dnssec-cds
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## Name

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

## Synopsis

`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; 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 are validly signed by a key represented in the existing DS records. This will typicially be the pre-existing key-signing key (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. This time 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 will be 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 go 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. You can use the `-u` and `-i` options together to maintain a `dsset-` file as well as emit an nsupdate script.

## OPTIONS

-a `algorithm`

  Specify a digest algorithm to use when converting CDNSKEY records to DS records. This option can be repeated, so that multiple DS records are created for each CDNSKEY record. This option has no effect when using CDS records.

  The `algorithm` must be one of SHA-1 (SHA1), SHA-256 (SHA256), GOST, or SHA-384 (SHA384). These values are case insensitive. If no algorithm is specified, the default is SHA-256.

-c `class`

  Specifies the DNS class of the zones.

-D

  Generate 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`

  Location of the parent DS records. The `path` can be the name of a file containing the DS records, or 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`

  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.

-i[`extension`]

  Update 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 you provide no `extension` then 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 is later than the file's current modification time.

-s `start-time`

  Specify the date and time after which RRSIG records become acceptable. This can be either an absolute or 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`

  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 then the new DS records also have no explicit TTL.

-u

  Write an nsupdate script to the standard output, instead of printing the new DS reords. The output will be empty if no change is needed.

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

-V

  Print version information.

-v `level`

  Sets the debugging level. Level 1 is intended to be usefully verbose for general users; higher levels are intended for developers.

`domain`

  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.

In the success case, the DS records might or might not need to be changed.

## EXAMPLES

Before running dnssec-signzone, you can 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 you can invoke dig as in the script below. It's okay 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, you can use dnssec-cds 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 keeps 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

dig(1) , dnssec-settime(8) , dnssec-signzone(8) , nsupdate(1) , BIND 9 Administrator Reference Manual, RFC 7344.

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