Configuration Reference¶
Description¶
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:
- INT – Integer
- STR – Textual string
- HEXSTR – Hexadecimal string (with
0xprefix) - BOOL – Boolean value (
on/offortrue/false) - TIME – Number of seconds, an integer with possible time multiplier suffix (
s~ 1,m~ 60,h~ 3600 ord~ 24 * 3600) - SIZE – Number of bytes, an integer with possible size multiplier suffix (
B~ 1,K~ 1024,M~ 1024^2 orG~ 1024^3) - BASE64 – Base64 encoded string
- ADDR – IPv4 or IPv6 address
- DNAME – Domain name
- … – Multi-valued item, order of the values is preserved
- [ ] – Optional value
- | – Choice
There are 12 main sections (module, server, control, log, statistics, keystore, policy, key, acl, remote, template, and zone) and module sections with the mod- prefix. Most of the sections (excluding server, control, and statistics) 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 " ".
Comments¶
A comment begins with a # character and is ignored during processing. Also each configuration section or sequence block allows a permanent comment using the comment item which is stored in the server beside the configuration.
Includes¶
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
Module section¶
Dynamic modules loading configuration.
Note
If configured with non-empty `--with-moduledir=path` parameter, all shared modules in this directory will be automatically loaded.
module:
- id: STR
file: STR
id¶
A module identifier in the form of the mod- prefix and module name suffix.
file¶
A path to a shared library file with the module implementation.
Warning
If the path is not absolute, the library is searched in the set of system directories. See man dlopen for more details.
Default: ${libdir}/knot/modules-${version}/module_name.so (or ${path}/module_name.so if configured with --with-moduledir=path)
Server section¶
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-handshake-timeout: TIME
tcp-idle-timeout: TIME
tcp-reply-timeout: TIME
max-tcp-clients: INT
max-udp-payload: SIZE
max-ipv4-udp-payload: SIZE
max-ipv6-udp-payload: SIZE
edns-client-subnet: BOOL
answer-rotation: BOOL
listen: ADDR[@INT] ...
identity¶
An identity of the server returned in the response to the query for TXT record id.server. or hostname.bind. in the CHAOS class (RFC 4892). Set empty value to disable.
Default: FQDN hostname
version¶
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 (RFC 4892). Set empty value to disable.
Default: server version
rundir¶
A path for storing run-time data (PID file, unix sockets, etc.).
Default: ${localstatedir}/run/knot (configured with --with-rundir=path)
user¶
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
udp-workers¶
A number of UDP workers (threads) used to process incoming queries over UDP.
Default: auto-estimated optimal value based on the number of online CPUs
tcp-workers¶
A number of TCP workers (threads) used to process incoming queries over TCP.
Default: auto-estimated optimal value based on the number of online CPUs
background-workers¶
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
async-start¶
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
tcp-handshake-timeout¶
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
tcp-idle-timeout¶
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
tcp-reply-timeout¶
Maximum time to wait for an outgoing connection or for a reply to an issued request (SOA, NOTIFY, AXFR…).
Default: 10
max-tcp-clients¶
A maximum number of TCP clients connected in parallel, set this below the file descriptor limit to avoid resource exhaustion.
Default: 100
edns-client-subnet¶
Enable or disable EDNS Client Subnet support. If enabled, responses to queries containing the EDNS Client Subnet option always contain a valid EDNS Client Subnet option according to RFC 7871.
Default: off
answer-rotation¶
Enable or disable sorted-rrset rotation in the answer section of normal replies. The rotation shift is simply determined by a query ID.
Default: off
listen¶
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. Non-local address binding is automatically enabled if supported by the operating system.
Default: not set
Key section¶
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
id¶
A key name identifier.
Note
This value MUST be exactly the same as the name of the TSIG key on the opposite master/slave server(s).
algorithm¶
A TSIG key algorithm. See TSIG Algorithm Numbers.
Possible values:
hmac-md5hmac-sha1hmac-sha224hmac-sha256hmac-sha384hmac-sha512
Default: not set
ACL section¶
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.).
acl:
- id: STR
address: ADDR[/INT] | ADDR-ADDR ...
key: key_id ...
action: notify | transfer | update ...
deny: BOOL
update-type: STR ...
update-owner: key | zone | name
update-owner-match: sub-or-equal | equal | sub
update-owner-name: STR ...
id¶
An ACL rule identifier.
address¶
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
key¶
An ordered list of references to TSIG keys. The query must match one of them. Empty value means that transaction authentication is not used.
Default: not set
action¶
An ordered list of allowed (or denied) actions.
Possible values:
notify– Allow incoming notify.transfer– Allow zone transfer.update– Allow zone updates.
Default: not set
deny¶
If enabled, instead of allowing, deny the specified action, address, key, or combination if these items. If no action is specified, deny all actions.
Default: off
update-type¶
A list of allowed types of Resource Records in a zone update. Every record in an update must match one of the specified types.
Default: not set
update-owner¶
This option restricts possible owners of Resource Records in a zone update by comparing them to either the TSIG key identity, the current zone name, or to a list of domain names given by the update-owner-name option. The comparison method is given by the update-owner-match option.
Possible values:
key— The owner of each updated RR must match the identity of the TSIG key if used.name— The owner of each updated RR must match at least one name in the update-owner-name list.zone— The owner of each updated RR must match the current zone name.
Default: not set
update-owner-match¶
This option defines how the owners of Resource Records in an update are matched to the domain name(s) set by the update-owner option.
Possible values:
sub-or-equal— The owner of each Resource Record in an update must either be equal to or be a subdomain of at least one domain set by update-owner.equal— The owner of each updated RR must be equal to at least one domain set by update-owner.sub— The owner of each updated RR must be a subdomain of, but MUST NOT be equal to at least one domain set by update-owner.
Default: sub-or-equal
update-owner-name¶
A list of allowed owners of RRs in a zone update used with update-owner set to name.
Default: not set
Statistics section¶
Periodic server statistics dumping.
statistics:
timer: TIME
file: STR
append: BOOL
Keystore section¶
DNSSEC keystore configuration.
keystore:
- id: STR
backend: pem | pkcs11
config: STR
id¶
A keystore identifier.
backend¶
A key storage backend type.
Possible values:
pem– PEM files.pkcs11– PKCS #11 storage.
Default: pem
config¶
A backend specific configuration. A directory with PEM files (the path can be specified as a relative path to kasp-db) or a configuration string for PKCS #11 storage (<pkcs11-url> <module-path>).
Note
Example configuration string for PKCS #11:
"pkcs11:token=knot;pin-value=1234 /usr/lib64/pkcs11/libsofthsm2.so"
Default: kasp-db/keys
Submission section¶
Parameters of KSK submission checks.
submission:
- id: STR
parent: remote_id ...
check-interval: TIME
timeout: TIME
id¶
A submission identifier.
parent¶
A list of references to parent’s DNS servers to be checked for presence of corresponding DS records in the case of KSK submission. All of them must have a corresponding DS for the rollover to continue. If none is specified, the rollover must be pushed forward manually.
Default: not set
Tip
A DNSSEC-validating resolver can be set as a parent.
check-interval¶
Interval for periodic checks of DS presence on parent’s DNS servers, in the case of the KSK submission.
Default: 1 hour
timeout¶
After this period, the KSK submission is automatically considered successful, even if all the checks were negative or no parents are configured. Set 0 for infinity.
Default: 0
Policy section¶
DNSSEC policy configuration.
policy:
- id: STR
keystore: STR
manual: BOOL
single-type-signing: BOOL
algorithm: rsasha1 | rsasha1-nsec3-sha1 | rsasha256 | rsasha512 | ecdsap256sha256 | ecdsap384sha384 | ed25519
ksk-size: SIZE
zsk-size: SIZE
ksk-shared: BOOL
dnskey-ttl: TIME
zone-max-ttl: TIME
zsk-lifetime: TIME
ksk-lifetime: TIME
propagation-delay: TIME
rrsig-lifetime: TIME
rrsig-refresh: TIME
nsec3: BOOL
nsec3-iterations: INT
nsec3-opt-out: BOOL
nsec3-salt-length: INT
nsec3-salt-lifetime: TIME
signing-threads: INT
ksk-submission: submission_id
ds-push: remote_id
cds-cdnskey-publish: none | delete-dnssec | rollover | always | double-ds
offline-ksk: BOOL
id¶
A policy identifier.
keystore¶
A reference to a keystore holding private key material for zones. A special default value can be used for the default keystore settings.
Default: default
single-type-signing¶
If enabled, Single-Type Signing Scheme is used in the automatic key management mode.
Default: off
algorithm¶
An algorithm of signing keys and issued signatures. See DNSSEC Algorithm Numbers.
Possible values:
rsasha1rsasha1-nsec3-sha1rsasha256rsasha512ecdsap256sha256ecdsap384sha384ed25519
Note
Ed25519 algorithm is only available when compiled with GnuTLS 3.6.0+.
Default: ecdsap256sha256
ksk-size¶
A length of newly generated KSK or CSK keys.
Default: 2048 (rsa*), 256 (ecdsap256), 384 (ecdsap384), 256 (ed25519)
dnskey-ttl¶
A TTL value for DNSKEY records added into zone apex.
Note
Has infuence over ZSK key lifetime.
Warning
Ensure all DNSKEYs with updated TTL are propagated before any subsequent DNSKEY rollover starts.
Default: zone SOA TTL
zone-max-ttl¶
Declare (override) maximal TTL value among all the records in zone.
Note
It’s generally recommended to override the maximal TTL computation by setting this explicitly whenever possible. It’s required for DNSSEC Offline KSK and really reasonable when records are generated dynamically (e.g. by a module).
Default: computed after zone is loaded
zsk-lifetime¶
A period between ZSK activation and the next rollover initiation.
Note
More exactly, this period is measured since a ZSK is activated, and after this, a new ZSK is generated to replace it within following roll-over.
ZSK key lifetime is also infuenced by propagation-delay and dnskey-ttl
Zero (aka infinity) value causes no ZSK rollover as a result.
Default: 30 days
ksk-lifetime¶
A period between KSK activation and the next rollover initiation.
Note
KSK key lifetime is also infuenced by propagation-delay, dnskey-ttl, and KSK submission delay.
Zero (aka infinity) value causes no KSK rollover as a result.
This applies for CSK lifetime if single-type-signing is enabled.
Default: 0
propagation-delay¶
An extra delay added for each key rollover step. This value should be high enough to cover propagation of data from the master server to all slaves.
Note
Has infuence over ZSK key lifetime.
Default: 1 hour
rrsig-lifetime¶
A validity period of newly issued signatures.
Note
The RRSIG’s signature inception time is set to 90 minutes in the past. This time period is not counted to the signature lifetime.
Default: 14 days
rrsig-refresh¶
A period how long before a signature expiration the signature will be refreshed.
Default: 7 days
nsec3-opt-out¶
If set, NSEC3 records won’t be created for insecure delegations. This speeds up the zone signing and reduces overall zone size.
Warning
NSEC3 with the Opt-Out bit set no longer works as a proof of non-existence in this zone.
Default: off
nsec3-salt-length¶
A length of a salt field in octets, which is appended to the original owner name before hashing.
Default: 8
nsec3-salt-lifetime¶
A validity period of newly issued salt field.
Zero value means infinity.
Default: 30 days
ksk-submission¶
A reference to submission section holding parameters of KSK submission checks.
Default: not set
ds-push¶
An optional reference to authoritative DNS server of the parent’s zone. The remote server must be configured to accept DS record updates via DDNS. Whenever a CDS record in the local zone is changed, the corresponding DS record is sent as a dynamic update (DDNS) to the parent DNS server. All previous DS records are deleted within the DDNS message. It’s possible to manage both child and parent zones by the same Knot DNS server.
Note
This feature requires cds-cdnskey-publish not to be set to none.
Note
Module Onlinesign doesn’t support DS push.
Default: not set
signing-threads¶
When signing zone or update, use this number of threads for parallel signing.
Those are extra threads independent of Background workers.
Note
Some steps of the DNSSEC signing operation are not parallelized.
Default: 1 (no extra threads)
cds-cdnskey-publish¶
Controls if and how shall the CDS and CDNSKEY be published in the zone.
Possible values:
none– Never publish any CDS or CDNSKEY records in the zone.delete-dnssec– Publish special CDS and CDNSKEY records indicating turning off DNSSEC.rollover– Publish CDS and CDNSKEY records only in the submission phase of KSK rollover.always– Always publish one CDS and one CDNSKEY records for the current KSK.double-ds– Always publish up to two CDS and two CDNSKEY records for ready and/or active KSKs.
Note
If the zone keys are managed manually, the CDS and CDNSKEY rrsets may contain more records depending on the keys available.
Default: rollover
Remote section¶
Definitions of remote servers for outgoing connections (source of a zone transfer, target for a notification, etc.).
remote:
- id: STR
address: ADDR[@INT] ...
via: ADDR[@INT] ...
key: key_id
id¶
A remote identifier.
address¶
An ordered list of destination IP addresses which are used for communication with the remote server. The addresses are tried in sequence until the remote is reached. Optional destination port (default is 53) can be appended to the address using @ separator.
Default: not set
Note
If the remote is contacted and it refuses to perform requested action, no more addresses will be tried for this remote.
via¶
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
Template section¶
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
timer-db: STR
max-timer-db-size: SIZE
journal-db: STR
journal-db-mode: robust | asynchronous
max-journal-db-size: SIZE
kasp-db: STR
max-kasp-db-size: SIZE
global-module: STR/STR ...
# All zone options (excluding 'template' item)
id¶
A template identifier.
timer-db¶
Specifies a path of the persistent timer database. The path can be specified as a relative path to the default template storage.
Note
This option is only available in the default template.
Default: storage/timers
max-timer-db-size¶
Hard limit for the timer database maximum size.
Note
This option is only available in the default template.
Default: 100 MiB
journal-db¶
Specifies a path of the persistent journal database. The path can be specified as a relative path to the default template storage.
Note
This option is only available in the default template.
Default: storage/journal
journal-db-mode¶
Specifies journal LMDB backend configuration, which influences performance and durability.
Possible values:
robust– The journal DB disk sychronization ensures DB durability but is generally slower.asynchronous– The journal DB disk synchronization is optimized for better performance at the expense of lower DB durability; this mode is recommended only on slave nodes with many zones.
Note
This option is only available in the default template.
Default: robust
max-journal-db-size¶
Hard limit for the common journal DB. There is no cleanup logic in journal to recover from reaching this limit: journal simply starts refusing changes across all zones. Decreasing this value has no effect if lower than actual DB file size.
It is recommended to limit max-journal-usage per-zone instead of max-journal-db-size in most cases. Please keep this value larger than the sum of all zones’ journal usage limits. See more details regarding journal behaviour.
This value also influences server’s usage of virtual memory.
Note
This option is only available in the default template.
Default: 20 GiB (1 GiB for 32-bit)
kasp-db¶
A KASP database path. Non-absolute path is relative to storage.
Note
This option is only available in the default template.
Default: storage/keys
max-kasp-db-size¶
Hard limit for the KASP database maximum size.
Note
This option is only available in the default template.
Default: 500 MiB
global-module¶
An ordered list of references to query modules in the form of module_name or module_name/module_id. These modules apply to all queries.
Note
This option is only available in the default template.
Default: not set
Zone section¶
Definition of zones served by the server.
zone:
- domain: DNAME
template: template_id
storage: STR
file: STR
master: remote_id ...
ddns-master: remote_id
notify: remote_id ...
acl: acl_id ...
semantic-checks: BOOL
disable-any: BOOL
zonefile-sync: TIME
zonefile-load: none | difference | difference-no-serial | whole
journal-content: none | changes | all
max-journal-usage: SIZE
max-journal-depth: INT
max-zone-size : SIZE
dnssec-signing: BOOL
dnssec-policy: STR
request-edns-option: INT:[HEXSTR]
serial-policy: increment | unixtime | dateserial
min-refresh-interval: TIME
max-refresh-interval: TIME
module: STR/STR ...
domain¶
A zone name identifier.
template¶
A reference to a configuration template.
Default: not set or default (if the template exists)
storage¶
A data directory for storing zone files, journal database, and timers database.
Default: ${localstatedir}/lib/knot (configured with --with-storage=path)
file¶
A path to the zone file. Non-absolute path is relative to storage. It is also possible to use the following formatters:
%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. All dots (including the terminal one) are considered. If the character is not available, the formatter has no effect.%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.%s– Means the current zone name in the textual representation. The zone name doesn’t include the terminating dot (the result for the root zone is the empty string!).%%– Means the%character.
Warning
Beware of special characters which are escaped or encoded in the \DDD form where DDD is corresponding decimal ASCII code.
Default: storage/%s.zone
ddns-master¶
A reference to zone primary master server. If not specified, the first master server is used.
Default: not set
notify¶
An ordered list of references to remotes to which notify message is sent if the zone changes.
Default: not set
acl¶
An ordered list of references to ACL rules which can allow or disallow zone transfers, updates or incoming notifies.
Default: not set
semantic-checks¶
If enabled, extra zone 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:
- SOA record missing in the zone (RFC 1034)
- An extra record together with CNAME record except for RRSIG and DS (RFC 1034)
- Multiple CNAME record with the same owner
- DNAME record having a record under it (RFC 2672)
Extra checks:
- Missing NS record at the zone apex
- Missing glue A or AAAA record
- Invalid DNSKEY, DS, or NSEC3PARAM record
- CDS or CDNSKEY inconsistency
- Missing, invalid, or unverifiable RRSIG record
- Invalid NSEC(3) record
- Broken or non-cyclic NSEC(3) chain
Default: off
disable-any¶
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
zonefile-sync¶
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 completely disable automatic zone file synchronization, set the value to -1. In that case, it is still possible to force a manual zone flush using the -f option.
Note
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)
zonefile-load¶
Selects how the zone file contents are applied during zone load.
Possible values:
none– The zone file is not used at all.difference– If the zone contents are already available during server start or reload, the difference is computed between them and the contents of the zone file. This difference is then checked for semantic errors and applied to the current zone contents.difference-no-serial– Same asdifference, but the SOA serial in the zone file is ignored, the server takes care of incrementing the serial automatically.whole– Zone contents are loaded from the zone file.
When difference is configured and there are no zone contents yet (cold start of Knot and no zone contents in journal), it behaves the same way like whole.
Default: whole
journal-content¶
Selects how the journal shall be used to store zone and its changes.
Possible values:
none– The journal is not used at all.changes– Zone changes history is stored in journal.all– Zone contents and history is stored in journal.
Default: changes
max-journal-usage¶
Policy how much space in journal DB will the zone’s journal occupy.
Note
Journal DB may grow far above the sum of max-journal-usage across all zones, because of DB free space fragmentation.
Default: 100 MiB
max-zone-size¶
Maximum size of the zone. The size is measured as size of the zone records in wire format without compression. The limit is enforced for incoming zone transfers and dynamic updates.
For incremental transfers (IXFR), the effective limit for the total size of the records in the transfer is twice the configured value. However the final size of the zone must satisfy the configured value.
Default: 2^64
dnssec-policy¶
A reference to DNSSEC signing policy. A special default value can be used for the default policy settings.
Required
request-edns-option¶
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
serial-policy¶
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:
increment– The serial is incremented according to serial number arithmetic.unixtime– The serial is set to the current unix time.dateserial– The 10-digit serial (YYYYMMDDnn) is incremented, the first 8 digits match the current iso-date.
Note
In case of unixtime, if the resulting serial is lower or equal than current zone (this happens e.g. in case of migrating from other policy or frequent updates) the serial is incremented instead.
Use dateserial only if you expect less than 100 updates per day per zone.
Default: increment
module¶
An ordered list of references to query modules in the form of module_name or module_name/module_id. These modules apply only to the current zone queries.
Default: not set
Logging section¶
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:
critical– Non-recoverable error resulting in server shutdown.error– Recoverable error, action should be taken.warning– Warning that might require user action.notice– Server notice or hint.info– Informational message.debug– Debug or detailed message.
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
control: critical | error | warning | notice | info | debug
zone: critical | error | warning | notice | info | debug
any: critical | error | warning | notice | info | debug
target¶
A logging output.
Possible values:
stdout– Standard output.stderr– Standard error output.syslog– Syslog or systemd journal.- file_name – A specific file.
With syslog target, syslog service is used. However, if Knot DNS has been compiled with systemd support and operating system has been booted with systemd, systemd journal is used for logging instead of syslog.
server¶
Minimum severity level for messages related to general operation of the server to be logged.
Default: not set
control¶
Minimum severity level for messages related to server control to be logged.
Default: not set
