Provided by: bind9-utils_9.18.30-0ubuntu0.22.04.2_amd64 
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 (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, :option`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.
-i extension
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
dig(1), dnssec-settime(8), dnssec-signzone(8), nsupdate(1), BIND 9 Administrator Reference Manual, RFC
7344.
AUTHOR
Internet Systems Consortium
COPYRIGHT
2025, Internet Systems Consortium