Backend writers’ guide

PowerDNS backends are implemented via a simple yet powerful C++ interface. If your needs are not met by the PipeBackend, you may want to write your own. Before doing any PowerDNS development, please read this blog post which has a FAQ and several pictures that help explain what a backend is.

A backend contains zero DNS logic. It need not look for CNAMEs, it need not return NS records unless explicitly asked for, etcetera. All DNS logic is contained within PowerDNS itself - backends should simply return records matching the description asked for.


However, please note that your backend can get queries in aNy CAsE! If your database is case sensitive, like most are (with the notable exception of MySQL), you must make sure that you do find answers which differ only in case.


PowerDNS may instantiate multiple instances of your backend, or destroy existing copies and instantiate new ones. Backend code should therefore be thread-safe with respect to its static data. Additionally, it is wise if instantiation is a fast operation, with the possible exception of the first construction.


Besides regular query types, the DNS also knows the ‘ANY’ query type. When a server receives a question for this ANY type, it should reply with all record types available.

Backends should therefore implement being able to answer ‘ANY’ queries in this way, and supply all record types they have when they receive such an ‘ANY’ query. This is reflected in the sample script above, which for every qtype answers if the type matches, or if the query is for ‘ANY’.

However, since backends need to implement the ANY query anyhow, PowerDNS makes use of this. Since almost all DNS queries internally need to be translated first into a CNAME query and then into the actual query, possibly followed by a SOA or NS query (this is how DNS works internally), it makes sense for PowerDNS to speed this up, and just ask the ANY query of a backend.

When it has done so, it gets the data about SOA, CNAME and NS records in one go. This speeds things up tremendously.

The upshot of the above is that for any backend, including the PIPE backend, implementing the ANY query is NOT optional. And in fact, a backend may see almost exclusively ANY queries. This is not a bug.

Simple read-only native backends

Implementing a backend consists of inheriting from the DNSBackend class. For read-only backends, which do not support slave operation, only the following methods are relevant:

class DNSBackend

virtual void lookup(const QType &qtype, const string &qdomain, DNSPacket *pkt_p=0, int zoneId=-1)=0;
virtual bool list(const string &target, int domain_id)=0;
virtual bool get(DNSResourceRecord &r)=0;
virtual bool getSOA(const string &name, SOAData &soadata, DNSPacket *p=0);

Note that the first three methods must be implemented. getSOA() has a useful default implementation.

The semantics are simple. Each instance of your class only handles one (1) query at a time. There is no need for locking as PowerDNS guarantees that your backend will never be called reentrantly.


Queries for wildcard names should be answered literally, without expansion. So, if a backend gets a question for “*”, it should only answer with data if there is an actual “*” name

Some examples, a more formal specification is down below. A normal lookup starts like this:

YourBackend yb;

Your class should now do everything to start this query. Perform as much preparation as possible - handling errors at this stage is better for PowerDNS than doing so later on. A real error should be reported by throwing an exception.

PowerDNS will then call the get() method to get DNSResourceRecords back. The following code illustrates a typical query:


DNSResourceRecord rr;
  cout<<"Found cname pointing to '"+rr.content+"'"<<endl;

Each zone starts with a Start of Authority (SOA) record. This record is special so many backends will choose to implement it specially. The default getSOA() method performs a regular lookup on your backend to figure out the SOA, so if you have no special treatment for SOA records, where is no need to implement your own getSOA().

Besides direct queries, PowerDNS also needs to be able to list a zone, to do zone transfers for example. Each zone has an id which should be unique within the backend. To list all records belonging to a zone id, the list() method is used. Conveniently, the domain_id is also available in the SOAData structure.

The following lists the contents of a zone called “”.

SOAData sd;
if(!yb.getSOA("",sd))  // are we authoritative over
  return RCode::NotAuth;           // no

  cout<<rr.qname<<"\t IN "<<rr.qtype.getName()<<"\t"<<rr.content<<endl;

A sample minimal backend

This backend only knows about the host “”, and furthermore, only about its A record:

class RandomBackend : public DNSBackend
  bool list(const string &target, int id)
    return false; // we don't support AXFR

  void lookup(const QType &type, const string &qdomain, DNSPacket *p, int zoneId)
    if(type.getCode()!=QType::A || qdomain!="")  // we only know about A
      d_answer="";                                                  // no answer
    else {
      ostringstream os;
      d_answer=os.str();                                           // our random ip address

  bool get(DNSResourceRecord &rr)
    if(!d_answer.empty()) {
      rr.qname="";                               // fill in details
      rr.qtype=QType::A;                                            // A record
      rr.ttl=86400;                                                 // 1 day

      d_answer="";                                                  // this was the last answer

      return true;
    return false;                                                   // no more data

  string d_answer;


class RandomFactory : public BackendFactory
  RandomFactory() : BackendFactory("random") {}

  DNSBackend *make(const string &suffix)
    return new RandomBackend();


class RandomLoader
    BackendMakers().report(new RandomFactory);
    g_log << Logger::Info << "[randombackend] This is the random backend version " VERSION " reporting" << endl;

static RandomLoader randomloader;

This simple backend can be used as an ‘overlay’. In other words, it only knows about a single record, another loaded backend would have to know about the SOA and NS records and such. But nothing prevents us from loading it without another backend.

The first part of the code contains the actual logic and should be pretty straightforward. The second part is a boilerplate ‘factory’ class which PowerDNS calls to create randombackend instances. Note that a ‘suffix’ parameter is passed. Real life backends also declare parameters for the configuration file; these get the ‘suffix’ appended to them. Note that the “random” in the constructor denotes the name by which the backend will be known.

The third part registers the RandomFactory with PowerDNS. This is a simple C++ trick which makes sure that this function is called on execution of the binary or when loading the dynamic module.

Please note that a RandomBackend is actually in most PowerDNS releases. By default it lives on, but you can change that by setting random-hostname.


This simple backend neglects to handle case properly!

Interface definition


class DNSResourceRecord
std::string DNSResourceRecord::qname

Name of this record

QType DNSResourceRecord::qtype

Query type of this record

std::string DNSResourceRecord::content

ASCII representation of the right-hand side

uint32_t DNSResourceRecord::ttl

Time To Live of this record

int DNSResourceRecord::domain_id

ID of the domain this record belongs to

time_t DNSResourceRecord::last_modified

If unzero, last time_t this record was changed

bool DNSResourceRecord::auth

Used for DNSSEC operations. See Migrating (Signed) Zones to PowerDNS. It is also useful to check out the rectifyZone() in

bool DNSResourceRecord::disabled

If set, this record is not to be served to DNS clients. Backends should not make these records available to PowerDNS unless indicated otherwise.

class SOAData
string SOAData::nameserver

Name of the master nameserver of this zone

string SOAData::hostmaster

Hostmaster of this domain. May contain an @

uint32_t SOAData::serial

Serial number of this zone

uint32_t SOAData::refresh

How often this zone should be refreshed

uint32_t SOAData::retry

How often a failed zone pull should be retried.

u_int32_t SOAData::expire

If zone pulls failed for this long, retire records

uint32_t SOAData::default_ttl


int SOAData::domain_id

The ID of the domain within this backend. Must be filled!

DNSBackend *SOAData::db

Pointer to the backend that feels authoritative for a domain and can act as a slave


void DNSBackend::lookup(const QType &qtype, const string &qdomain, DNSPacket *pkt = 0, int zoneId = -1)

This function is used to initiate a straight lookup for a record of name ‘qdomain’ and type ‘qtype’. A QType can be converted into an integer by invoking its getCode() method and into a string with the getCode().

The original question may or may not be passed in the pointer pkt. If it is, you can retrieve information about who asked the question with the pkt->getRemote() method.

Note that qdomain can be of any case and that your backend should make sure it is in effect case insensitive. Furthermore, the case of the original question should be retained in answers returned by get()!

Finally, the domain_id might also be passed indicating that only answers from the indicated zone need apply. This can both be used as a restriction or as a possible speedup, hinting your backend where the answer might be found.

If initiated successfully, as indicated by returning true, answers should be made available over the get() method.

Should throw an PDNSException if an error occurred accessing the database. Returning otherwise indicates that the query was started successfully. If it is known that no data is available, no exception should be thrown! An exception indicates that the backend considers itself broken - not that no answers are available for a question.

It is legal to return here, and have the first call to get() return false. This is interpreted as ‘no data’.

bool DNSBackend::list(int domain_id, bool include_disabled = false)

Initiates a list of the indicated domain. Records should then be made available via the get() method. Need not include the SOA record. If it is, PowerDNS will not get confused. If include_disabled is given as true, records that are configured but should not be served to DNS clients must also be made available.

Should return false if the backend does not consider itself authoritative for this zone. Should throw an PDNSException if an error occurred accessing the database. Returning true indicates that data is or should be available.

bool DNSBackend::get(DNSResourceRecord &rr)

Request a DNSResourceRecord from a query started by get() of list(). If this functions returns true, rr has been filled with data. When it returns false, no more data is available, and rr does not contain new data. A backend should make sure that it either fills out all fields of the DNSResourceRecord or resets them to their default values.

The qname field of the DNSResourceRecord should be filled out with the exact qdomain passed to lookup, preserving its case. So if a query for ‘’ comes in and your database contains data for ‘’, the qname field of rr should contain ‘’!

Should throw an PDNSException in case a database error occurred.

bool DNSBackend::getSOA(const string &name, SOAData &soadata)

If the backend considers itself authoritative over domain name, this method should fill out the passed SOAData structure and return a positive number. If the backend is functioning correctly, but does not consider itself authoritative, it should return 0. In case of errors, an PDNSException should be thrown.

Reporting errors

To report errors, the Logger class is available which works mostly like an iostream. Example usage is as shown above in the RandomBackend. Note that it is very important that each line is ended with endl as your message won’t be visible otherwise.

To indicate the importance of an error, the standard syslog errorlevels are available. They can be set by outputting Logger::Critical, Logger::Error, Logger::Warning, Logger::Notice, Logger::Info or Logger::Debug to L, in descending order of graveness.

Declaring and reading configuration details

It is highly likely that a backend needs configuration details. On launch, these parameters need to be declared with PowerDNS so it knows it should accept them in the configuration file and on the command line. Furthermore, they will be listed in the output of --help.

Declaring arguments is done by implementing the member function declareArguments() in the factory class of your backend. PowerDNS will call this method after launching the backend.

In the declareArguments() method, the function declare() is available. The exact definitions:

void DNSBackend::declareArguments(const string &suffix = "")

This method is called to allow a backend to register configurable parameters. The suffix is the sub-name of this module. There is no need to touch this suffix, just pass it on to the declare method.

void DNSBackend::declare(const string &suffix, const string &param, const string &explanation, const string &value)

The suffix is passed to your method, and can be passed on to declare. param is the name of your parameter. explanation is what will appear in the output of –help. Furthermore, a default value can be supplied in the value parameter.

A sample implementation:

void declareArguments(const string &suffix)
  declare(suffix,"dbname","Pdns backend database name to connect to","powerdns");
  declare(suffix,"user","Pdns backend user to connect as","powerdns");
  declare(suffix,"host","Pdns backend host to connect to","");
  declare(suffix,"password","Pdns backend password to connect with","");

After the arguments have been declared, they can be accessed from your backend using the mustDo(), getArg() and getArgAsNum() methods. The are defined as follows in the DNSBackend class:

void DNSBackend::setArgPrefix(const string &prefix)

Must be called before any of the other accessing functions are used. Typical usage is ‘setArgPrefix("mybackend"+suffix)’ in the constructor of a backend.

bool DNSBackend::mustDo(const string &key)

Returns true if the variable key is set to anything but ‘no’.

const string &DNSBackend::getArg(const string &key)

Returns the exact value of a parameter.

int DNSBackend::getArgAsNum(const string &key)

Returns the numerical value of a parameter. Uses atoi() internally

Sample usage from the BindBackend: getting the ‘check-interval’ setting:

if(!safeGetBBDomainInfo(i->name, &bbd)) {

Read/write slave-capable backends

The backends above are ‘natively capable’ in that they contain all data relevant for a domain and do not pull in data from other nameservers. To enable storage of information, a backend must be able to do more.

Before diving into the details of the implementation some theory is in order. Slave domains are pulled from the master. PowerDNS needs to know for which domains it is to be a slave, and for each slave domain, what the IP address of the master is.

A slave zone is pulled from a master, after which it is ‘fresh’, but this is only temporary. In the SOA record of a zone there is a field which specifies the ‘refresh’ interval. After that interval has elapsed, the slave nameserver needs to check at the master ff the serial number there is higher than what is stored in the backend locally.

If this is the case, PowerDNS dubs the domain ‘stale’, and schedules a transfer of data from the remote. This transfer remains scheduled until the serial numbers remote and locally are identical again.

This theory is implemented by the getUnfreshSlaveInfos method, which is called on all backends periodically. This method fills a vector of SlaveDomains with domains that are unfresh and possibly stale.

PowerDNS then retrieves the SOA of those domains remotely and locally and creates a list of stale domains. For each of these domains, PowerDNS starts a zone transfer to resynchronise. Because zone transfers can fail, it is important that the interface to the backend allows for transaction semantics because a zone might otherwise be left in a halfway updated situation.

The following excerpt from the DNSBackend shows the relevant functions:

 class DNSBackend {
      /* ... */
      virtual bool getDomainInfo(const string &domain, DomainInfo &di);
  virtual bool isMaster(const string &name, const string &ip);
  virtual bool startTransaction(const string &qname, int id);
  virtual bool commitTransaction();
  virtual bool abortTransaction();
  virtual bool feedRecord(const DNSResourceRecord &rr, string *ordername=0);
  virtual void getUnfreshSlaveInfos(vector<DomainInfo>* domains);
  virtual void setFresh(uint32_t id);
      /* ... */

The mentioned DomainInfo struct looks like this:

class DomainInfo
uint32_t DomainInfo::id

ID of this zone within this backend

string DomainInfo::master

IP address of the master of this domain, if any

uint32_t DomainInfo::serial

Serial number of this zone

uint32_t DomainInfo::notified_serial

Last serial number of this zone that slaves have seen

time_t DomainInfo::last_check

Last time this zone was checked over at the master for changes

enum DomainKind DomainInfo::kind

Type of zone

DNSBackend *DomainInfo::backend

Pointer to the backend that feels authoritative for a domain and can act as a slave

enum DomainKind

The kind of domain, one of {Master,Slave,Native}.

These functions all have a default implementation that returns false - which explains that these methods can be omitted in simple backends. Furthermore, unlike with simple backends, a slave capable backend must make sure that the ‘DNSBackend *db’ field of the SOAData record is filled out correctly - it is used to determine which backend will house this zone.

bool DomainInfo::isMaster(const string &name, const string &ip)

If a backend considers itself a slave for the domain name and if the IP address in ip is indeed a master, it should return true. False otherwise. This is a first line of checks to guard against reloading a domain unnecessarily.

void DomainInfo::getUnfreshSlaveInfos(vector<DomainInfo> *domains)

When called, the backend should examine its list of slave domains and add any unfresh ones to the domains vector.

bool DomainInfo::getDomainInfo(const string &name, DomainInfo &di)

This is like getUnfreshSlaveInfos, but for a specific domain. If the backend considers itself authoritative for the named zone, di should be filled out, and ‘true’ be returned. Otherwise return false.

bool DomainInfo::startTransaction(const string &qname, int id)

When called, the backend should start a transaction that can be committed or rolled back atomically later on. In SQL terms, this function should BEGIN a transaction and DELETE all records.

bool DomainInfo::feedRecord(const DNSResourceRecord &rr, string *ordername)

Insert this record.

bool DomainInfo::commitTransaction()

Make the changes effective. In SQL terms, execute COMMIT.

bool DomainInfo::abortTransaction()

Abort changes. In SQL terms, execute ABORT.

bool DomainInfo::setFresh()

Indicate that a domain has either been updated or refreshed without the need for a retransfer. This causes the domain to vanish from the vector modified by getUnfreshSlaveInfos().

PowerDNS will always call startTransaction() before making calls to feedRecord(). Although it is likely that abortTransaction() will be called in case of problems, backends should also be prepared to abort from their destructor.

The actual code in PowerDNS is currently:

Resolver resolver;

db->startTransaction(domain, domain_id);
g_log<<Logger::Error<<"AXFR started for '"<<domain<<"'"<<endl;
Resolver::res_t recs;

while(resolver.axfrChunk(recs)) {
  for(Resolver::res_t::const_iterator i=recs.begin();i!=recs.end();++i) {
g_log<<Logger::Error<<"AXFR done for '"<<domain<<"'"<<endl;

Supermaster/Superslave capability

A backend that wants to act as a ‘superslave’ for a master should implement the following method:

class DNSBackend
   virtual bool superMasterBackend(const string &ip, const string &domain, const vector<DNSResourceRecord>&nsset, string *account, DNSBackend **db)

This function gets called with the IP address of the potential supermaster, the domain it is sending a notification for and the set of NS records for this domain at that IP address.

Using the supplied data, the backend needs to determine if this is a bonafide ‘supernotification’ which should be honoured. If it decides that it should, the supplied pointer to ‘account’ needs to be filled with the configured name of the supermaster (if accounting is desired), and the db needs to be filled with a pointer to your backend.

Supermaster/superslave is a complicated concept, if this is all unclear see the Supermaster and Superslave documentation.

Read/write master-capable backends

In order to be a useful master for a domain, notifies must be sent out whenever a domain is changed. Periodically, PowerDNS queries backends for domains that may have changed, and sends out notifications for slave nameservers.

In order to do so, PowerDNS calls the getUpdatedMasters() method. Like the getUnfreshSlaveInfos() function mentioned above, this should add changed domain names to the vector passed.

The following excerpt from the DNSBackend shows the relevant functions:

 class DNSBackend {
      /* ... */
  virtual void getUpdatedMasters(vector<DomainInfo>* domains);
  virtual void setNotified(uint32_t id, uint32_t serial);
      /* ... */

These functions all have a default implementation that returns false - which explains that these methods can be omitted in simple backends. Furthermore, unlike with simple backends, a slave capable backend must make sure that the ‘DNSBackend *db’ field of the SOAData record is filled out correctly - it is used to determine which backend will house this zone.

void DNSBackend::getUpdatedMasters(vector<DomainInfo> *domains)

When called, the backend should examine its list of master domains and add any changed ones to the DomainInfo vector.

bool DNSBackend::setNotified(uint32_t domain_id, uint32_t serial)

Indicate that notifications have been queued for this domain and that it need not be considered ‘updated’ anymore

DNS update support

To make your backend DNS update compatible, it needs to implement a number of new functions and functions already used for slave-operation. The new functions are not DNS update specific and might be used for other update/remove functionality at a later stage.

class DNSBackend {
  /* ... */
  virtual bool startTransaction(const string &qname, int id);
  virtual bool commitTransaction();
  virtual bool abortTransaction();
  virtual bool feedRecord(const DNSResourceRecord &rr, string *ordername);
  virtual bool replaceRRSet(uint32_t domain_id, const string& qname, const QType& qt, const vector<DNSResourceRecord>& rrset)
  virtual bool listSubZone(const string &zone, int domain_id);
  /* ... */
virtual bool DNSBackend::startTransaction(const string &qname, int id)

See above. Please note that this function now receives a negative number (-1), which indicates that the current zone data should NOT be deleted.

virtual bool DNSBackend::commitTransaction()

See above.

virtual bool DNSBackend::abortTransaction()

See cpp:func:above <DNSBackend::abortTransaction>. Method is called when an exception is received.

virtual bool DNSBackend::feedRecord(const DNSResourceRecord &rr, string *ordername)

See above. Please keep in mind that the zone is not empty because startTransaction() was called different.

virtual bool DNSBackend::listSubZone(const string &name, int domain_id)

This method is needed for rectification of a zone after NS-records have been added. For DNSSEC, we need to know which records are below the currently added record. listSubZone() is used like list() which means PowerDNS will call get() after this method. The default SQL query looks something like this:

// First %s is '', second %s is '*'
select content,ttl,prio,type,domain_id,name from records where (name='%s' OR name like '%s') and domain_id=%d

The method is not only used when adding records, but also to correct ENT-records in powerdns. Make sure it returns every record in the tree below the given record.

virtual bool DNSBackend::replaceRRSet(uint32_t domain_id, const string &qname, const QType &qt, const vector<DNSResourceRecord> &rrset)

This method should remove all the records with qname of type qt. qt might also be ANY, which means all the records with that qname need to be removed. After removal, the records in rrset must be added to the zone. rrset can be empty in which case the method is used to remove a RRset.


ENT (Empty Non-Terminal)

You are expected to reply with a DNSResourceRecord having qtype = 0, ttl = 0 and content should be empty string (string length 0)