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:
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: [10.0.0.1, 10.0.0.2]
or as more single-valued items each on an extra line:
address: 10.0.0.1
address: 10.0.0.2
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.
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-idle-timeout: TIME
tcp-handshake-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 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't wait for the zones to be loaded and starts responding immediately with SERVFAIL answers until the zone loads.
Default: off
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 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
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, qtype, 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 rejected or enters SLIP (server responds with a truncated response). Number of available tokens is recalculated each second.
Default: 0 (disabled)
Size of the hashtable in a number of buckets. The larger the hashtable, 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 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't be truncated (e.g. SERVFAIL).
It is advisable not to set the slip interval to a value larger than 2, as too large slip value means more denial of service for legitimate requestors, and introduces excessive timeouts during resolution. On the other hand, slipping truncated answer gives the legitimate requestors a chance to reconnect over TCP.
Default: 1
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 0.0.0.0 for all configured IPv4 addresses or :: for all configured IPv6 addresses.
Default: empty
Shared TSIG keys used to authenticate communication with the server.
key:
- id: DNAME
algorithm: hmac-md5 | hmac-sha1 | hmac-sha224 | hmac-sha256 | hmac-sha384 | hmac-sha512
secret: BASE64
A key name identifier.
Access control list rule definition.
acl:
- id: STR
address: ADDR[/INT] ...
key: key_id ...
action: transfer | notify | update | control ...
deny: BOOL
An ACL rule identifier.
An ordered list of IP addresses or network subnets. The query must match one of them. Empty value means that address match is not required.
Default: empty
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: empty
An ordered list of allowed actions.
Possible values:
Default: empty
Configuration of the server remote control.
Caution: The control protocol is not encrypted and is susceptible to replay attacks in a short timeframe until message digest expires. For that reason, it is recommended to use default UNIX socket.
control:
listen: ADDR[@INT]
acl: acl_id ...
A UNIX socket path or IP address where the server listens for remote control commands. Optional port specification (default is 5533) can be appended to the address using @ separator.
Default: rundir/knot.sock
An ordered list of references to ACL rules allowing the remote control.
Caution: This option has no effect with UNIX socket.
Default: empty
Definition of remote servers for zone transfers or notifications.
remote:
- 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: empty
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: empty
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't have another template specified.
template:
- id: STR
global-module: STR/STR ...
# All zone options (excluding 'template' item)
A template identifier.
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 available only for the default template.
Default: empty
Definition of zones served by the server.
zone:
- 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. If not specified and the default template exists, the default template is used.
Default: empty
A path to the zone file. Non absolute path is relative to storage. It is also possible to use the following formatters:
Default: storage/%s.zone
A data directory for storing zone files, journal files and timers database.
Default: ${localstatedir}/lib/knot (configured with --with-storage=path)
A reference to zone primary master server. If not specified, the first master server is used.
Default: empty
An ordered list of references to remotes to which notify message is sent if the zone changes.
Default: empty
An ordered list of references to ACL rules which can allow or disallow zone transfers, updates or incoming notifies.
Default: empty
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:
Extra checks:
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
If enabled, automatic DNSSEC signing for the zone is turned on.
Caution: Cannot be enabled on a slave zone.
Default: off
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: empty
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:
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 'lower' 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: empty
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:
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.
log:
- 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:
Minimum severity level for messages related to general operation of the server that are logged.
Default: empty
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.
mod-dnstap:
- id: STR
sink: STR
A module identifier.
A sink path, which can be either a file or a UNIX socket when prefixed with unix:.
Default: empty
This module is able to synthesize either forward or reverse records for the given prefix and subnet.
mod-synth-record:
- id: STR
type: forward | reverse
prefix: STR
origin: DNAME
ttl: INT
network: ADDR[/INT]
A module identifier.
The type of generated records.
Possible values:
Default: empty
A record owner prefix.
Caution: prefix doesn’t allow dots, address parts in the synthetic names are separated with a dash.
Default: empty
The module catches all unsatisfied queries and forwards them to the indicated server for resolution.
mod-dnsproxy:
- id: STR
remote: remote_id
catch-nxdomain: BOOL
A module identifier.
If enabled, all unsatisfied queries (also applies to local zone lookups) are forwarded.
Default: off