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. Each template option can be explicitly overridden in zone-specific configurations. A 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

  - 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

  - domain: example1.eu
    template: slave
    master: master1

  - domain: example2.eu
    template: slave
    master: master2

Access control list (ACL)

An ACL list specifies which remotes are allowed to send the server a specific request. A remote can be a single IP address or a network subnet. Also a TSIG key can be assigned:

acl:
  - id: address_rule
    address: [2001:db8::1, 192.168.2.0/24] # Allowed IP address list
    action: [transfer, update]  # Allow zone transfers and updates

  - id: deny_rule             # Negative match rule
    address: 192.168.2.100
    action: transfer
    deny: on                  # The request is denied

  - id: key_rule
    key: key1                 # Access based just on TSIG key
    action: transfer

These rules can then be referenced from a zone acl:

zone:
  - domain: example.com
    acl: [address_rule, deny_rule, key_rule]

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

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

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

Response rate limiting

Response rate limiting (RRL) is a method to combat DNS reflection amplification attacks. These attacks rely on the fact that source address of a UDP query can be forged, and without a worldwide deployment of BCP38, such a forgery cannot be prevented. An attacker can use a DNS server (or multiple servers) as an amplification source and can flood a victim with a large number of unsolicited DNS responses.

The RRL lowers the amplification factor of these attacks by sending some of the responses as truncated or by dropping them altogether.

You can enable RRL by setting the rate-limit option in the server section. The option controls how many responses per second are permitted for each flow. Responses exceeding this rate are limited. The option rate-limit-slip then configures how many limited responses are sent as truncated (slip) instead of being dropped.

server:
    rate-limit: 200     # Allow 200 resp/s for each flow
    rate-limit-slip: 2  # Every other response slips

Automatic DNSSEC signing

Knot DNS supports automatic DNSSEC signing for static zones. The signing can operate in two modes:

  1. 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 by the zone operator.
  2. Automatic key management. In this mode, the server also 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.

The DNSSEC signing is controlled by the dnssec-signing and kasp-db configuration options. The first option states if the signing is enabled for a particular zone, the second option points to a KASP database holding the signing configuration.

Example configuration

The example configuration enables automatic signing for all zones in the default template, but the signing is explicitly disabled for zone example.dev. The KASP database is common for all zones:

template:
  - id: default
    dnssec-signing: on
    kasp-db: /var/lib/knot/kasp

zone:
  - domain: example.com
    file: example.com.zone

  - domain: example.dev
    file: example.dev.zone
    dnssec-signing: off

DNSSEC KASP database

The configuration for DNSSEC is stored in the KASP database. The database is simply a directory in the file-system containing files in the JSON format. The database contains

  • definitions of signing policies;
  • private key stores configuration; and
  • zones configuration and signing metadata.

The keymgr utility serves for the database maintenance. To initialize the database, run:

$ mkdir -p /var/lib/knot/kasp
$ cd /var/lib/knot/kasp
$ keymgr init

The init command initializes the database, defines a default signing policy named default with default signing parameters, and defines a default key store named default with file-backed key store within the KASP database directory.

Attention

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. The database also contains private key material – don't set the permissions too loose.

Automatic key management

For automatic key management, a signing policy has to be defined in the first place. This policy specifies how a zone is signed (i.e. signing algorithm, key size, signature lifetime, key lifetime, etc.).

To create a new policy named rsa using RSA-SHA-256 algorithm for signing keys, 1024-bit long ZSK, and 2048-bit long KSK, run:

$ keymgr policy add rsa algorithm RSASHA256 zsk-size 1024 ksk-size 2048

The unspecified policy parameters are set to defaults. The complete definition of the policy will be printed after executing the command.

Next, create a zone entry for zone myzone.test and assign it the newly created policy:

$ keymgr zone add myzone.test policy rsa

Make sure everything is set correctly:

$ keymgr policy show rsa
$ keymgr zone show myzone.test

Add the zone into the server configuration and enable DNSSEC for that zone. The configuration fragment might look similar to:

template:
  - id: default
    storage: /var/lib/knot
    kasp-db: kasp

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

Finally, reload the server:

$ knotc reload

The server will generate initial signing keys and sign the zone properly. Check the server logs to see whether everything went well.

Attention

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 zone key import command before reloading the server. Also the algorithm in the policy must match the algorithm of all imported keys.

Manual key management

For automatic DNSSEC signing with manual key management, a signing policy with manual key management flag has to be set.

Define a signing policy named man with disabled automatic key management:

$ keymgr policy add man manual true

Create a zone entry for the zone myzone.test with the created policy:

$ keymgr zone add myzone.test policy man

Generate signing keys for the zone. Let's use the Single-Type Signing scheme with two algorithms, which is a scheme currently not supported by the automatic key management. Run:

$ keymgr zone key generate myzone.test algorithm RSASHA256 size 1024
$ keymgr zone key generate myzone.test algorithm ECDSAP256SHA256 size 256

Enable automatic DNSSEC signing for the zone in the server configuration and reload the server. Use the same steps as in Automatic key management.

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 zone key generate myzone.test algorithm RSASHA256 size 1024 activate +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 zone key set myzone.test <old_key_id> retire +1d remove +1d

Reload the server again. The new key gets published. Do not forget to update the DS record in the parent zone to include the 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.

Signing policy

The signing policy used in the KASP database defines parameters, how the zone signatures and keys should be handled. At the moment, the policy comprises of the following parameters:

Signing algorithm
An algorithm of signing keys and issued signatures. The default value is ECDSA-P256-SHA256.
KSK size
Desired length of the newly generated ZSK keys. The default value is 256 bits (the only feasible value for the default signing algorithm).
ZSK size
Desired length of the newly generated ZSK keys. The default value is 256 bits.
DNSKEY TTL
TTL value for DNSKEY records added into zone apex. This parameter is temporarily overridden by the TTL value of the zone SOA record and thus has no default value.
ZSK lifetime
Period between ZSK publication and the next rollover initiation. The default value is 30 days.
RRSIG lifetime
Validity period of newly issued signatures. The default value is 14 days.
RRSIG refresh
Specifies how long before a signature expiration the signature will be refreshed. The default value is 7 days.
NSEC3
Specifies if NSEC3 will be used instead of NSEC. This value is temporarily ignored. The setting is derived from the NSEC3PARAM record presence in the zone. The default value has not been decided yet.
SOA minimum TTL
Specifies the SOA Minimum TTL field value. This option is required for correct key rollovers. The value has no real meaning with Knot DNS because the server will use a real value from the zone.
Zone maximum TTL
Maximum TTL value present in the zone. This option is required for correct key rollovers. Knot DNS will determine the value automatically in the future.
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. The default value is 1 hour.
Key store name
A name of a key store holding private key material for zones which use the policy. The default value is default.
Manual key management
An option to disable key management for all zones which use the policy. The option is disabled by default.

Zone signing

The signing process consists of the following steps:

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

The signing is initiated on the following occasions:

  • Start of the server
  • Zone reload
  • Reaching the signature refresh period
  • Received DDNS update
  • Forced zone resign issued with knotc signzone

On a forced zone resign, all signatures in the zone are dropped and recreated.

The knotc zonestatus command can be used to see when the next scheduled DNSSEC resign will happen.

Limitations

The current DNSSEC implementation in Knot DNS has a bunch of limitations. Most of the limitations will be hopefully removed in the near future.

  • Automatic key management:
    • Only one DNSSEC algorithm can be used per zone.
    • Single-Type Signing scheme is not supported.
    • ZSK rollover always uses key pre-publish method (actually a feature).
    • KSK rollover is not implemented.
  • Manual key management:
    • Default values for signature lifetime are forced.
  • NSEC3:
    • Use of NSEC3 is determined by the presence of NSEC3PARAM in the zone.
    • Automatic re-salt is not implemented.
  • KASP policy:
    • DNSKEY TTL value is overridden by the SOA TTL.
    • NSEC3 related parameters are ignored.
    • Zone maximum TTL is not determined automatically.
  • Signing:
    • Signature expiration jitter is not implemented.
    • Signature expiration skew is not implemented.
  • Utilities:
    • Legacy key import requires a private key.
    • Legacy key export is not implemented.
    • DS record export is not implemented.

Query modules

Knot DNS supports configurable query modules that can alter the way queries are processed. The concept is quite simple – 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.

Each module is configured in the corresponding module section and is identified for the subsequent usage. Then the identifier is referenced in the form of module_name/module_id through a zone/template module option or through the default template global-module option if it is used for all queries.

dnstap – dnstap-enabled query logging

A module for query and response logging based on dnstap library. You can capture either all or zone-specific queries and responses; usually you want to do the former. The configuration comprises only a sink path parameter, which can be either a file or a UNIX socket:

mod-dnstap:
  - id: capture_all
    sink: /tmp/capture.tap

template:
  - id: default
    global-module: mod-dnstap/capture_all

Note: to be able to use a Unix socket you need an external program to create it. Knot DNS connects to it as a client using the libfstrm library. It operates exactly like syslog. See here for more details.

Note: dnstap log files can also be created or read using kdig.

synth-record – Automatic forward/reverse records

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

Records are synthesized only if the query can't be satisfied from the zone. Both IPv4 and IPv6 are supported.

Automatic forward records

Example:

mod-synth-record:
  - id: test1
    type: forward
    prefix: dynamic-
    ttl: 400
    network: 2620:0:b61::/52

zone:
  - domain: test.
    file: test.zone # Must exist
    module: mod-synth-record/test1

Result:

$ kdig AAAA dynamic-2620-0000-0b61-0100-0000-0000-0000-0001.test.
...
;; QUESTION SECTION:
;; dynamic-2620-0000-0b61-0100-0000-0000-0000-0001.test. IN AAAA

;; ANSWER SECTION:
dynamic-2620-0000-0b61-0100-0000-0000-0000-0001.test. 400 IN AAAA 2620:0:b61:100::1

You can also have CNAME aliases to the dynamic records, which are going to be further resolved:

$ kdig AAAA alias.test.
...
;; QUESTION SECTION:
;; alias.test. IN AAAA

;; ANSWER SECTION:
alias.test. 3600 IN CNAME dynamic-2620-0000-0b61-0100-0000-0000-0000-0002.test.
dynamic-2620-0000-0b61-0100-0000-0000-0000-0002.test. 400 IN AAAA 2620:0:b61:100::2

Automatic reverse records

Example:

mod-synth-record:
  - id: test2
    type: reverse
    prefix: dynamic-
    origin: test
    ttl: 400
    network: 2620:0:b61::/52

zone:
  - domain: 1.6.b.0.0.0.0.0.0.2.6.2.ip6.arpa.
    file: 1.6.b.0.0.0.0.0.0.2.6.2.ip6.arpa.zone # Must exist
    module: mod-synth-record/test2

Result:

$ kdig -x 2620:0:b61::1
...
;; QUESTION SECTION:
;; 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.6.b.0.0.0.0.0.0.2.6.2.ip6.arpa. IN PTR

;; ANSWER SECTION:
1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.6.b.0.0.0.0.0.0.2.6.2.ip6.arpa. 400 IN PTR
                               dynamic-2620-0000-0b61-0000-0000-0000-0000-0001.test.

dnsproxy – Tiny DNS proxy

The module catches all unsatisfied queries and forwards them to the indicated server for resolution, i.e. a tiny DNS proxy. There are several uses of this feature:

  • A substitute public-facing server in front of the real one
  • Local zones (poor man's "views"), rest is forwarded to the public-facing server
  • etc.

Note: The module does not alter the query/response as the resolver would, and the original transport protocol is kept as well.

The configuration is straightforward and just a single remote server is required:

remote:
  - id: hidden
    address: 10.0.1.1

mod-dnsproxy:
  - id: default
    remote: hidden

template:
  - id: default
    global-module: mod-dnsproxy/default

zone:
  - domain: local.zone

When clients query for anything in the local.zone, they will be responded to locally. The rest of the requests will be forwarded to the specified server (10.0.1.1 in this case).

rosedb – Static resource records

The module provides a mean to override responses for certain queries before the record is searched in the available zones. The module comes with the rosedb_tool tool used to manipulate the database of static records. Neither the tool nor the module are enabled by default, recompile with the --enable-rosedb configuration flag to enable them.

For example, let's suppose we have a database of following records:

myrecord.com.      3600 IN A 127.0.0.1
www.myrecord.com.  3600 IN A 127.0.0.2
ipv6.myrecord.com. 3600 IN AAAA ::1

And we query the nameserver with the following:

$ kdig IN A myrecord.com
  ... returns NOERROR, 127.0.0.1
$ kdig IN A www.myrecord.com
  ... returns NOERROR, 127.0.0.2
$ kdig IN A stuff.myrecord.com
  ... returns NOERROR, 127.0.0.1
$ kdig IN AAAA myrecord.com
  ... returns NOERROR, NODATA
$ kdig IN AAAA ipv6.myrecord.com
  ... returns NOERROR, ::1

Note: An entry in the database matches anything at the same or a lower domain level, i.e. 'myrecord.com' matches 'a.a.myrecord.com' as well. This can be utilized to create catch-all entries.

You can also add authority information for the entries, provided you create SOA + NS records for a name, like so:

myrecord.com.     3600 IN SOA master host 1 3600 60 3600 3600
myrecord.com.     3600 IN NS ns1.myrecord.com.
myrecord.com.     3600 IN NS ns2.myrecord.com.
ns1.myrecord.com. 3600 IN A 127.0.0.1
ns2.myrecord.com. 3600 IN A 127.0.0.2

In this case, the responses will:

  1. Be authoritative (AA flag set)
  2. Provide an authority section (SOA + NS)
  3. Be NXDOMAIN if the name is found (i.e. the 'IN AAAA myrecord.com' from the example), but not the RR type (this is to allow the synthesis of negative responses)

Note: The SOA record applies only to the 'myrecord.com.', not to any other record (not even those of its subdomains). From this point of view, all records in the database are unrelated and not hierarchical. The idea is to provide subtree isolation for each entry.

In addition, the module is able to log matching queries via remote syslog if you specify a syslog address endpoint and an optional string code.

Here is an example on how to use the module:

  • Create the entries in the database:

    $ mkdir /tmp/static_rrdb
$ # No logging
$ rosedb_tool /tmp/static_rrdb add myrecord.com. A 3600 "127.0.0.1" "-" "-"
$ # Logging as 'www_query' to Syslog at 10.0.0.1
$ rosedb_tool /tmp/static_rrdb add www.myrecord.com. A 3600 "127.0.0.1" \
                                                 "www_query" "10.0.0.1"
$ # Logging as 'ipv6_query' to Syslog at 10.0.0.1
$ rosedb_tool /tmp/static_rrdb add ipv6.myrecord.com. AAAA 3600 "::1" \
                                              "ipv6_query" "10.0.0.1"
$ # Verify settings
$ rosedb_tool /tmp/static_rrdb list
www.myrecord.com.       A RDATA=10B     www_query       10.0.0.1
ipv6.myrecord.com.      AAAA RDATA=22B  ipv6_query      10.0.0.1
myrecord.com.           A RDATA=10B     -               -

    Note: The database may be modified later on while the server is running.

  • Configure the query module:

    mod-rosedb:
  - id: default
    dbdir: /tmp/static_rrdb

template:
  - id: default
    global-module: mod-rosedb/default

    Note: The module accepts just one parameter – the path to the directory where the database will be stored.

  • Start the server:

    $ knotd -c knot.conf

  • Verify the running instance:

    $ kdig @127.0.0.1#6667 A myrecord.com

online-sign — Online DNSSEC signing

The module provides online DNSSEC signing. Instead of pre-computing the zone signatures when the zone is loaded into the server or instead of loading an externally signed zone, the signatures are computed on-the-fly during answering.

The main purpose of the module is to enable authenticated responses with zones which use other dynamic module (e.g., automatic reverse record synthesis) because these zones cannot be pre-signed. However, it can be also used as a simple signing solution for zones with low traffic and also as a protection against zone content enumeration (zone walking).

In order to minimize the number of computed signatures per query, the module produces a bit different responses from the responses that would be sent if the zone was pre-signed. Still, the responses should be perfectly valid for a DNSSEC validating resolver.

Differences from statically signed zones:

  • The NSEC records are constructed as Minimally Covering NSEC Records (see Appendix A in RFC 7129). Therefore the generated domain names cover the complete domain name space in the zone's authority.
  • NXDOMAIN responses are promoted to NODATA responses. The module proves that the query type does not exist rather than that the domain name does not exist.
  • Domain names matching a wildcard are expanded. The module pretends and proves that the domain name exists rather than proving a presence of the wildcard.

Records synthesized by the module:

  • DNSKEY record is synthesized in the zone apex and includes public key material for the active signing key.
  • NSEC records are synthesized as needed.
  • RRSIG records are synthesized for authoritative content of the zone.

How to use the online signing module:

  • First add the zone into the server's KASP database and generate a key to be used for signing:

    $ cd /path/to/kasp
$ keymgr zone add example.com
$ keymgr zone key generate example.com algorithm ecdsap256sha256 size 256

  • Enable the module in server configuration and hook it to the zone:

    mod-online-sign:
  - id: default

zone:
  - domain: example.com
    module: mod-online-sign/default
    dnssec-signing: false

  • Make sure the zone is not signed and also that the automatic signing is disabled. All is set, you are good to go. Reload (or start) the server:

    $ knotc reload

The following example stacks the online signing with reverse record synthesis module:

mod-online-sign:
  - id: default

mod-synth-record:
  - id: lan-forward
    type: forward
    prefix: ip-
    ttl: 1200
    network: 192.168.100.0/24

template:
  - id: default
    dnssec-signing: false

zone:
  - domain: corp.example.net
    module: mod-synth-record/lan-forward
    module: mod-online-sign/default

Known issues:

  • The delegations are not signed correctly.
  • Some CNAME records are not signed correctly.

Limitations:

  • Only a Single-Type Signing scheme is supported.
  • Only one active signing key can be used.
  • Key rollover is not possible.
  • The NSEC records may differ for one domain name if queried for different types. This is an implementation shortcoming as the dynamic modules cooperate loosely. Possible synthesis of a type by other module cannot be predicted. This dissimilarity should not affect response validation, even with validators performing aggressive negative caching.
  • The NSEC proofs will work well with other dynamic modules only if the modules synthesize only A and AAAA records. If synthesis of other type is required, please, report this information to Knot DNS developers.