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Scripting The Recursor

In the PowerDNS recursor, it is possible to modify resolving behaviour using simple scripts written in the Lua programming language. This page documents the Recursor 4.0.0 and beyond version of the scripting API.

Note: This describes the Lua scripts as supported by 4.x. They are very different than the ones from 3.x, but tend to be faster and more correct.

These scripts can be used to quickly override dangerous domains, fix things that are wrong, for load balancing or for legal or commercial purposes. The scripts can also protect you or your users from malicious traffic.

Lua is extremely fast and lightweight, easily supporting hundreds of thousands of queries per second. The Lua language is explained very well in the excellent book Programming in Lua. If you already have programming experience, Learn Lua in 15 Minutes is a great primer.

For extra performance, a Just In Time compiled version of Lua called LuaJIT is supported.

Queries can be intercepted in many places:

  • before any packet parsing begins (ipfilter)
  • before any filtering policy have been applied (prerpz)
  • before the resolving logic starts to work (preresolve)
  • after the resolving process failed to find a correct answer for a domain (nodata, nxdomain)
  • after the whole process is done and an answer is ready for the client (postresolve)
  • before an outgoing query is made to an authoritative server (preoutquery)

Configuring Lua scripts

In order to load scripts, the PowerDNS Recursor must have Lua support built in. The packages distributed from the PowerDNS website have this language enabled, other distributions may differ. By default, the Recursor's configure script will attempt to detect if Lua is available.

note: Only one script can be loaded at the same time. If you load a different script, the current one will be replaced (safely)!

If Lua support is available, a script can be configured either via the configuration file, or at runtime via the rec_control tool. Scripts can be reloaded or unloaded at runtime with no interruption in operations. If a new script contains syntax errors, the old script remains in force.

On the command line, or in the configuration file, the setting lua-dns-script can be used to supply a full path to the Lua script.

At runtime, rec_control reload-lua-script can be used to either reload the script from its current location, or, when passed a new file name, load one from a new location. A failure to parse the new script will leave the old script in working order.

Note: It is also possible to precompile scripts using luac, and have PowerDNS load the result. This means that switching scripts is faster, and also that you'll be informed about syntax errors at compile time.

Finally, rec_control unload-lua-script can be used to remove the currently installed script, and revert to unmodified behaviour.

Writing Lua PowerDNS Recursor scripts

To get a quick start, we have supplied a sample script that showcases all functionality described below. Please find it here.

Addresses and DNS Names are not passed as strings but as native objects. This allows for easy checking against netmasks and domain sets. It also means that to print such names, the :toString method must be used (or even :toStringWithPort for addresses).

Comparing IP addresses and DNSNames is not done with '==' but with the :equal method.

Once a script is loaded, PowerDNS looks for several functions, as detailed below. All of these functions are optional.

The DNSQuestion (dq) object

Apart from the ipfilter-function, all functions work on a dq (DNSQuestion) object. This object contains details about the current state of the question. This state can be modified from the various hooks. If a function returns 'true', it will indicate that it handled a query. If it returns false, the Recursor will continue processing unchanged (with one minor exception).

The DNSQuestion object contains at least the following fields:

  • qname - DNS native version of the name this query is for
  • qtype - type this query is for, can be compared against pdns.A, pdns.AAAA etc
  • rcode - current DNS Result Code, which can be overridden, including to several magical values
  • isTcp - whether the query have been received over TCP or UDP
  • remoteaddr - address of the requestor
  • localaddr - address this query was received on
  • variable - a boolean which, if set, indicates the recursor should not packet cache this answer. Honored even when returning 'false'! Important when providing answers that vary over time or based on sender details.
  • followupFunction - a string that signals the nameserver to take one of the following additional actions:
    • followCNAMERecords: When adding a CNAME to the answer, this tells the recursor to follow that CNAME. See CNAME chain resolution
    • getFakeAAAARecords: Get a fake AAAA record, see DNS64
    • getFakePTRRecords: Get a fake PTR record, see DNS64
    • udpQueryResponse: Do a UDP query and call a handler, see udpQueryResponse
  • appliedPolicy - The decision that was made by the policy engine, see Modifying policy decisions. It has the following fields:
    • policyName: The name of the policy (used in e.g. protobuf logging)
    • policyAction: The action taken by the engine
    • policyCustom: The CNAME content for the pdns.policyactions.Custom response, a string
    • policyTTL: The TTL in seconds for the pdns.policyactions.Custom response
  • wantsRPZ - A boolean that indicates the use of the Policy Engine, can be set to false in preresolve to disable RPZ for this query

It also supports the following methods:

  • addAnswer(type, content, [ttl, name]): add an answer to the record of type with content. Optionally supply TTL and the name of the answer too, which defaults to the name of the question
  • addPolicyTag(tag): add a policy tag.
  • discardPolicy(policyname): skip the filtering policy (for example RPZ) named policyname for this query. This is mostly useful in the prerpz hook.
  • getPolicyTags(): get the current policy tags as a table of strings.
  • getRecords(): get a table of DNS Records in this DNS Question (or answer by now)
  • setPolicyTags(tags): update the policy tags, taking a table of strings.
  • setRecords(records): after your edits, update the answers of this question
  • getEDNSOption(num): get the EDNS Option with number num
  • getEDNSOptions(): get a map of all EDNS Options
  • getEDNSSubnet(): returns the netmask specified in the EDNSSubnet option, or empty if there was none

function ipfilter ( remoteip, localip, dh )

This hook gets queried immediately after consulting the packet cache, but before parsing the DNS packet. If this hook returns something else than false, the packet is dropped. However, because this check is after the packet cache, the IP address might still receive answers that require no packet parsing.

With this hook, undesired traffic can be dropped rapidly before using precious CPU cycles for parsing.

remoteip is the IP(v6) address of the requestor, localip is the address on which the query arrived. dh is the DNS Header of the query, and it offers the following methods:

  • getRD(), getAA(), getAD(), getCD(), getRD(), getRD(), getTC(): query these bits from the DNS Header
  • getRCODE(): get the RCODE of the query
  • getOPCODE(): get the OPCODE of the query
  • getID(): get the ID of the query

As an example, to filter all queries coming from 1.2.3.0/24, or with the AD bit set:

badips = newNMG()
badips:addMask("1.2.3.0/24")

function ipfilter(rem, loc, dh)
	return badips:match(rem) or dh:getAD()
end

This hook does not get the full DNSQuestion object, since filling out the fields would require packet parsing, which is what we are trying to prevent with ipfilter.

function gettag(remote, ednssubnet, local, qname, qtype)

The gettag function is invoked when the Recursor attempts to discover in which packetcache an answer is available. This function must return an integer, which is the tag number of the packetcache. In addition to this integer, this function can return a table of policy tags.

The resulting tag number can be accessed via dq.tag in the preresolve hook, and the policy tags via dq:getPolicyTags() in every hook.

The tagged packetcache can e.g. be used to answer queries from cache that have e.g. been filtered for certain IPs (this logic should be implemented in the gettag function). This ensure that queries are answered quickly compared to setting dq.variable to true. In the latter case, repeated queries will pass through the entire Lua script.

function prerpz(dq)

This hook is called before any filtering policy have been applied, making it possible to completely disable filtering by setting wantsRPZ to false. Using the discardPolicy() function, it is also possible to selectively disable one or more filtering policy, for example RPZ zones, based on the content of the dq object.

As an example, to disable the malware policy for example.com queries:

function prerpz(dq)
  -- disable the RPZ policy named 'malware' for example.com
  if dq.qname:equal('example.com') then
    dq:discardPolicy('malware')
  end
end

function preresolve(dq)

is called before any DNS resolution is attempted, and if this function indicates it, it can supply a direct answer to the DNS query, overriding the internet. This is useful to combat botnets, or to disable domains unacceptable to an organization for whatever reason.

The rcode can be set to pdns.DROP to drop the query. Other statuses are normal DNS return codes, like no error, NXDOMDAIN etc.

function postresolve(dq)

is called right before returning a response to a client (and, unless variable is set, to the packet cache too). It allows inspection and modification of almost any detail in the return packet.

function nxdomain(dq)

is called after the DNS resolution process has run its course, but ended in an 'NXDOMAIN' situation, indicating that the domain or the specific record does not exist. Works entirely like postresolve, but saves a trip through Lua for answers which are not NXDOMAIN.

function nodata(dq)

is just like nxdomain, except it gets called when a domain exists, but the requested type does not. This is where one would implement DNS64.

function preoutquery(dq)

This hook is not called in response to a client packet, but fires when the Recursor wants to talk to an authoritative server. When this hook sets the special result code -3, the whole DNS client query causing this outquery gets dropped.

However, this function can also return records like the preresolve query above.

Semantics

The functions must return true if they have taken over the query and wish that the nameserver should not proceed with its regular query-processing. When a function returns false, the nameserver will process the query normally until a new function is called.

If a function has taken over a request, it should set an rcode (usually 0), and specify a table with records to be put in the answer section of a packet. An interesting rcode is NXDOMAIN (3, or pdns.NXDOMAIN), which specifies the non-existence of a domain.

The ipfilter and preoutquery hooks are different, in that ipfilter can only return a true of false value, and that preoutquery can also set rcode -3 to signify that the whole query should be terminated.

A minimal sample script:

function nxdomain(dq)
	print("Intercepting NXDOMAIN for: ",dq.qname:toString())
	if dq.qtype == pdns.A
	then
		dq.rcode=0 -- make it a normal answer
		dq:addAnswer(pdns.A, "192.168.1.1")
		return true
	end
	return false
end

Warning: Please do NOT use the above sample script in production! Responsible NXDomain redirection requires more attention to detail.

Useful 'rcodes' include 0 for "no error", pdns.NXDOMAIN for "NXDOMAIN", pdns.DROP to drop the question from further processing (since 3.6, and such a drop is accounted in the 'policy-drops' metric).

Helpful functions

Netmask Groups

IP addresses are passed to Lua in native format. They can be matched against netmasks objects like this:

nmg = newNMG()
nmg:addMask("127.0.0.0/8")
nmg:addMasks({"213.244.168.0/24", "130.161.0.0/16"})
nmg:addMasks(dofile("bad.ips")) -- contains return {"ip1","ip2"..}

if nmg:match(dq.remoteaddr) then
	print("Intercepting query from ", dq.remoteaddr)
end

IP Addresses

We move IP addresses around in native format, called ComboAddress within PowerDNS. ComboAddresses can be IPv4 or IPv6, and unless you want to know, you don't need to. You can make a ComboAddress with: newCA("::1"), and you can compare it against a NetmaskGroup as described above.

To compare the address (so not the port) of two ComboAddresses, use :equal.

To convert an address to human-friendly representation, use :toString() or :toStringWithPort(). To get only the port number, use :getPort().

Other functions that can be called on a ComboAddress are:

  • isIPv4 - true if the address is an IPv4 address
  • isIPv6 - true if the address is an IPv6 address
  • getRaw - returns the bytestring representing the address
  • isMappedIPv4 - true if the address is an IPv4 address mapped into an IPv6 one
  • mapToIPv4 - if the address is an IPv4 mapped into an IPv6 one, return the corresponding IPv4
  • truncate(bits) - truncate to the supplied number of bits

Netmask

IP addresses can be matched against a Netmask object, which can be created with newNetmask("192.0.2.1/24") and supports the following methods:

  • empty - true if the netmask doesn't contain a valid address
  • getBits - the number of bits in the address
  • getNetwork - return a ComboAddress representing the network (no mask applied)
  • getMaskedNetwork - return a ComboAddress representing the network (truncating according to the mask)
  • isIpv4 - true if the address is an IPv4 address
  • isIpv6 - true if the address is an IPv6 address
  • match(str) - true if the address passed in str matches
  • toString - human-friendly representation

DNSName

DNSNames are passed to various functions, and they sport the following methods:

  • :equal: use this to compare two DNSNames in DNS native fashion. So 'PoWeRdNs.COM' matches 'powerdns.com'
  • :isPartOf: returns true if a is a part of b. So: newDN("www.powerdns.com"):isPartOf(newDN("CoM.")) returns true
  • :toString and :toStringNoDot: return a string representation of the name, with or without trailing dot.
  • :chopOff: removes the leftmost label from the name, returns true if this succeeded.
  • :countLabels: returns the number of labels
  • :wirelength: returns the length on the wire

You can compare DNSNames using :equal or the == operator.

To make your own DNSName, use newDN("domain.name"). To copy an existing DNSName (please remember to do this before using chopOff), use newDN(mydn).

DNS Suffix Match groups

The newDS function creates a "Suffix Match group" that allows fast checking if a DNSName is part of a group. Add domains to this group with the :add(domain) function of the object: myDS:add("example.net"), or with a list: `myDS:add({"example.net", "example.com"}).

To check e.g. the dq.qname against this list, use :check(dq.qname). This will be true if dq.qname is part of any of the Suffix Match group domains.

This could e.g. be used to answer questions for known malware domains.

To see the set of suffixes matched by a Suffix Match Group, use :toString().

DNS Record

DNS record objects are returned by dq:getRecords(), and have the following members:

  • name: the name of the record as a DNSName
  • place: the place where the record is located, 1 for the answer section, 2 for the authority and 3 for the additional one
  • ttl: the TTL of the record
  • type: the type of the record, for example pdns.A

and the following methods:

  • changeContent(newcontent): replace the record content with the string representation passed as newcontent. The type and class cannot be changed.
  • getCA(): if the record type is A or AAAA, a ComboAddress representing the content is returned, nil otherwise.
  • getContent(): return a string representation of the record content

Metrics

You can custom metrics which will be shown in the output of 'rec_control get-all' and sent to the metrics server over the Carbon protocol, and also appear in the JSON HTTP API.

Create a custom metric with: myMetric= getMetric("name"). This metric sports the following metrics:

  • :inc(): increase metric by 1
  • :incBy(amount): increase metric by amount
  • :set(to): set metric to value to
  • :get(): get value of metric

Metrics are shared across all of PowerDNS and are fully atomic and high performance. The myMetric object is effectively a pointer to an atomic value.

Note that metrics live in the same namespace as 'system' metrics. So if you generate one that overlaps with a PowerDNS stock metric, you will get double output and weird results.

Logging

To log messages with the main PowerDNS Recursor process, use pdnslog(message). pdnslog can also write out to a syslog loglevel if specified. Use pdnslog(message, pdns.loglevels.LEVEL) with the correct pdns.loglevels entry. Entries are listed in the following table:

  • All - pdns.loglevels.All
  • Alert - pdns.loglevels.Alert
  • Critical - pdns.loglevels.Critical
  • Error - pdns.loglevels.Error
  • Warning - pdns.loglevels.Warning
  • Notice - pdns.loglevels.Notice
  • Info - pdns.loglevels.Info
  • Debug - pdns.loglevels.Debug
  • None - pdns.loglevels.None

pdnslog(message) will write out to Info by default.

getregisteredname('www.powerdns.com') returns powerdns.com., based on Mozilla's Public Suffix List. In general it will tell you the 'registered domain' for a given name.

getRecursorThreadId() returns an unsigned integer identifying the thread handling the current request.

DNS64

The getFakeAAAARecords and getFakePTRRecords followupFunctions can be used to implement DNS64. See DNS64 support in the PowerDNS Recursor for more information.

To get fake AAAA records for DNS64 usage, set dq.followupFunction to getFakeAAAARecords, dq.followupPrefix to e.g. "64:ff9b::" and dq.followupName to the name you want to synthesize an IPv6 address for.

For fake reverse (PTR) records, set dq.followupFunction to getFakePTRRecords and set dq.followupName to the name to look up and dq.followupPrefix to the same prefix as used with getFakeAAAARecords.

CNAME chain resolution

It may be useful to return a CNAME record for Lua, and then have the PowerDNS Recursor continue resolving that CNAME. This can be achieved by setting dq.followupFunction to followCNAMERecords and dq.followupDomain to "www.powerdns.com". PowerDNS will do the rest.

udpQueryResponse

The udpQueryResponse dq.followupFunction allows you to query a simple key-value store over UDP asynchronously.

Several dq variables can be set:

  • udpQueryDest: destination IP address to send the UDP packet to
  • udpQuery: The content of the UDP payload
  • udpCallback: The name of the callback function that is called when an answer is received

The callback function must accept the dq object and can find the response to the UDP query in dq.udpAnswer.

In this callback function, dq.followupFunction can be set again to any of the available functions for further processing.

This example script queries a simple key/value store over UDP to decide on whether or not to filter a query:

!!include=../pdns/kv-example-script.lua

Example Script

!!include=../pdns/powerdns-example-script.lua

Dropping all traffic from botnet-infected users

Frequently, DoS attacks are performed where specific IP addresses are attacked, often by queries coming in from open resolvers. These queries then lead to a lot of queries to 'authoritative servers' which actually often aren't nameservers at all, but just targets of attack.

The following script will add a requestor's IP address to a blocking set if they've sent a query that caused PowerDNS to attempt to talk to a certain subnet.

This specific script is, as of January 2015, useful to prevent traffic to ezdns.it related traffic from creating CPU load. This script requires PowerDNS Recursor 4.x or later.

lethalgroup=newNMG()
lethalgroup:addMask("192.121.121.0/24") -- touch these nameservers and you die

function preoutquery(dq)
	print("pdns wants to ask "..dq.remoteaddr:toString().." about "..dq.qname:toString().." "..dq.qtype.." on behalf of requestor "..dq.localaddr:toString())
	if(lethalgroup:match(dq.remoteaddr))
	then
		print("We matched the group "..lethalgroup:tostring().."!", "killing query dead & adding requestor "..dq.localaddr:toString().." to block list")
		dq.rcode = -3 -- "kill"	
		return true
	end
	return false
end

Modifying Policy Decisions

The PowerDNS Recursor has a policy engine based on Response Policy Zones (RPZ). Starting with version 4.0.1 of the recursor, it is possible to alter this decision inside the Lua hooks. If the decision is modified in a Lua hook, false should be returned, as the query is not actually handled by Lua so the decision is picked up by the Recursor. The result of the policy decision is checked after preresolve and postresolve.

For example, if a decision is set to pdns.policykinds.NODATA by the policy engine and is unchanged in preresolve, the query is replied to with a NODATA response immediately after preresolve.

Example script

-- Dont ever block my own domain and IPs
myDomain = newDN("example.com")

myNetblock = newNMG()
myNetblock:addMasks("192.0.2.0/24")

function preresolve(dq)
  if dq.qname:isPartOf(myDomain) and dq.appliedPolicy.policyKind != pdns.policykinds.NoAction then
    pdnslog("Not blocking our own domain!")
    dq.appliedPolicy.policyKind = pdns.policykinds.NoAction
  end
end

function postresolve(dq)
  if dq.appliedPolicy.policyKind != pdns.policykinds.NoAction then
    local records = dq:getRecords()
    for k,v in pairs(records) do
      if v.type == pdns.A then
        local blockedIP = newCA(v:getContent())
        if myNetblock:match(blockedIP) then
          pdnslog("Not blocking our IP space")
          dq.appliedPolicy.policyKind = pdns.policykinds.NoAction
        end
      end
    end
  end
end

The decision is contained in the dq object under dq.appliedPolicy and features 4 fields:

dq.appliedPolicy.policyName

A string with the name of the policy (set by polName= in the rpzFile and rpzMaster configuration items). It is advised to overwrite this when modifying the policyKind

dq.appliedPolicy.policyKind

The kind of policy response, there are several policy kinds:

  • pdns.policykinds.Custom will return a NoError, CNAME answer with the value specified in dq.appliedPolicy.policyCustom
  • pdns.policykinds.Drop will simply cause the query to be dropped
  • pdns.policykinds.NoAction will continue normal processing of the query
  • pdns.policykinds.NODATA will return a NoError response with no value in the answer section
  • pdns.policykinds.NXDOMAIN will return a response with a NXDomain rcode
  • pdns.policykinds.Truncate will return a NoError, no answer, truncated response over UDP. Normal processing will continue over TCP

dq.appliedPolicy.policyCustom and dq.appliedPolicy.policyTTL

These fields are only used when dq.appliedPolicy.policyKind is set to pdns.policykinds.Custom. dq.appliedPolicy.policyCustom contains the name for the CNAME target as a string. And dq.appliedPolicy.policyTTL is the TTL field (in seconds) for the CNAME response.