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 (or systemd journal).

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. There is no inheritance between templates; they are exclusive. 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)

Some types of incoming DNS requests must be authorized before they can be processed by the server. A zone can have configured acl which is a sequence of rules describing what requests are authorized. By default if automatic ACL is not enabled, all requests, which require authorization, are denied.

Every ACL rule can allow or deny one or more request types based on the source IP address, network subnet, or address range and/or if the request is secured by a given TSIG key. See keymgr -t on how to generate a TSIG key.

If there are multiple ACL rules assigned to a zone, they are applied in the specified order of the acl configuration. The first rule that matches the given request is applied and the remaining rules are ignored. Some examples:

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]     # Allow some addresses with an exception
key:
  - id: key1                           # The real TSIG key name
    algorithm: hmac-sha256
    secret: 4Tc0K1QkcMCs7cOW2LuSWnxQY0qysdvsZlSb4yTN9pA=

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]          # Allow with the TSIG except for the subnet

For dynamic DNS updates, additional conditions may be specified for more granular filtering. Example:

acl:
    - id: owner_type_rule
      action: update
      update-type: [A, AAAA, MX]             # Updated records must match one of the specified types
      update-owner: name                     # Updated record owners are restricted by the next conditions
      update-owner-match: equal              # The record owner must exactly match one name from the next list
      update-owner-name: [a, b.example.com.] # Note that non-FQDN names are relative to the effective zone name

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.

Tip

In order to restrict regular DNS queries, use module queryacl.

Secondary (slave) zone

Knot DNS doesn't strictly differ between primary (formerly known as master) and secondary (formerly known as slave) zones. The only requirement for a secondary zone is to have a master statement set. For effective zone synchronization, incoming zone change notifications (NOTIFY), which require authorization, can be enabled using automatic ACL or explicit ACL configuration. Optional transaction authentication (TSIG) is supported for both zone transfers and zone notifications:

server:
    automatic-acl: on                     # Enabled automatic ACL

key:
  - id: xfr_notify_key                    # Common TSIG key for XFR an NOTIFY
    algorithm: hmac-sha256
    secret: VFRejzw8h4M7mb0xZKRFiZAfhhd1eDGybjqHr2FV3vc=

remote:
  - id: primary
    address: [2001:DB8:1::1, 192.168.1.1] # Primary server IP addresses
    # via: [2001:DB8:2::1, 10.0.0.1]      # Local source addresses (optional)
    key: xfr_notify_key                   # TSIG key (optional)

zone:
  - domain: example.com
    master: primary                       # Primary remote(s)

An example of explicit ACL with different TSIG keys for zone transfers and notifications:

key:
  - id: notify_key                        # TSIG key for NOTIFY
    algorithm: hmac-sha256
    secret: uBbhV4aeSS4fPd+wF2ZIn5pxOMF35xEtdq2ibi2hHEQ=

  - id: xfr_key                           # TSIG key for XFR
    algorithm: hmac-sha256
    secret: VFRejzw8h4M7mb0xZKRFiZAfhhd1eDGybjqHr2FV3vc=

remote:
  - id: primary
    address: [2001:DB8:1::1, 192.168.1.1] # Primary server IP addresses
    # via: [2001:DB8:2::1, 10.0.0.1]      # Local source addresses if needed
    key: xfr_key                          # Optional TSIG key

acl:
  - id: notify_from_primary               # ACL rule for NOTIFY from primary
    address: [2001:DB8:1::1, 192.168.1.1] # Primary addresses (optional)
    key: notify_key                       # TSIG key (optional)
    action: notify

zone:
  - domain: example.com
    master: primary                       # Primary remote(s)
    acl: notify_from_primary              # Explicit ACL(s)

Note that the master option accepts a list of remotes, which are queried for a zone refresh sequentially in the specified order. When the server receives a zone change notification from a listed remote, only that remote is used for a subsequent zone transfer.

Note

When transferring a lot of zones, the server may easily get into a state where all available ports are in the TIME_WAIT state, thus transfers cease 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

Primary (master) zone

A zone is considered primary if it doesn't have master set. As outgoing zone transfers (XFR) require authorization, it must be enabled using automatic ACL or explicit ACL configuration. Outgoing zone change notifications (NOTIFY) to remotes can be set by configuring notify. Transaction authentication (TSIG) is supported for both zone transfers and zone notifications:

server:
    automatic-acl: on                     # Enabled automatic ACL

key:
  - id: xfr_notify_key                    # Common TSIG key for XFR an NOTIFY
    algorithm: hmac-sha256
    secret: VFRejzw8h4M7mb0xZKRFiZAfhhd1eDGybjqHr2FV3vc=

remote:
  - id: secondary
    address: [2001:DB8:1::1, 192.168.1.1] # Secondary server IP addresses
    # via: [2001:DB8:2::1, 10.0.0.1]      # Local source addresses (optional)
    key: xfr_notify_key                   # TSIG key (optional)

acl:
  - id: local_xfr                         # Allow XFR to localhost without TSIG
    address: [::1, 127.0.0.1]
    action: transfer

zone:
  - domain: example.com
    notify: secondary                     # Secondary remote(s)
    acl: local_xfr                        # Explicit ACL for local XFR

Note that the notify option accepts a list of remotes, which are all notified sequentially in the specified order.

A secondary zone may serve as a primary zone for a different set of remotes at the same time.

Dynamic updates

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

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

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

zone:
  - domain: example.com
    acl: update_acl

Automatic DNSSEC signing

Knot DNS supports automatic DNSSEC signing of 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: custom_policy
    signing-threads: 4
    algorithm: ECDSAP256SHA256
    zsk-lifetime: 60d

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

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.

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: parent_zone_server
    address: 192.168.12.1@53

submission:
  - id: parent_zone_sbm
    parent: [parent_zone_server]

policy:
  - id: custom_policy
    signing-threads: 4
    algorithm: ECDSAP256SHA256
    zsk-lifetime: 60d
    ksk-lifetime: 365d
    ksk-submission: parent_zone_sbm

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

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 parent zone and when positive, finishes the rollover.

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. For example, we can use Single-Type Signing:

$ keymgr myzone.test. generate algorithm=ECDSAP256SHA256 ksk=yes zsk=yes

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 key. Generate a new key, but do not activate it yet:

$ keymgr myzone.test. generate algorithm=ECDSAP256SHA256 ksk=yes zsk=yes active=+1d

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

$ keymgr myzone.test. set <old_key_id> retire=+2d remove=+3d

Reload the server again. The new key will be published (i.e. the DNSKEY record will be added into the zone). Remember to update the DS record in the parent zone to include a reference to the new key. This must happen within one day (in this case) including a delay required to propagate the new DS to caches.

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-secondary (on-slave) signing

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

It is strongly recommended to block any outside access to the primary server, so that only the secondary server's signed version of the zone is served.

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

Catalog zones

Catalog zones are a concept whereby a list of zones to be configured is maintained as contents of a separate, special zone. This approach has the benefit of simple propagation of a zone list to secondary servers, especially when the list is frequently updated. Currently, catalog zones are described in this Internet Draft.

Terminology first. Catalog zone is a meta-zone which shall not be a part of the DNS tree, but it contains information about the set of member zones and is transferable to secondary servers using common AXFR/IXFR techniques. Catalog-member zone (or just member zone) is a zone based on information from the catalog zone and not from configuration file/database. Member properties are some additional information related to each member zone, also distributed by the catalog zone.

A catalog zone is handled almost in the same way as a regular zone: It can be configured using all the standard options (but for example DNSSEC signing would be useless), including primary/secondary configuration and ACLs. A catalog zone is indicated by setting the option catalog-role. The difference is that standard DNS queries to a catalog zone are answered with REFUSED as though the zone doesn't exist, unless querying over TCP from an address with transfers enabled by ACL. The name of the catalog zone is arbitrary. It's possible to configure multiple catalog zones.

Warning

Don't choose a name for a catalog zone below a name of any other existing zones configured on the server as it would effectively "shadow" part of your DNS subtree.

Upon catalog zone (re)load or change, all the PTR records in the format unique-id.zones.catalog. 0 IN PTR member.com. (but not too.deep.zones.catalog.!) are processed and member zones created, with zone names taken from the PTR records' RData, and zone settings taken from the configuration templates specified by catalog-template.

The owner names of the PTR records shall follow this scheme:

<unique-id>.zones.<catalog-zone>.

where the mentioned labels shall match:

  • <unique-id> — Single label that is recommended to be unique among member zones.

  • zones — Required label.

  • <catalog-zone> — Name of the catalog zone.

Additionally, records in the format group.unique-id.zones.catalog. 0 IN TXT "conf-template" are processed as a definition of the member's group property. The unique-id must match the one of the PTR record defining the member.

All other records and other member properties are ignored. They remain in the catalog zone, however, and might be for example transferred to a secondary server, which may interpret catalog zones differently. SOA still needs to be present in the catalog zone and its serial handled appropriately. An apex NS record should be present for the sake of interoperability. The version record version 0 IN TXT "2" is required at the catalog zone apex.

A catalog zone may be modified using any standard means (e.g. AXFR/IXFR, DDNS, zone file reload). In the case of incremental change, only affected member zones are reloaded.

The catalog zone must have at least one catalog-template configured. The configuration for any defined member zone is taken from its group property value, which should match some catalog-template name. If the group property is not defined for a member, is empty, or doesn't match any of defined catalog-template names, the first catalog-template (in the order from configuration) is used.

Any de-cataloged member zone is purged immediately, including its zone file, journal, timers, and DNSSEC keys. The zone file is not deleted if zonefile-sync is set to -1 for member zones. Any member zone, whose PTR record's owner has been changed, is purged immediately if and only if the <unique-id> has been changed.

When setting up catalog zones, it might be useful to set catalog-db and catalog-db-max-size to non-default values.

Note

Whenever a catalog zone is updated, the server reloads itself with all configured zones, including possibly existing other catalog zones. It's similar to calling knotc zone-reload (for all zones). The consequence is that new zone files might be discovered and reloaded, even for zones that do not relate to updated catalog zone.

Warning

The server does not work well if one member zone appears in two catalog zones concurrently. The user is encouraged to avoid this situation whatsoever. Thus, there is no way a member zone can be migrated from one catalog to another while preserving its metadata. Following steps may be used as a workaround:

  • Back up the member zone's metadata (on each server separately).

  • Remove the member zone from the catalog it's a member of.

  • Wait for the catalog zone to be propagated to all servers.

  • Add the member zone to the other catalog.

  • Restore the backed up metadata (on each server separately).

Catalog zones configuration examples

Below are configuration snippets (e.g. server and log sections missing) of very simple catalog zone setups, in order to illustrate the relations between catalog-related configuration options.

First setup represents a very simple scenario where the primary is the catalog zone generator and the secondary is the catalog zone consumer.

Primary configuration:

acl:
  - id: slave_xfr
    address: ...
    action: transfer

template:
  - id: mmemb
    catalog-role: member
    catalog-zone: catz.
    acl: slave_xfr

zone:
  - domain: catz.
    catalog-role: generate
    acl: slave_xfr

  - domain: foo.com.
    template: mmemb

  - domain: bar.com.
    template: mmemb

Secondary configuration:

acl:
  - id: master_notify
    address: ...
    action: notify

template:
  - id: smemb
    master: master
    acl: master_notify

zone:
  - domain: catz.
    master: master
    acl: master_notify
    catalog-role: interpret
    catalog-template: smemb

When new zones are added (or removed) to the primary configuration with assigned mmemb template, they will automatically propagate to the secondary and have the smemb template assigned there.

Second example is with a hand-written (or script-generated) catalog zone, while employing configuration groups:

catz.                   0       SOA     invalid. invalid. 1625079950 3600 600 2147483646 0
catz.                   0       NS      invalid.
version.catz.           0       TXT     "2"
nj2xg5bnmz2w4ltd.zones.catz.       0       PTR     just-fun.com.
group.nj2xg5bnmz2w4ltd.zones.catz. 0       TXT     unsigned
nvxxezjnmz2w4ltd.zones.catz.       0       PTR     more-fun.com.
group.nvxxezjnmz2w4ltd.zones.catz. 0       TXT     unsigned
nfwxa33sorqw45bo.zones.catz.       0       PTR     important.com.
group.nfwxa33sorqw45bo.zones.catz. 0       TXT     signed
mjqw42zomnxw2lq0.zones.catz.       0       PTR     bank.com.
group.mjqw42zomnxw2lq0.zones.catz. 0       TXT     signed

And the server in this case is configured to distinguish the groups by applying different templates:

template:
  - id: unsigned
    ...

  - id: signed
    dnssec-signing: on
    dnssec-policy: ...
    ...

zone:
  - domain: catz.
    file: ...
    catalog-role: interpret
    catalog-template: [ unsigned, signed ]

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-cookies, 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 via the UDP and TCP protocol (respectively) and do the response jobs for common queries. Background workers process changes to the zone.

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

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

Number of available file descriptors

A name server configured for a large number of zones (hundreds or more) needs enough file descriptors available for zone transfers and zone file updates, which default OS settings often don't provide. It's necessary to check with the OS configuration and documentation and ensure the number of file descriptors (sometimes called a number of concurrently open files) effective for the knotd process is set suitably high. The number of concurrently open incoming TCP connections must be taken into account too. In other words, the required setting is affected by the tcp-max-clients 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 improve or impact 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 in advance 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.