PowerDNS offers full master and slave semantics for replicating domain information. Furthermore, PowerDNS can benefit from native database replication.

Native replication is the default, unless other operation is specifically configured. Native replication basically means that PowerDNS will not send out DNS update notifications, nor will react to them. PowerDNS assumes that the backend is taking care of replication unaided.

MySQL replication has proven to be very robust and well suited, even over transatlantic connections between badly peering ISPs.

To use native replication, configure your backend storage to do the replication and do not configure PowerDNS to do so.

Typically, a database slave will be configured as read-only as uni-directional database replication is usually sufficient. A PowerDNS server only requires database write access if it is participating as a master or slave in zone transfers, or has a frontend attached for managing records etc.

When operating as a master, PowerDNS sends out notifications of changes to slaves, which react to these notifications by querying PowerDNS to see if the zone changed, and transferring its contents if it has. Notifications are a way to promptly propagate zone changes to slaves, as described in RFC 1996. Since version 4.0.0, the NOTIFY messages have a TSIG record added (transaction signature) if zone has been configured to use TSIG and feature has been enabled.

Left open by RFC 1996 is who is to be notified - which is harder to figure out than it sounds. All slaves for this domain must receive a notification but the nameserver only knows the names of the slaves - not the IP addresses, which is where the problem lies. The nameserver itself might be authoritative for the name of its secondary, but not have the data available.

To resolve this issue, PowerDNS tries multiple tactics to figure out the IP addresses of the slaves, and notifies everybody. In contrived configurations this may lead to duplicate notifications being sent out, which shouldn't hurt.

Some backends may be able to detect zone changes, others may chose to let the operator indicate which zones have changed and which haven't. Consult the documentation for your backend to see how it processes changes in zones.

To help deal with slaves that may have missed notifications, or have failed to respond to them, several override commands are available via the :ref:`pdns_control <running-pdnscontrol>` tool:

• pdns_control notify <domain> This instructs PowerDNS to notify all IP addresses it considers to be slaves of this domain.
• pdns_control notify-host <domain> <ip-address> This is truly an override and sends a notification to an arbitrary IP address. Can be used in :ref:`setting-also-notify` situations or when PowerDNS has trouble figuring out who to notify - which may happen in contrived configurations.

On launch, PowerDNS requests from all backends a list of domains which have not been checked recently for changes. This should happen every 'refresh' seconds, as specified in the SOA record. All domains that are unfresh are then checked for changes over at their master. If the :ref:`types-SOA` serial number there is higher, the domain is retrieved and inserted into the database. In any case, after the check the domain is declared 'fresh', and will only be checked again after 'refresh' seconds have passed.

When the freshness of a domain cannot be checked, e.g. because the master is offline, PowerDNS will retry the domain after :ref:`setting-slave-cycle-interval` seconds. Every time the domain fails it's freshness check, PowerDNS will hold back on checking the domain for amount of failures * slave-cycle-interval seconds, with a maximum of :ref:`setting-soa-retry-default` seconds between checks. With default settings, this means that PowerDNS will back off for 1, then 2, then 3 etc. minutes, to a maximum of 60 minutes between checks. The same hold back algorithm is also applied if the zone transfer fails due to problems on the master, i.e. if zone transfer is not allowed.

Receiving a NOTIFY immediately clears the back off period for the respective domain to allow immediately freshness checks for this domain.

PowerDNS also reacts to notifies by immediately checking if the zone has updated and if so, retransfering it.

All backends which implement this feature must make sure that they can handle transactions so as to not leave the zone in a half updated state. MySQL configured with either BerkeleyDB or InnoDB meets this requirement, as do PostgreSQL. The BIND backend implements transaction semantics by renaming files if and only if they have been retrieved completely and parsed correctly.

Slave operation can also be programmed using several :ref:`running-pdnscontrol` commands. The retrieve command is especially useful as it triggers an immediate retrieval of the zone from the configured master.

PowerDNS supports multiple masters. For the BIND backend, the native BIND configuration language suffices to specify multiple masters, for SQL based backends, list all master servers separated by commas in the 'master' field of the domains table.

Since version 4.0.0, PowerDNS requires that masters sign their notifications. During transition and interoperation with other nameservers, you can use options :ref:`setting-allow-unsigned-notify` to permit unsigned notifications. For 4.0.0 this is turned on by default, but it might be turned off permanently in future releases.

Generally to enable a Master/Slave setup you have to take care of following properties.

• The :ref:`setting-master`/:ref:`setting-slave` state has to be enabled in the respective /etc/powerdns/pdns.conf config files.
• The nameservers have to be set up correctly as NS domain records i.e. defining a NS and A record for each slave.
• Master/Slave state has to be configured on a per domain basis in the domains table. Namely the type column has to be either MASTER or SLAVE respectively and the slave needs a comma separated list of master node IP addresses in the master column in the domains table. :doc:`more to this topic <backends/generic-sql>`.

If the 'IXFR' zone metadata item is set to 1 for a zone, PowerDNS will attempt to retrieve zone updates via IXFR.

In such cases, make sure to delete the zone contents to force a fresh retrieval.

Finally, IXFR updates that "plug" Empty Non Terminals do not yet remove ENT records. A 'pdnsutil rectify-zone' may be required.

PowerDNS itself is currently only able to retrieve updates via IXFR. It can not serve IXFR updates.

.. versionchanged:: 4.2.0
  Supermaster support needs to be explicitly enabled with the
  :ref:`setting-superslave` setting.

PowerDNS can recognize so called 'supermasters'. A supermaster is a host which is master for domains and for which we are to be a slave. When a master (re)loads a domain, it sends out a notification to its slaves. Normally, such a notification is only accepted if PowerDNS already knows that it is a slave for a domain.

However, a notification from a supermaster carries more persuasion. When PowerDNS determines that a notification comes from a supermaster and it is bonafide, it can provision the domain automatically, and configure itself as a slave for that zone.

Before a supermaster notification succeeds, the following conditions must be met:

• :ref:`setting-superslave` support must be enabled
• The supermaster must carry a SOA record for the notified domain
• The supermaster IP must be present in the 'supermaster' table
• The set of NS records for the domain, as retrieved by the slave from the supermaster, must include the name that goes with the IP address in the supermaster table
• If your master sends signed NOTIFY it will mark that TSIG key as the TSIG key used for retrieval as well
• If you turn off :ref:`setting-allow-unsigned-supermaster`, then your supermaster(s) are required to sign their notifications.

So, to benefit from this feature, a backend needs to know about the IP address of the supermaster, and how PowerDNS will be listed in the set of NS records remotely, and the 'account' name of your supermaster. There is no need to fill the account name out but it does help keep track of where a domain comes from. Adding a supermaster can be done either directly in the database, or by using the 'pdnsutil add-supermaster' command.

The PowerDNS Authoritative Server can invoke a Lua script on an incoming AXFR zone transfer. The user-defined function axfrfilter within your script is invoked for each resource record read during the transfer, and the outcome of the function defines what PowerDNS does with the records.

What you can accomplish using a Lua script: - Ensure consistent values on SOA - Change incoming SOA serial number to a YYYYMMDDnn format - Ensure consistent NS RRset - Timestamp the zone transfer with a TXT record

To enable a Lua script for a particular slave zone, determine the domain_id for the zone from the domains table, and add a row to the domainmetadata table for the domain. Supposing the domain we want has an id of 3, the following SQL statement will enable the Lua script my.lua for that domain:

INSERT INTO domainmetadata (domain_id, kind, content) VALUES (3, "LUA-AXFR-SCRIPT", "/lua/my.lua");

Your Lua functions have access to the query codes through a pre-defined Lua table called pdns. For example if you want to check for a CNAME record you can either compare qtype to the numeric constant 5 or the value pdns.CNAME -- they are equivalent.

If your function decides to handle a resource record it must return a result code of 0 together with a Lua table containing one or more replacement records to be stored in the back-end database (if the table is empty, no record is added). If you want your record(s) to be appended after the matching record, return 1 and table of record(s). If, on the other hand, your function decides not to modify a record, it must return -1 and an empty table indicating that PowerDNS should handle the incoming record as normal.

Consider the following simple example:

function axfrfilter(remoteip, zone, record)

   -- Replace each HINFO records with this TXT
   if record:qtype() == pdns.HINFO then
      resp = {}
      resp[1] = {
        qname   = record:qname():toString(),
        qtype   = pdns.TXT,
        ttl     = 99,
        content = "Hello Ahu!"
     }
      return 0, resp
   end

   -- Grab each _tstamp TXT record and add a time stamp
   if record:qtype() == pdns.TXT and string.starts(record:qname():toString(), "_tstamp.") then
      resp = {}
      resp[1] = {
        qname   = record:qname():toString(),
        qtype   = record:qtype(),
        ttl     = record:ttl(),
        content = os.date("Ver %Y%m%d-%H:%M")
      }
      return 0, resp
   end

   -- Append A records with this TXT
   if record:qtype() == pdns.A then
      resp = {}
      resp[1] = {
        qname   = record:qname():toString(),
        qtype   = pdns.TXT,
        ttl     = 99,
        content = "Hello Ahu, again!"
      }
      return 1, resp
   end

   resp = {}
   return -1, resp
end

function string.starts(s, start)
   return s.sub(s, 1, s.len(start)) == start
end

Upon an incoming AXFR, PowerDNS calls our axfrfilter function for each record. All HINFO records are replaced by a TXT record with a TTL of 99 seconds and the specified string. TXT Records with names starting with _tstamp. get their value (rdata) set to the current time stamp. A records are appended with a TXT record. All other records are unhandled.