The shared key’s creation timestamp will be equal for all zones, but other timers (e.g. activate, retire) may get out of sync.

policy:
  - id: shared
    ...
    ksk-shared: true

zone:
  - domain: firstzone.test
    dnssec-signing: on
    dnssec-policy: shared

zone:
  - domain: secondzone.test
    dnssec-signing: on
    dnssec-policy: shared

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. Run:

$ keymgr myzone.test. generate algorithm=ECDSAP256SHA256
$ keymgr myzone.test. generate algorithm=ED25519

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 myzone.test. generate 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 myzone.test. set <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.

Warning

If you ever decide to switch from manual key management to automatic key management, note that the automatic key management uses zsk-lifetime and ksk-lifetime policy configuration options to schedule key rollovers and it internally uses timestamps of keys differently than in the manual case. As a consequence it might break if the retire or remove timestamps are set for the manually generated keys currently in use. Make sure to set these timestamps to zero using keymgr:

$ keymgr myzone.test. set <key_id> retire=0 remove=0

and configure your policy suitably according to Automatic ZSK management and Automatic KSK management.

Zone signing

The signing process consists of the following steps:

  1. Processing KASP database events. (e.g. performing a step of a rollover).
  2. 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).
  3. Fixing the NSEC or NSEC3 chain.
  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 re-signing SOA record.

The signing is initiated on the following occasions:

  • Start of the server
  • Zone reload
  • Reaching the signature refresh period
  • Key set changed due to rollover event
  • Received DDNS update
  • Forced zone re-sign via server control interface

On a forced zone re-sign, all signatures in the zone are dropped and recreated.

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

On-slave signing

It is possible to enable automatic DNSSEC zone signing even on a slave server. If enabled, the zone is signed after every AXFR/IXFR transfer from master, so that the slave always serves a signed up-to-date version of the zone.

It is strongly recommended to block any outside access to the master server, so that only the slave’s signed version of the zone is served.

Enabled on-slave signing introduces events when the slave zone changes while the master zone remains unchanged, such as a key rollover or refreshing of RRSIG records, which cause inequality of zone SOA serial between master and slave. The slave server handles this by saving the master’s SOA serial in a special variable inside KASP DB and appropriately modifiying AXFR/IXFR queries/answers to keep the communication with master consistent while applying the changes with a different serial.

It is recommended to use UNIX time serial policy on master and incremental serial policy on slave so that their SOA serials are equal most of the time.

Query modules

Knot DNS supports configurable query modules that can alter the way queries are processed. Each query requires a finite number of steps to be resolved. We call this set of steps a query plan, an abstraction that groups these steps into several stages.

  • Before-query processing
  • Answer, Authority, Additional records packet sections processing
  • After-query processing

For example, processing an Internet-class query needs to find an answer. Then based on the previous state, it may also append an authority SOA or provide additional records. Each of these actions represents a ‘processing step’. Now, if a query module is loaded for a zone, it is provided with an implicit query plan which can be extended by the module or even changed altogether.

A module is active if its name, which includes the mod- prefix, is assigned to the zone/template module option or to the default template global-module option if activating for all queries. If the module is configurable, a corresponding module section with an identifier must be created and then referenced in the form of module_name/module_id. See Modules for the list of available modules.

Note

Query modules are processed in the order they are specified in the zone/template configuration. In most cases, the recommended order is:

mod-synthrecord, mod-onlinesign, mod-rrl, mod-dnstap, mod-stats

Performance Tuning

Numbers of Workers

There are three types of workers ready for parallel execution of performance-oriented tasks: UDP workers, TCP workers, and Background workers. The first two types handle all network requests coming through UDP and TCP protocol (respectively) and do all the response job for common queries. Background workers process changes to the zone.

By default, Knot determines well-fitting number of workers based on the number of CPU cores. The user can specify the numbers of workers for each type with configuration/server section: udp-workers, tcp-workers, background-workers.

An indication on when to increase number of workers is a situation when the server is lagging behind the expected performance, while the CPU usage is low. This is usually because of waiting for network or I/O response during the operation. It may be caused by Knot design not fitting well the usecase. The user should try increasing the number of workers (of the related type) slightly above 100 and if the performance gets better, he can decide about further exact setting.

Sysctl and NIC optimizations

There are several recommendations based on Knot developers’ experience with their specific HW and SW (mainstream Intel-based servers, Debian-based GNU/Linux distribution). They may or may not positively (or negatively) influence performance in common use cases.

If your NIC driver allows it (see /proc/interrupts for hint), set CPU affinity (/proc/irq/$IRQ/smp_affinity) manually so that each NIC channel is served by unique CPU core(s). You must turn off irqbalance service before to avoid configuration override.

Configure sysctl as follows:

socket_bufsize=1048576
busy_latency=0
backlog=40000
optmem_max=20480

net.core.wmem_max     = $socket_bufsize
net.core.wmem_default = $socket_bufsize
net.core.rmem_max     = $socket_bufsize
net.core.rmem_default = $socket_bufsize
net.core.busy_read = $busy_latency
net.core.busy_poll = $busy_latency
net.core.netdev_max_backlog = $backlog
net.core.optmem_max = $optmem_max

Disable huge pages.

Configure your CPU to “performance” mode. This can be achieved depending on architecture, e.g. in BIOS, or e.g. configuring /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor to “performance”.

Tune your NIC device with ethtool:

ethtool -A $dev autoneg off rx off tx off
ethtool -K $dev tso off gro off ufo off
ethtool -G $dev rx 4096 tx 4096
ethtool -C $dev rx-usecs 75
ethtool -C $dev tx-usecs 75
ethtool -N $dev rx-flow-hash udp4 sdfn
ethtool -N $dev rx-flow-hash udp6 sdfn

On FreeBSD you can just:

ifconfig ${dev} -rxcsum -txcsum -lro -tso

Knot developers are open to hear about users’ further suggestions about network devices tuning/optimization.