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Manual Reference Pages  -  KNOT.CONF (5)


knot.conf - Knot DNS configuration file \$1 \n[an-margin] level \n[rst2man-indent-level] level margin: \n[rst2man-indent\n[rst2man-indent-level]] - \n[rst2man-indent0] \n[rst2man-indent1] \n[rst2man-indent2]


Server Section
Key Section
Acl Section
Control Section
Remote Section
Template Section
Zone Section
Logging Section
Module Dnstap
Module Synth-record
Module Dnsproxy
Module Rosedb
Module Online-sign


Configuration files for Knot DNS use simplified YAML format. Simplified means that not all of the features are supported.

For the description of configuration items, we have to declare a meaning of the following symbols:
o INT – Integer
o STR – Textual string
o HEXSTR – Hexadecimal string (with 0x prefix)
o BOOL – Boolean value (on/off or true/false)
o TIME – Number of seconds, an integer with possible time multiplier suffix (s ~ 1, m ~ 60, h ~ 3600 or d ~ 24 * 3600)
o SIZE – Number of bytes, an integer with possible size multiplier suffix (B ~ 1, K ~ 1024, M ~ 1024^2 or G ~ 1024^3)
o BASE64 – Base64 encoded string
o ADDR – IPv4 or IPv6 address
o DNAME – Domain name
o ... – Multi-valued item, order of the values is preserved
o [ ] – Optional value
o | – Choice

There are 8 main sections (server, key, acl, control, remote, template, zone and log) and module sections with the mod- prefix. The most of the sections (excluding server and control) are sequences of settings blocks. Each settings block begins with a unique identifier, which can be used as a reference from other sections (such identifier must be defined in advance).

A multi-valued item can be specified either as a YAML sequence:

address: [,]

or as more single-valued items each on an extra line:


If an item value contains spaces or other special characters, it is necessary to enclose such value within double quotes " ".


A comment begins with a # character and is ignored during processing. Also each configuration section or sequence block allows to specify permanent comment using comment item which is stored in the server beside the configuration.


Another configuration file or files, matching a pattern, can be included at the top level in the current file. If the path is not absolute, then it is considered to be relative to the current file. The pattern can be an arbitrary string meeting POSIX glob requirements, e.g. dir/*.conf. Matching files are processed in sorted order.

include: STR


General options related to the server.

    identity: [STR]
    version: [STR]
    nsid: [STR|HEXSTR]
    rundir: STR
    user: STR[:STR]
    pidfile: STR
    udp-workers: INT
    tcp-workers: INT
    background-workers: INT
    async-start: BOOL
    tcp-handshake-timeout: TIME
    tcp-idle-timeout: TIME
    tcp-reply-timeout: TIME
    max-tcp-clients: INT
    max-udp-payload: SIZE
    rate-limit: INT
    rate-limit-slip: INT
    rate-limit-table-size: INT
    listen: ADDR[@INT] ...


An identity of the server returned in the response to the query for TXT record id.server. or hostname.bind. in the CHAOS class (see RFC 4892). Set empty value to disable.

Default: FQDN hostname


A version of the server software returned in the response to the query for TXT record version.server. or version.bind. in the CHAOS class (see RFC 4892). Set empty value to disable.

Default: server version


A DNS name server identifier (see RFC 5001). Set empty value to disable.

Default: FQDN hostname


A path for storing run-time data (PID file, unix sockets, etc.).

Default: ${localstatedir}/run/knot (configured with --with-rundir=path)


A system user with an optional system group (user:group) under which the server is run after starting and binding to interfaces. Linux capabilities are employed if supported.

Default: root:root


A PID file location.

Default:  rundir/


A number of quering UDP workers (threads).

Default: auto-estimated optimal value based on the number of online CPUs


A number of quering TCP workers (threads).

Default: auto-estimated optimal value based on the number of online CPUs


A number of workers (threads) used to execute background operations (zone loading, zone updates, etc.).

Default: auto-estimated optimal value based on the number of online CPUs


If enabled, server doesn\(aqt wait for the zones to be loaded and starts responding immediately with SERVFAIL answers until the zone loads.

Default: off


Maximum time between newly accepted TCP connection and the first query. This is useful to disconnect inactive connections faster than connections that already made at least 1 meaningful query.

Default: 5


Maximum idle time between requests on a TCP connection. This also limits receiving of a single query, each query must be received in this time limit.

Default: 20


Maximum time to wait for an outgoing connection or for a reply to an issued request (SOA, NOTIFY, AXFR...). This limit also applies to knotc remote operation over an internet socket.

Default: 10


A maximum number of TCP clients connected in parallel, set this below the file descriptor limit to avoid resource exhaustion.

Default: 100


Rate limiting is based on the token bucket scheme. A rate basically represents a number of tokens available each second. Each response is processed and classified (based on several discriminators, e.g. source netblock, query type, zone name, rcode, etc.). Classified responses are then hashed and assigned to a bucket containing number of available tokens, timestamp and metadata. When available tokens are exhausted, response is dropped or sent as truncated (see  rate-limit-slip). Number of available tokens is recalculated each second.

Default: 0 (disabled)


Size of the hash table in a number of buckets. The larger the hash table, the lesser the probability of a hash collision, but at the expense of additional memory costs. Each bucket is estimated roughly to 32 bytes. The size should be selected as a reasonably large prime due to better hash function distribution properties. Hash table is internally chained and works well up to a fill rate of 90 %, general rule of thumb is to select a prime near 1.2 * maximum_qps.

Default: 393241


As attacks using DNS/UDP are usually based on a forged source address, an attacker could deny services to the victim\(aqs netblock if all responses would be completely blocked. The idea behind SLIP mechanism is to send each Nth response as truncated, thus allowing client to reconnect via TCP for at least some degree of service. It is worth noting, that some responses can\(aqt be truncated (e.g. SERVFAIL).
o Setting the value to 0 will cause that all rate-limited responses will be dropped. The outbound bandwidth and packet rate will be strictly capped by the  rate-limit option. All legitimate requestors affected by the limit will face denial of service and will observe excessive timeouts. Therefore this setting is not recommended.
o Setting the value to 1 will cause that all rate-limited responses will be sent as truncated. The amplification factor of the attack will be reduced, but the outbound data bandwidth won\(aqt be lower than the incoming bandwidth. Also the outbound packet rate will be the same as without RRL.
o Setting the value to 2 will cause that half of the rate-limited responses will be dropped, the other half will be sent as truncated. With this configuration, both outbound bandwidth and packet rate will be lower than the inbound. On the other hand, the dropped responses enlarge the time window for possible cache poisoning attack on the resolver.
o Setting the value to anything larger than 2 will keep on decreasing the outgoing rate-limited bandwidth, packet rate, and chances to notify legitimate requestors to reconnect using TCP. These attributes are inversely proportional to the configured value. Setting the value high is not advisable.

Default: 1


Maximum EDNS0 UDP payload size.

Default: 4096


One or more IP addresses where the server listens for incoming queries. Optional port specification (default is 53) can be appended to each address using @ separator. Use for all configured IPv4 addresses or :: for all configured IPv6 addresses.

Default: not set


Shared TSIG keys used to authenticate communication with the server.

  - id: DNAME
    algorithm: hmac-md5 | hmac-sha1 | hmac-sha224 | hmac-sha256 | hmac-sha384 | hmac-sha512
    secret: BASE64


A key name identifier.


A key algorithm.

Default: not set


Shared key secret.

Default: not set


Access control list rule definitions. The ACLs are used to match incoming connections to allow or deny requested operation (zone transfer request, DDNS update, etc.).

  - id: STR
    address: ADDR[/INT] | ADDR-ADDR ...
    key: key_id ...
    action: notify | transfer | update ...
    deny: BOOL


An ACL rule identifier.


An ordered list of IP addresses, network subnets, or network ranges. The query must match one of them. Empty value means that address match is not required.

Default: not set


An ordered list of  references to TSIG keys. The query must match one of them. Empty value means that TSIG key is not required.

Default: not set


An ordered list of allowed actions. Empty action list is only allowed if  deny is set.

Possible values:
o transfer – Allow zone transfer
o notify – Allow incoming notify
o update – Allow zone updates

Default: not set


Deny if  address,  key and  action match.

Default: off


Configuration of the server control interface.

    listen: STR


A UNIX socket path where the server listens for remote control commands.

Default:  rundir/knot.sock


Definitions of remote servers for outgoing connections (source of a zone transfer, target for a notification, etc.).

  - id: STR
    address: ADDR[@INT] ...
    via: ADDR[@INT] ...
    key: key_id


A remote identifier.


An ordered list of destination IP addresses which are used for communication with the remote server. The addresses are tried in sequence unless the operation is successful. Optional destination port (default is 53) can be appended to the address using @ separator.

Default: not set


An ordered list of source IP addresses. The first address with the same family as the destination address is used. Optional source port (default is random) can be appended to the address using @ separator.

Default: not set


A  reference to the TSIG key which ise used to autenticate the communication with the remote server.

Default: not set


A template is a shareable zone setting which can be used for configuration of many zones in one place. A special default template (with the default identifier) can be used for global querying configuration or as an implicit configuration if a zone doesn\(aqt have another template specified.

  - id: STR
    timer-db: STR
    global-module: STR/STR ...
    # All zone options (excluding \(aqtemplate\(aq item)


A template identifier.


Specifies a path of the persistent timer database. The path can be specified as a relative path to the default template  storage.

Caution: This option is only available in the default template.

Default:  storage/timers


An ordered list of references to query modules in the form module_name/module_id. These modules apply to all queries.

Caution: This option is only available in the default template.

Default: not set


Definition of zones served by the server.

  - domain: DNAME
    template: template_id
    file: STR
    storage: STR
    master: remote_id ...
    ddns-master: remote_id
    notify: remote_id ...
    acl: acl_id ...
    semantic-checks: BOOL
    disable-any: BOOL
    zonefile-sync: TIME
    ixfr-from-differences: BOOL
    max-journal-size: SIZE
    dnssec-signing: BOOL
    kasp-db: STR
    request-edns-option: INT:[HEXSTR]
    serial-policy: increment | unixtime
    module: STR/STR ...


A zone name identifier.


A  reference to a configuration template.

Default: not set or default (if the template exists)


A path to the zone file. Non absolute path is relative to  storage. It is also possible to use the following formatters:
o %c[N] or %c[N-M] – means the Nth character or a sequence of characters beginning from the Nth and ending with the Mth character of the textual zone name (see %s). The indexes are counted from 0 from the left. If the character is not available, the formatter has no effect.
o %l[N] – means the Nth label of the textual zone name (see %s). The index is counted from 0 from the right (0 ~ TLD). If the label is not available, the formatter has no effect.
o %s – means the current zone name in the textual representation (beware of special characters which are escaped or encoded in the \DDD form where DDD is corresponding decimal ASCII code). The zone name doesn\(aqt include the terminating dot (the result for the root zone is the empty string!).
o %% – means the % character

Default:  storage/


A data directory for storing zone files, journal files and timers database.

Default: ${localstatedir}/lib/knot (configured with --with-storage=path)


An ordered list of  references to zone master servers.

Default: not set


A  reference to zone primary master server. If not specified, the first  master server is used.

Default: not set


An ordered list of  references to remotes to which notify message is sent if the zone changes.

Default: not set


An ordered list of  references to ACL rules which can allow or disallow zone transfers, updates or incoming notifies.

Default: not set


If enabled, extra zone file semantic checks are turned on.

Several checks are enabled by default and cannot be turned off. An error in mandatory checks causes zone not to be loaded. An error in extra checks is logged only.

Mandatory checks:
o An extra record together with CNAME record (except for RRSIG and DS)
o CNAME link chain length greater than 10 (including infinite cycles)
o DNAME and CNAME records under the same owner (RFC 2672)
o CNAME and DNAME wildcards pointing to themselves
o SOA record missing in the zone (RFC 1034)
o DNAME records having records under it (DNAME children) (RFC 2672)

Extra checks:
o Missing NS record at the zone apex
o Missing glue A or AAAA records
o Broken or non-cyclic NSEC(3) chain
o Wrong NSEC(3) type bitmap
o Multiple NSEC records at the same node
o Missing NSEC records at authoritative nodes
o Extra record types under the same name as NSEC3 record (this is RFC-valid, but Knot will not serve such a zone correctly)
o NSEC3-unsecured delegation that is not part of Opt-out span
o Wrong original TTL value in NSEC3 records
o Wrong RDATA TTL value in RRSIG record
o Signer name in RRSIG RR not the same as in DNSKEY
o Signed RRSIG
o Not all RRs in the node are signed
o Wrong key flags or wrong key in RRSIG record (not the same as ZSK)

Default: off


If enabled, all authoritative ANY queries sent over UDP will be answered with an empty response and with the TC bit set. Use this option to minimize the risk of DNS reflection attack.

Default: off


The time after which the current zone in memory will be synced with a zone file on the disk (see  file). The server will serve the latest zone even after a restart using zone journal, but the zone file on the disk will only be synced after zonefile-sync time has expired (or after manual zone flush). This is applicable when the zone is updated via IXFR, DDNS or automatic DNSSEC signing. In order to disable automatic zonefile synchronization, -1 value can be used (manual zone flush is still possible).

Caution: If you are serving large zones with frequent updates where the immediate sync with a zone file is not desirable, increase the value.

Default: 0 (immediate)


If enabled, the server creates zone differences from changes you made to the zone file upon server reload. This option is relevant only if the server is a master server for the zone.

Caution: This option has no effect with enabled  dnssec-signing.

Default: off


Maximum size of the zone journal file.

Default: 2^64


If enabled, automatic DNSSEC signing for the zone is turned on.

Caution: Cannot be enabled on a slave zone.

Default: off


A KASP database path. Non absolute path is relative to  storage.

Default:  storage/keys


An arbitrary EDNS0 option which is included into a server request (AXFR, IXFR, SOA, or NOTIFY). The value is in the option_code:option_data format.

Default: not set


Specifies how the zone serial is updated after a dynamic update or automatic DNSSEC signing. If the serial is changed by the dynamic update, no change is made.

Possible values:
o increment – The serial is incremented according to serial number arithmetic
o unixtime – The serial is set to the current unix time

Caution: If your serial was in other than unix time format, be careful with the transition to unix time. It may happen that the new serial will be \(aqlower\(aq than the old one. If this is the case, the transition should be done by hand (see RFC 1982).

Default: increment


An ordered list of references to query modules in the form module_name/module_id. These modules apply only to the current zone queries.

Default: not set


Server can be configured to log to the standard output, standard error output, syslog (or systemd journal if systemd is enabled) or into an arbitrary file.

There are 6 logging severity levels:
o critical – Non-recoverable error resulting in server shutdown
o error – Recoverable error, action should be taken
o warning – Warning that might require user action
o notice – Server notice or hint
o info – Informational message
o debug – Debug messages (must be turned on at compile time)

In the case of missing log section, warning or more serious messages will be logged to both standard error output and syslog. The info and notice messages will be logged to standard output.

  - target: stdout | stderr | syslog | STR
    server: critical | error | warning | notice | info | debug
    zone: critical | error | warning | notice | info | debug
    any: critical | error | warning | notice | info | debug


A logging output.

Possible values:
o stdout – Standard output
o stderr – Standard error output
o syslog – Syslog
o file_name – File


Minimum severity level for messages related to general operation of the server that are logged.

Default: not set


Minimum severity level for messages related to zones that are logged.

Default: not set


Minimum severity level for all message types that are logged.

Default: not set


The module dnstap allows query and response logging.

For all queries logging, use this module in the default template. For zone-specific logging, use this module in the proper zone configuration.

  - id: STR
    sink: STR


A module identifier.


A sink path, which can be either a file or a UNIX socket when prefixed with unix:.



This module is able to synthesize either forward or reverse records for the given prefix and subnet.

  - id: STR
    type: forward | reverse
    prefix: STR
    origin: DNAME
    ttl: INT
    network: ADDR[/INT] | ADDR-ADDR


A module identifier.


The type of generated records.

Possible values:
o forward – Forward records
o reverse – Reverse records



A record owner prefix.

Caution: prefix doesn’t allow dots, address parts in the synthetic names are separated with a dash.

Default: empty


A zone origin (only valid for the  reverse type).



Time to live of the generated records.

Default: 3600


An IP address, a network subnet, or a network range the query must match.



The module catches all unsatisfied queries and forwards them to the indicated server for resolution.

  - id: STR
    remote: remote_id
    catch-nxdomain: BOOL


A module identifier.


A  reference to a remote server where the queries are forwarded to.



If enabled, all unsatisfied queries (also applies to local zone lookups) are forwarded.

Default: off


The module provides a mean to override responses for certain queries before the available zones are searched for the record.

  - id: STR
    dbdir: STR


A module identifier.


A path to the directory where the database is stored.



The module provides online DNSSEC signing.

  - id: STR


A module identifier.


CZ.NIC Labs <>


Copyright 2010–2016, CZ.NIC, z.s.p.o.
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2.1.1 KNOT.CONF (5) 2016-02-10

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