.. SPDX-License-Identifier: GPL-3.0-or-later

Forwarding
----------

*Forwarding* configuration instructs resolver to forward cache-miss queries from clients to manually specified DNS resolvers *(upstream servers)*. In other words the *forwarding* mode does exact opposite of the default *recursive* mode because resolver in *recursive* mode automatically selects which servers to ask.

Main use-cases are:

  - Building a tree structure of DNS resolvers to improve performance (by improving cache hit rate).
  - Accessing domains which are not available using recursion (e.g. if internal company servers return different answers than public ones).
  - Forwarding through a central DNS traffic filter.

Forwarding implementation in Knot Resolver has following properties:

  - Answers from *upstream* servers are cached.
  - Answers from *upstream* servers are locally DNSSEC-validated, unless :func:`policy.STUB` is used.
  - Resolver automatically selects which IP address from given set of IP addresses will be used (based on performance characteristics).
  - Forwarding can use either unencrypted DNS protocol, or :ref:`tls-forwarding`.

.. warning::

        We strongly discourage use of "fake top-level domains" like ``corp.`` because these made-up domains are indistinguishable from an attack, so DNSSEC validation will prevent such domains from working. If you *really* need a variant of forwarding which does not DNSSEC-validate received data please see chapter :ref:`dns-graft`. In long-term it is better to migrate data into a legitimate, properly delegated domains which do not suffer from these security problems.


Simple examples for **unencrypted** forwarding:

.. code-block:: lua

        -- forward all traffic to specified IP addresses (selected automatically)
        policy.add(policy.all(policy.FORWARD({'2001:db8::1', '192.0.2.1'})))

        -- forward only queries for names under domain example.com to a single IP address
        policy.add(policy.suffix(policy.FORWARD('192.0.2.1'), {todname('example.com.')}))

To configure encrypted version please see chapter :ref:`tls-forwarding`.

Forwarding is documented in depth together with rest of :ref:`mod-policy`.