Modules¶
dnsproxy – Tiny DNS proxy¶
The module forwards all queries, or all specific zone queries if configured per zone, to the indicated server for resolution. If configured in the fallback mode, only localy unsatisfied queries are forwarded. 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.
Example¶
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
fallback: on
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).
Module reference¶
mod-dnsproxy:
- id: STR
remote: remote_id
timeout: INT
fallback: BOOL
catch-nxdomain: BOOL
id¶
A module identifier.
fallback¶
If enabled, localy unsatisfied queries leading to REFUSED (no zone) are forwarded. If disabled, all queries are directly forwarded without any local attempts to resolve them.
Default: on
catch-nxdomain¶
If enabled, localy unsatisfied queries leading to NXDOMAIN are forwarded. This option is only relevant in the fallback mode.
Default: off
dnstap – Dnstap traffic logging¶
A module for query and response logging based on the dnstap library. You can capture either all or zone-specific queries and responses; usually you want to do the former.
Example¶
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.
Module reference¶
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
identity: STR
version: STR
log-queries: BOOL
log-responses: BOOL
id¶
A module identifier.
sink¶
A sink path, which can be either a file or a UNIX socket when prefixed with unix:.
Required
Warning
File is overwritten on server startup or reload.
noudp — No UDP response¶
The module sends empty truncated response to any UDP query. TCP queries are not affected.
Example¶
To enable this module globally, you need to add something like the following to the configuration file:
template:
- id: default
global-module: mod-noudp
Note
This module is not configurable.
onlinesign — 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 (RFC 7129#appendix-A). 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.
Known issues:
- The delegations are not signed correctly.
- Some CNAME records are not signed correctly.
- The automatic policy-based key rotation does not work. The rotation events are invoked just at server (re)load.
Limitations:
- Online-sign module always enforces Single-Type Signing scheme.
- 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.
Example¶
Enable the module in the zone configuration with the default signing policy:
zone: - domain: example.com module: mod-onlinesignOr with an explicit signing policy:
policy: - id: rsa algorithm: RSASHA256 zsk-size: 2048 mod-onlinesign: - id: explicit policy: rsa zone: - domain: example.com module: mod-onlinesign/explicitOr use manual policy in an analogous manner, see Manual key management.
Note
Only id, manual, keystore, algorithm, ksk-size, and rrsig-lifetime policy items are relevant to this module. If no rrsig-lifetime is configured, the default value is 25 hours.
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-synthrecord:
- id: lan-forward
type: forward
prefix: ip-
ttl: 1200
network: 192.168.100.0/24
zone:
- domain: corp.example.net
module: [mod-synthrecord/lan-forward, mod-onlinesign]
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.
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
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:
- Be authoritative (AA flag set)
- Provide an authority section (SOA + NS)
- 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)
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.
Example¶
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
The module accepts just one parameter – the path to the directory where the database will be stored.
Start the server:
$ knotd -c knot.confVerify the running instance:
$ kdig @127.0.0.1#6667 A myrecord.com
rrl — 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.
Note
The module introduces two statistics counters. The number of slipped and dropped responses.
Example¶
You can enable RRL by setting the module globally or per zone.
mod-rrl:
- id: default
rate-limit: 200 # Allow 200 resp/s for each flow
slip: 2 # Every other response slips
template:
- id: default
global-module: mod-rrl/default # Enable RRL globally
Module reference¶
mod-rrl:
- id: STR
rate-limit: INT
slip: INT
table-size: INT
whitelist: ADDR[/INT] | ADDR-ADDR ...
id¶
A module identifier.
rate-limit¶
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 slip). Number of available tokens is recalculated each second.
Required
table-size¶
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
slip¶
As attacks using DNS/UDP are usually based on a forged source address, an attacker could deny services to the victim’s 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).
- 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.
- 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’t be lower than the incoming bandwidth. Also the outbound packet rate will be the same as without RRL.
- 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.
- 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
whitelist¶
A list of IP addresses, network subnets, or network ranges to exempt from rate limiting. Empty list means that no incoming connection will be white-listed.
Default: not set
stats — Query statistics¶
The module extends server statistics with incoming DNS request and corresponding response counters, such as used network protocol, total number of responded bytes, etc (see module reference for full list of supported counters). This module should be configured as the last module.
Note
Server initiated communication (outgoing NOTIFY, incoming *XFR,...) is not counted by this module.
Note
Leading 16-bit message size over TCP is not considered.
Example¶
Common statistics with default module configuration:
template:
- id: default
global-module: mod-stats
Per zone statistics with explicit module configuration:
mod-stats:
- id: custom
edns-presence: on
query-type: on
template:
- id: default
module: mod-stats/custom
Module reference¶
mod-stats:
- id: STR
request-protocol: BOOL
server-operation: BOOL
request-bytes: BOOL
response-bytes: BOOL
edns-presence: BOOL
flag-presence: BOOL
response-code: BOOL
request-edns-option: BOOL
response-edns-option: BOOL
reply-nodata: BOOL
query-type: BOOL
query-size: BOOL
reply-size: BOOL
id¶
A module identifier.
request-protocol¶
If enabled, all incoming requests are counted by the network protocol:
- udp4 - UDP over IPv4
- tcp4 - TCP over IPv4
- udp6 - UDP over IPv6
- tcp6 - TCP over IPv6
Default: on
server-operation¶
If enabled, all incoming requests are counted by the server operation. The server operation is based on message header OpCode and message query (meta) type:
- query - Normal query operation
- update - Dynamic update operation
- notify - NOTIFY request operation
- axfr - Full zone transfer operation
- ixfr - Incremental zone transfer operation
- invalid - Invalid server operation
Default: on
request-bytes¶
If enabled, all incoming request bytes are counted by the server operation:
- query - Normal query bytes
- update - Dynamic update bytes
- other - Other request bytes
Default: on
response-bytes¶
If enabled, outgoing response bytes are counted by the server operation:
- reply - Normal response bytes
- transfer - Zone transfer bytes
- other - Other response bytes
Warning
Dynamic update response bytes are not counted by this module.
Default: on
edns-presence¶
If enabled, EDNS pseudo section presence is counted by the message direction:
- request - EDNS present in request
- response - EDNS present in response
Default: off
flag-presence¶
If enabled, some message header flags are counted:
- TC - Truncated Answer in response
- DO - DNSSEC OK in request
Default: off
response-code¶
If enabled, outgoing response code is counted:
- NOERROR
- ...
- NOTZONE
- BADVERS
- ...
- BADCOOKIE
- other - All other codes
Note
In the case of multi-message zone transfer response, just one counter is incremented.
Warning
Dynamic update response code is not counted by this module.
Default: on
request-edns-option¶
If enabled, EDNS options in requests are counted by their code:
- CODE0
- ...
- EDNS-KEY-TAG (CODE14)
- other - All other codes
Default: off
response-edns-option¶
If enabled, EDNS options in responses are counted by their code. See request-edns-option.
Default: off
reply-nodata¶
If enabled, NODATA pseudo RCODE (RFC 2308#section-2.2) is counted by the query type:
- A
- AAAA
- other - All other types
Default: off
query-type¶
If enabled, normal query type is counted:
- A (TYPE1)
- ...
- TYPE65
- SPF (TYPE99)
- ...
- TYPE110
- ANY (TYPE255)
- ...
- TYPE260
- other - All other types
Note
Not all assigned meta types (IXFR, AXFR,...) have their own counters, because such types are not processed as normal query.
Default: off
query-size¶
If enabled, normal query message size distribution is counted by the size range in bytes:
- 0-15
- 16-31
- ...
- 272-287
- 288-65535
Default: off
reply-size¶
If enabled, normal reply message size distribution is counted by the size range in bytes:
- 0-15
- 16-31
- ...
- 4080-4095
- 4096-65535
Default: off
synthrecord – 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.
Example¶
Automatic forward records¶
mod-synthrecord:
- id: test1
type: forward
prefix: dynamic-
ttl: 400
network: 2620:0:b61::/52
zone:
- domain: test.
file: test.zone # Must exist
module: mod-synthrecord/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¶
mod-synthrecord:
- 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-synthrecord/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.
Module reference¶
mod-synthrecord:
- id: STR
type: forward | reverse
prefix: STR
origin: DNAME
ttl: INT
network: ADDR[/INT] | ADDR-ADDR ...
id¶
A module identifier.
type¶
The type of generated records.
Possible values:
forward– Forward recordsreverse– Reverse records
Required
prefix¶
A record owner prefix.
Note
The value doesn’t allow dots, address parts in the synthetic names are separated with a dash.
Default: empty
whoami — Whoami response¶
The module synthesizes an A or AAAA record containing the query source IP address, at the apex of the zone being served. It makes sure to allow Knot DNS to generate cacheable negative responses, and to allow fallback to extra records defined in the underlying zone file. The TTL of the synthesized record is copied from the TTL of the SOA record in the zone file.
Because a DNS query for type A or AAAA has nothing to do with whether the query occurs over IPv4 or IPv6, this module requires a special zone configuration to support both address families. For A queries, the underlying zone must have a set of nameservers that only have IPv4 addresses, and for AAAA queries, the underlying zone must have a set of nameservers that only have IPv6 addresses.
Example¶
To enable this module, you need to add something like the following to the Knot DNS configuration file:
zone:
- domain: whoami.domain.example
file: "/path/to/whoami.domain.example"
module: mod-whoami
zone:
- domain: whoami6.domain.example
file: "/path/to/whoami6.domain.example"
module: mod-whoami
The whoami.domain.example zone file example:
$TTL 1 @ SOA ( whoami.domain.example. ; MNAME hostmaster.domain.example. ; RNAME 2016051300 ; SERIAL 86400 ; REFRESH 86400 ; RETRY 86400 ; EXPIRE 1 ; MINIMUM ) $TTL 86400 @ NS ns1.whoami.domain.example. @ NS ns2.whoami.domain.example. @ NS ns3.whoami.domain.example. @ NS ns4.whoami.domain.example. ns1 A 198.51.100.53 ns2 A 192.0.2.53 ns3 A 203.0.113.53 ns4 A 198.19.123.53
The whoami6.domain.example zone file example:
$TTL 1 @ SOA ( whoami6.domain.example. ; MNAME hostmaster.domain.example. ; RNAME 2016051300 ; SERIAL 86400 ; REFRESH 86400 ; RETRY 86400 ; EXPIRE 1 ; MINIMUM ) $TTL 86400 @ NS ns1.whoami6.domain.example. @ NS ns2.whoami6.domain.example. @ NS ns3.whoami6.domain.example. @ NS ns4.whoami6.domain.example. ns1 AAAA 2001:db8:100::53 ns2 AAAA 2001:db8:200::53 ns3 AAAA 2001:db8:300::53 ns4 AAAA 2001:db8:400::53
The parent domain would then delegate whoami.domain.example to ns[1-4].whoami.domain.example and whoami6.domain.example to ns[1-4].whoami6.domain.example, and include the corresponding A-only or AAAA-only glue records.
Note
This module is not configurable.
