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.. _man_dnssec-cds:

dnssec-cds - change DS records for a child zone based on CDS/CDNSKEY
--------------------------------------------------------------------

Synopsis
~~~~~~~~

:program:`dnssec-cds` [**-a** alg...] [**-c** class] [**-D**] {**-d** dsset-file} {**-f** child-file} [**-i**[extension]] [**-s** start-time] [**-T** ttl] [**-u**] [**-v** level] [**-V**] {domain}

Description
~~~~~~~~~~~

The ``dnssec-cds`` command changes DS records at a delegation point
based on CDS or CDNSKEY records published in the child zone. If both CDS
and CDNSKEY records are present in the child zone, the CDS is preferred.
This enables a child zone to inform its parent of upcoming changes to
its key-signing keys (KSKs); by polling periodically with ``dnssec-cds``, the
parent can keep the DS records up-to-date and enable automatic rolling
of KSKs.

Two input files are required. The ``-f child-file`` option specifies a
file containing the child's CDS and/or CDNSKEY records, plus RRSIG and
DNSKEY records so that they can be authenticated. The ``-d path`` option
specifies the location of a file containing the current DS records. For
example, this could be a ``dsset-`` file generated by
``dnssec-signzone``, or the output of ``dnssec-dsfromkey``, or the
output of a previous run of ``dnssec-cds``.

The ``dnssec-cds`` command uses special DNSSEC validation logic
specified by :rfc:`7344`. It requires that the CDS and/or CDNSKEY records
be validly signed by a key represented in the existing DS records. This
is typically the pre-existing KSK.

For protection against replay attacks, the signatures on the child
records must not be older than they were on a previous run of
``dnssec-cds``. Their age is obtained from the modification time of the
``dsset-`` file, or from the ``-s`` option.

To protect against breaking the delegation, ``dnssec-cds`` ensures that
the DNSKEY RRset can be verified by every key algorithm in the new DS
RRset, and that the same set of keys are covered by every DS digest
type.

By default, replacement DS records are written to the standard output;
with the ``-i`` option the input file is overwritten in place. The
replacement DS records are the same as the existing records, when no
change is required. The output can be empty if the CDS/CDNSKEY records
specify that the child zone wants to be insecure.

.. warning::

   Be careful not to delete the DS records when ``dnssec-cds`` fails!

Alternatively, ``dnssec-cds -u`` writes an ``nsupdate`` script to the
standard output. The ``-u`` and ``-i`` options can be used together to
maintain a ``dsset-`` file as well as emit an ``nsupdate`` script.

Options
~~~~~~~

``-a algorithm``
   When converting CDS records to DS records, this option specifies
   the acceptable digest algorithms. This option can be repeated, so
   that multiple digest types are allowed. If none of the CDS records
   use an acceptable digest type, ``dnssec-cds`` will try to use CDNSKEY
   records instead; if there are no CDNSKEY records, it reports an error.

   When converting CDNSKEY records to DS records, this option specifies the
   digest algorithm to use. It can be repeated, so that multiple DS records
   are created for each CDNSKEY records.

   The algorithm must be one of SHA-1, SHA-256, or SHA-384. These values
   are case-insensitive, and the hyphen may be omitted. If no algorithm
   is specified, the default is SHA-256 only.

``-c class``
   This option specifies the DNS class of the zones.

``-D``
   This option generates DS records from CDNSKEY records if both CDS and CDNSKEY
   records are present in the child zone. By default CDS records are
   preferred.

``-d path``
   This specifies the location of the parent DS records. The path can be the name of a file
   containing the DS records; if it is a directory, ``dnssec-cds``
   looks for a ``dsset-`` file for the domain inside the directory.

   To protect against replay attacks, child records are rejected if they
   were signed earlier than the modification time of the ``dsset-``
   file. This can be adjusted with the ``-s`` option.

``-f child-file``
   This option specifies the file containing the child's CDS and/or CDNSKEY records, plus its
   DNSKEY records and the covering RRSIG records, so that they can be
   authenticated.

   The examples below describe how to generate this file.

``-iextension``
   This option updates the ``dsset-`` file in place, instead of writing DS records to
   the standard output.

   There must be no space between the ``-i`` and the extension. If
   no extension is provided, the old ``dsset-`` is discarded. If an
   extension is present, a backup of the old ``dsset-`` file is kept
   with the extension appended to its filename.

   To protect against replay attacks, the modification time of the
   ``dsset-`` file is set to match the signature inception time of the
   child records, provided that it is later than the file's current
   modification time.

``-s start-time``
   This option specifies the date and time after which RRSIG records become
   acceptable. This can be either an absolute or a relative time. An
   absolute start time is indicated by a number in YYYYMMDDHHMMSS
   notation; 20170827133700 denotes 13:37:00 UTC on August 27th, 2017. A
   time relative to the ``dsset-`` file is indicated with ``-N``, which is N
   seconds before the file modification time. A time relative to the
   current time is indicated with ``now+N``.

   If no start-time is specified, the modification time of the
   ``dsset-`` file is used.

``-T ttl``
   This option specifies a TTL to be used for new DS records. If not specified, the
   default is the TTL of the old DS records. If they had no explicit TTL,
   the new DS records also have no explicit TTL.

``-u``
   This option writes an ``nsupdate`` script to the standard output, instead of
   printing the new DS reords. The output is empty if no change is
   needed.

   Note: The TTL of new records needs to be specified: it can be done in the
   original ``dsset-`` file, with the ``-T`` option, or using the
   ``nsupdate`` ``ttl`` command.

``-V``
   This option prints version information.

``-v level``
   This option sets the debugging level. Level 1 is intended to be usefully verbose
   for general users; higher levels are intended for developers.

``domain``
   This indicates the name of the delegation point/child zone apex.

Exit Status
~~~~~~~~~~~

The ``dnssec-cds`` command exits 0 on success, or non-zero if an error
occurred.

If successful, the DS records may or may not need to be
changed.

Examples
~~~~~~~~

Before running ``dnssec-signzone``, ensure that the delegations
are up-to-date by running ``dnssec-cds`` on every ``dsset-`` file.

To fetch the child records required by ``dnssec-cds``, invoke
``dig`` as in the script below. It is acceptable if the ``dig`` fails, since
``dnssec-cds`` performs all the necessary checking.

::

   for f in dsset-*
   do
       d=${f#dsset-}
       dig +dnssec +noall +answer $d DNSKEY $d CDNSKEY $d CDS |
       dnssec-cds -i -f /dev/stdin -d $f $d
   done

When the parent zone is automatically signed by ``named``,
``dnssec-cds`` can be used with ``nsupdate`` to maintain a delegation as follows.
The ``dsset-`` file allows the script to avoid having to fetch and
validate the parent DS records, and it maintains the replay attack
protection time.

::

   dig +dnssec +noall +answer $d DNSKEY $d CDNSKEY $d CDS |
   dnssec-cds -u -i -f /dev/stdin -d $f $d |
   nsupdate -l

See Also
~~~~~~~~

:manpage:`dig(1)`, :manpage:`dnssec-settime(8)`, :manpage:`dnssec-signzone(8)`, :manpage:`nsupdate(1)`, BIND 9 Administrator
Reference Manual, :rfc:`7344`.
