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# `_dns` – DNS resolution utilities

This module contains a few experimental utilities to interact with the DNS server before performing a connection.

Warning

This module is experimental and its interface could change in the future, without warning or respect for the version scheme. It is provided here to allow experimentation before making it more stable.

Warning

This module depends on the dnspython package. The package is currently not installed automatically as a Psycopg dependency and must be installed manually:

    $ pip install "dnspython >= 2.1"

psycopg._dns.resolve_srv(params)

  Apply SRV DNS lookup as defined in RFC 2782.

  Parameters:

    params (`dict`) – The input parameters, for instance as returned by `conninfo_to_dict()`.

  Returns:

    An updated list of connection parameters.

  For every host defined in the `params["host"]` list (comma-separated), perform SRV lookup if the host is in the form `_Service._Proto.Target`. If lookup is successful, return a params dict with hosts and ports replaced with the looked-up entries.

  Raise `OperationalError` if no lookup is successful and no host (looked up or unchanged) could be returned.

  In addition to the rules defined by RFC 2782 about the host name pattern, perform SRV lookup also if the the port is the string `SRV` (case insensitive).

  Warning

  This is an experimental functionality.

  Note

  One possible way to use this function automatically is to subclass `Connection`, extending the `_get_connection_params()` method:

      import psycopg._dns  # not imported automatically

      class SrvCognizantConnection(psycopg.Connection):
          @classmethod
          def _get_connection_params(cls, conninfo, **kwargs):
              params = super()._get_connection_params(conninfo, **kwargs)
              params = psycopg._dns.resolve_srv(params)
              return params

      # The name will be resolved to db1.example.com
      cnn = SrvCognizantConnection.connect("host=_postgres._tcp.db.psycopg.org")

async psycopg._dns.resolve_srv_async(params)

  Async equivalent of `resolve_srv()`.

classmethod Connection._get_connection_params(conninfo: str, **kwargs: Any) → dict[str, str | int | None]

  Manipulate connection parameters before connecting.

  Warning

  This is an experimental method.

  This method is a subclass hook allowing to manipulate the connection parameters before performing the connection. Make sure to call the `super()` implementation before further manipulation of the arguments:

      @classmethod
      def _get_connection_params(cls, conninfo, **kwargs):
          params = super()._get_connection_params(conninfo, **kwargs)
          # do something with the params
          return params

async classmethod AsyncConnection._get_connection_params(conninfo: str, **kwargs: Any) → dict[str, str | int | None]

  Manipulate connection parameters before connecting.

  Warning

  This is an experimental method.

async psycopg._dns.resolve_hostaddr_async(params)

  Perform async DNS lookup of the hosts and return a new params dict.

  Deprecated since version 3.1: The use of this function is not necessary anymore, because `psycopg.AsyncConnection.connect()` performs non-blocking name resolution automatically.

  Parameters:

    params (`dict`) – The input parameters, for instance as returned by `conninfo_to_dict()`.

  If a `host` param is present but not `hostname`, resolve the host addresses dynamically.

  The function may change the input `host`, `hostname`, `port` to allow connecting without further DNS lookups, eventually removing hosts that are not resolved, keeping the lists of hosts and ports consistent.

  Raise `OperationalError` if connection is not possible (e.g. no host resolve, inconsistent lists length).

  See the PostgreSQL docs for explanation of how these params are used, and how they support multiple entries.

  Warning

  Before psycopg 3.1, this function doesn’t handle the `/etc/hosts` file.

  Note

  Starting from psycopg 3.1, a similar operation is performed automatically by `AsyncConnection._get_connection_params()`, so this function is unneeded.

  In psycopg 3.0, one possible way to use this function automatically is to subclass `AsyncConnection`, extending the `_get_connection_params()` method:

      import psycopg._dns  # not imported automatically

      class AsyncDnsConnection(psycopg.AsyncConnection):
          @classmethod
          async def _get_connection_params(cls, conninfo, **kwargs):
              params = await super()._get_connection_params(conninfo, **kwargs)
              params = await psycopg._dns.resolve_hostaddr_async(params)
              return params