Internet Engineering Task Force (IETF) W. Hardaker
Request for Comments: 7477 Parsons, Inc.
Category: Standards Track March 2015
Child-to-Parent Synchronization in DNS
This document specifies how a child zone in the DNS can publish a
record to indicate to a parental agent that the parental agent may
copy and process certain records from the child zone. The existence
of the record and any change in its value can be monitored by a
parental agent and acted on depending on local policy.
Status of This Memo
This is an Internet Standards Track document.
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(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
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Table of Contents
1. Introduction ....................................................21.1. Terminology Used in This Document ..........................32. Definition of the CSYNC RRType ..................................32.1. The CSYNC Resource Record Format ...........................42.1.1. The CSYNC Resource Record Wire Format ...............42.1.2. The CSYNC Presentation Format .......................62.1.3. CSYNC RR Example ....................................63. CSYNC Data Processing ...........................................63.1. Processing Procedure .......................................73.2. CSYNC Record Types .........................................83.2.1. The NS type .........................................83.2.2. The A and AAAA Types ................................94. Operational Considerations ......................................94.1. Error Reporting ...........................................104.2. Child Nameserver Selection ................................104.3. Out-of-Bailiwick NS Records ...............................104.4. Documented Parental Agent Type Support ....................114.5. Removal of the CSYNC Records ..............................114.6. Parent/Child/Grandchild Glue Synchronization ..............125. Security Considerations ........................................126. IANA Considerations ............................................127. References .....................................................137.1. Normative References ......................................137.2. Informative References ....................................14
Author's Address ..................................................151. Introduction
This document specifies how a child zone in the DNS ([RFC1034]
[RFC1035]) can publish a record to indicate to a parental agent (see
Section 1.1 for a definition of "parental agent") that it can copy
and process certain records from the child zone. The existence of
the record and any change in its value can be monitored by a parental
agent and acted on depending on local policy.
Currently, some resource records (RRs) in a parent zone are typically
expected to be in sync with the source data in the child's zone. The
most common records that should match are the nameserver (NS) records
and any necessary associated address records (A and AAAA), also known
as "glue records". These records are referred to as "delegation
It has been challenging for operators of child DNS zones to update
their delegation records within the parent's set in a timely fashion.
These difficulties may stem from operator laziness as well as from
the complexities of maintaining a large number of DNS zones. Having
an automated mechanism for signaling updates will greatly ease the
child zone operator's maintenance burden and improve the robustness
of the DNS as a whole.
This document introduces a new Resource Record Type (RRType) named
"CSYNC" that indicates which delegation records published by a child
DNS operator should be processed by a parental agent and used to
update the parent zone's DNS data.
This specification was not designed to synchronize DNSSEC security
records, such as DS RRsets. For a solution to this problem, see the
complementary solution [RFC7344], which is designed to maintain
security delegation information. In addition, this specification
does not address how to perform bootstrapping operations, including
to get the required initial DNSSEC-secured operating environment in
1.1. Terminology Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Terminology describing relationships between the interacting roles
involved in this document are defined in the following list:
Child: The entity on record that has the delegation of the domain
from the parent
Parent: The domain in which the child is registered
Child DNS operator: The entity that maintains and publishes the zone
information for the child DNS
Parental agent: The entity that the child has relationship with, to
change its delegation information
2. Definition of the CSYNC RRType
The CSYNC RRType contains, in its RDATA component, these parts: an
SOA serial number, a set of flags, and a simple bit-list indicating
the DNS RRTypes in the child that should be processed by the parental
agent in order to modify the DNS delegation records within the
parent's zone for the child DNS operator. Child DNS operators
wanting a parental agent to perform the synchronization steps
outlined in this document MUST publish a CSYNC record at the apex of
the child zone. Parental agent implementations MAY choose to query
child zones for this record and process DNS record data as indicated
by the Type Bit Map field in the RDATA of the CSYNC record. How the
data is processed is described in Section 3.
Parental agents MUST process the entire set of child data indicated
by the Type Bit Map field (i.e., all record types indicated along
with all of the necessary records to support processing of that type)
or else parental agents MUST NOT make any changes to parental records
at all. Errors due to unsupported Type Bit Map bits, or otherwise
nonpunishable data, SHALL result in no change to the parent zone's
delegation information for the child. Parental agents MUST ignore a
child's CSYNC RDATA set if multiple CSYNC resource records are found;
only a single CSYNC record should ever be present.
The parental agent MUST perform DNSSEC validation ([RFC4033]
[RFC4034] [RFC4035]), of the CSYNC RRType data and MUST perform
DNSSEC validation of any data to be copied from the child to the
parent. Parents MUST NOT process any data from any of these records
if any of the validation results indicate anything other than
"Secure" [RFC4034] or if any the required data cannot be successfully
2.1. The CSYNC Resource Record Format
2.1.1. The CSYNC Resource Record Wire Format
The CSYNC RDATA consists of the following fields:
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| SOA Serial |
| Flags | Type Bit Map /
/ Type Bit Map (continued) /
184.108.40.206. The SOA Serial Field
The SOA Serial field contains a copy of the 32-bit SOA serial number
from the child zone. If the soaminimum flag is set, parental agents
querying children's authoritative servers MUST NOT act on data from
zones advertising an SOA serial number less than this value. See
[RFC1982] for properly implementing "less than" logic. If the
soaminimum flag is not set, parental agents MUST ignore the value in
the SOA Serial field. Clients can set the field to any value if the
soaminimum flag is unset, such as the number zero.
Note that a child zone's current SOA serial number may be greater
than the number indicated by the CSYNC record. A child SHOULD update
the SOA Serial field in the CSYNC record every time the data being
referenced by the CSYNC record is changed (e.g., an NS record or
associated address record is changed). A child MAY choose to update
the SOA Serial field to always match the current SOA Serial field.
Parental agents MAY cache SOA serial numbers from data they use and
refuse to process data from zones older than the last instance from
which they pulled data.
Although Section 3.2 of [RFC1982] describes how to properly implement
a less-than comparison operation with SOA serial numbers that may
wrap beyond the 32-bit value in both the SOA record and the CSYNC
record, it is important that a child using the soaminimum flag must
not increment its SOA serial number value more than 2^16 within the
period of time that a parent might wait between polling the child for
the CSYNC record.
220.127.116.11. The Flags Field
The Flags field contains 16 bits of boolean flags that define
operations that affect the processing of the CSYNC record. The flags
defined in this document are as follows:
0x00 0x01: "immediate"
0x00 0x02: "soaminimum"
The definitions for how the flags are to be used can be found in
The remaining flags are reserved for use by future specifications.
Undefined flags MUST be set to 0 by CSYNC publishers. Parental
agents MUST NOT process a CSYNC record if it contains a 1 value for a
flag that is unknown to or unsupported by the parental agent.
18.104.22.168.1. The Type Bit Map Field
The Type Bit Map field indicates the record types to be processed by
the parental agent, according to the procedures in Section 3. The
Type Bit Map field is encoded in the same way as the Type Bit Map
field of the NSEC record, described in [RFC4034], Section 4.1.2. If
a bit has been set that a parental agent implementation does not
understand, the parental agent MUST NOT act upon the record.
Specifically, a parental agent must not simply copy the data, and it
must understand the semantics associated with a bit in the Type Bit
Map field that has been set to 1.
2.1.2. The CSYNC Presentation Format
The CSYNC presentation format is as follows:
The SOA Serial field is represented as an integer.
The Flags field is represented as an integer.
The Type Bit Map field is represented as a sequence of RRType
mnemonics. When the mnemonic is not known, the TYPE
representation described in [RFC3597], Section 5, MUST be used.
Implementations that support parsing of presentation format
records SHOULD be able to read and understand these TYPE
representations as well.
2.1.3. CSYNC RR Example
The following CSYNC RR shows an example entry for "example.com" that
indicates the NS, A, and AAAA bits are set and should be processed by
the parental agent for example.com. The parental agent should pull
data only from a zone using a minimum SOA serial number of 66 (0x42
example.com. 3600 IN CSYNC 66 3 A NS AAAA
The RDATA component of the example CSYNC RR would be encoded on the
wire as follows:
0x00 0x00 0x00 0x42 (SOA Serial)
0x00 0x03 (Flags = immediate | soaminimum)
0x00 0x04 0x60 0x00 0x00 0x08 (Type Bit Map)
3. CSYNC Data Processing
The CSYNC record and associated data must be processed as an "all or
nothing" operation set. If a parental agent fails to successfully
query for any of the required records, the whole operation MUST be
aborted. (Note that a query resulting in "no records exist" as
proven by NSEC or NSEC3 is to be considered successful).
Parental agents MAY:
Process the CSYNC record immediately if the "immediate" flag is
set. If the "immediate" flag is not set, the parental agent MUST
NOT act until the zone administrator approves the operation
through an out-of-band mechanism (such as through pushing a button
via a web interface).
Choose not to process the CSYNC record immediately, even if the
"immediate" flag is set. That is, a parental agent might require
the child zone administrator approve the operation through an out-
of-band mechanism (such as through pushing a button via a web
Note: how the approval is done out of band is outside the scope of
this document and is implementation specific to parental agents.
3.1. Processing Procedure
The following shows a sequence of steps that SHOULD be used when
collecting and processing CSYNC records from a child zone. Because
DNS queries are not allowed to contain more than one "question" at a
time, a sequence of requests is needed. When processing a CSYNC
transaction request, all DNS queries should be sent to a single
authoritative name server for the child zone. To ensure a single
host is being addressed, DNS over TCP SHOULD be used to avoid
conversing with multiple nodes at an anycast address.
1. Query for the child zone's SOA record
2. Query for the child zone's CSYNC record
3. Query for the child zone's data records, as required by the CSYNC
record's Type Bit Map field
* Note: if any of the resulting records being queried are not
authoritative within the child zone but rather in a grandchild
or deeper, SOA record queries must be made for the
grandchildren. This will require the parental agent to
determine where the child/grandchild zone cuts occur. Because
of the additional operational complexity, parental agents MAY
choose not to support this protocol with children making use
of records that are authoritative in the grandchildren.
4. Query for the collected SOA records again, starting with the
deepest and ending with the SOA of the child's.
If the SOA records from the first, middle, and last steps for a given
zone have different serial numbers (for example, because the zone was
edited and republished during the interval between steps 1 and 4),
then the CSYNC record obtained in the second set SHOULD NOT be
processed (rapidly changing child zones may need special
consideration or processing). The operation MAY be restarted or
retried in the future.
If the soaminimum flag is set and the SOA serial numbers are equal
but less than the CSYNC record's SOA Serial field [RFC1982], the
record MUST NOT be processed. If state is being kept by the parental
agent and the SOA serial number is less than the last time a CSYNC
record was processed, this CSYNC record SHOULD NOT be processed.
Similarly, if state is being kept by the parental agent and the SOA
Serial field of the CSYNC record is less than the SOA Serial field of
the CSYNC record from last time, then this CSYNC record SHOULD NOT be
If a failure of any kind occurs while trying to obtain any of the
required data, or if DNSSEC fails to validate all of the data
returned for these queries as "secure", then this CSYNC record MUST
NOT be processed.
See the "Operational Consideration" section (Section 4) for
additional guidance about processing.
3.2. CSYNC Record Types
This document defines how the following record types may be processed
if the CSYNC Type Bit Map field indicates they are to be processed.
3.2.1. The NS type
The NS type flag indicates that the NS records from the child zone
should be copied into the parent's delegation information records for
NS records found within the child's zone should be copied verbatim
(with the exception of the Time to Live (TTL) field, for which the
parent MAY want to select a different value) and the result published
within the parent zone should be a set of NS records that match
exactly. If the child has published a new NS record within their
set, this record should be added to the parent zone. Similarly, if
NS records in the parent's delegation records for the child contain
records that have been removed in the child's NS set, then they
should be removed in the parent's set as well.
Parental agents MAY refuse to perform NS updates if the replacement
records fail to meet NS record policies required by the parent zone
(e.g., "every child zone must have at least two NS records").
Parental agents MUST NOT perform NS updates if there are no NS
records returned in a query, as verified by DNSSEC denial-of-
existence protection. This situation should never happen unless the
child nameservers are misconfigured.
Note that it is permissible for a child's nameserver to return a
CSYNC record that removes the queried nameserver itself from the
future NS or address set.
3.2.2. The A and AAAA Types
The A and AAAA type flags indicates that the A and AAAA address glue
records for in-bailiwick NS records within the child zone should be
copied verbatim (with the exception of the TTL field, for which the
parent MAY want to select a different value) into the parent's
Queries should be sent by the parental agent to determine the A and
AAAA record addresses for each NS record within a NS set for the
child that are in bailiwick.
Note: only the matching types should be queried. For example, if the
AAAA bit has not been set, then the AAAA records (if any) in the
parent's delegation should remain as is. If a given address type is
set and the child's zone contains no data for that type (as proven by
appropriate NSEC or NSEC3 records), then the result in the parent's
delegation records for the child should be an empty set. However, if
the end result of processing would leave no glue records present in
the parent zone for any of the of the in-bailiwick NS records, then
the parent MUST NOT update the glue address records. That is, if the
result of the processing would leave no in-bailiwick A or AAAA
records when there are in-bailiwick NS records, then processing of
the address records cannot happen as it would leave the parent/child
relationship without any address linkage.
The procedure for querying for A and AAAA records MUST occur after
the procedure, if required, for querying for NS records as defined in
Section 3.2.1. This ensures that the right set of NS records is used
as provided by the current NS set of the child. That is, for CSYNC
records that have the NS bit set, the NS set used should be the one
pulled from the child while processing the CSYNC record. For CSYNC
records without the NS bit set, the existing NS records within the
parent should be used to determine which A and/or AAAA records to
4. Operational Considerations
There are a number of important operational aspects to consider when
deploying a CSYNC RRType.
4.1. Error Reporting
There is no inline mechanism for a parental agent to report errors to
operators of child zones. Thus, the only error reporting mechanisms
must be out of band, such as through a web console or over email.
Parental agents should, at a minimum, at least log errors encountered
when processing CSYNC records. Child operators utilizing the
"immediate" flag that fail to see an update within the parental
agent's specified operational window should access the parental
agent's error logging interface to determine why an update failed to
4.2. Child Nameserver Selection
Parental agents will need to poll child nameservers in search of
CSYNC records and related data records.
Parental agents MAY perform best-possible verification by querying
all NS records for available data to determine which has the most
recent SOA and CSYNC version (in an ideal world, they would all be
equal, but this is not possible in practice due to synchronization
delays and transfer failures).
Parental agents may offer a configuration interface to allow child
operators to specify which nameserver should be considered the master
to send data queries, too. Note that this master could be a
different nameserver than the publicly listed nameservers in the NS
set (i.e., it may be a "hidden master").
Parental agents with a large number of clients may choose to offer a
programmatic interface to let their children indicate that new CSYNC
records and data are available for polling rather than polling every
child on a frequent basis.
Children that wish to phase out a nameserver will need to publish the
CSYNC record to remove the nameserver and then wait for the parental
agent to process the published record before turning off the service.
This is required because the child cannot control which nameserver in
the existing NS set the parental agent may choose to query when
performing CSYNC processing.
4.3. Out-of-Bailiwick NS Records
When a zone contains NS records where the domain name pointed at does
not fall within the zone itself, there is no way for the parent to
safely update the associated glue records. Thus, the child DNS
operator MAY indicate that the NS records should be synchronized, and
MAY set any glue record flags (A, AAAA) as well, but the parent will
only update those glue records that are below the child's delegation
Children deploying NS records pointing to domain names within their
own children (the "grandchildren") SHOULD ensure the grandchildren's
associated glue records are properly set before publishing the CSYNC
record. That is, it is imperative that proper communication and
synchronization exist between the child and the grandchild.
4.4. Documented Parental Agent Type Support
Parental agents that support processing CSYNC records SHOULD publicly
document the following minimum processing characteristics:
The fact that they support CSYNC processing
The Type Bit Map bits they support
The frequency with which they poll clients (which may also be
configurable by the client)
If they support the "immediate" flag
If they poll a child's single nameserver, a configured list of
nameservers, or all of the advertised nameservers when querying
If they support SOA serial number caching to avoid issues with
regression and/or replay
Where errors for CSYNC processing are published
If they support sending queries to a "hidden master"
4.5. Removal of the CSYNC Records
Children MAY remove the CSYNC record upon noticing that the parent
zone has published the required records, thus eliminating the need
for the parent to continually query for the CSYNC record and all
corresponding records. By removing the CSYNC record from the child
zone, the parental agent will only need to perform the query for the
CSYNC record and can stop processing when it finds it missing. This
will reduce resource usage by both the child and the parental agent.
4.6. Parent/Child/Grandchild Glue Synchronization
When a child needs to publish a CSYNC record that synchronizes NS and
A/AAAA glue records and the NS record is actually pointing to a child
of the child (a grandchild of the parent), then it is critical that
the glue records in the child point to the proper real addresses
records published by the grandchild. It is assumed that if a child
is using a grandchild's nameserver that they must be in careful
synchronization. Specifically, this specification requires this to
be the case.
5. Security Considerations
This specification requires the use of DNSSEC in order to determine
that the data being updated was unmodified by third parties.
Parental agents implementing CSYNC processing MUST ensure all DNS
transactions are validated by DNSSEC as "secure". Clients deploying
CSYNC MUST ensure their zones are signed, current and properly linked
to the parent zone with a DS record that points to an appropriate
DNSKEY of the child's zone.
This specification does not address how to perform bootstrapping
operations to get the required initial DNSSEC-secured operating
environment in place. Additionally, this specification was not
designed to synchronize DNSSEC security records, such as DS pointers,
or the CSYNC record itself. Thus, implementations of this protocol
MUST NOT use it to synchronize DS records, DNSKEY materials, CDS
records, CDNSKEY records, or CSYNC records. Similarly, future
documents extending this protocol MUST NOT offer the ability to
synchronize DS, DNSKEY materials, CDS records, CDNSKEY records, or
CSYNC records. For such a solution, please see the complimentary
solution [RFC7344] for maintaining security delegation information.
To ensure that an older CSYNC record making use of the soaminimum
flag cannot be replayed to revert values, the SOA serial number MUST
NOT be incremented by more than 2^16 during the lifetime of the
signature window of the associated RRSIGs signing the SOA and CSYNC
records. Note that this is independent of whether or not the
increment causes the 2^32 bit serial number field to wrap.
6. IANA Considerations
This document defines a new DNS Resource Record Type, named "CSYNC".
The IANA has assigned a code point from the "Resource Record (RR)
TYPEs" sub-registry of the "Domain Name System (DNS) Parameters"
registry (http://www.iana.org/assignments/dns-parameters) for this
TYPE Value Meaning Reference
----- ------ -------------------------- -----------
CSYNC 62 Child-to-Parent Synchronization [RFC7477]
The IANA has created and maintains a sub-registry (the "Child
Synchronization (CSYNC) Flags" registry) of the "Domain Name System
(DNS) Parameters" registry. The initial values for this registry are
A "Standards Action" [RFC5226] is required for the assignment of new
This registry holds a set of single-bit "Flags" for use in the CSYNC
record within the 16-bit Flags field. Thus, a maximum of 16 flags
may be defined.
The initial assignments in this registry are:
Bit Flag Description Reference
---- ------ ------------- -----------
Bit 0 immediate Immediately process this [RFC7477],
CSYNC record. Section 3
Bit 1 soaminimum Require a SOA serial [RFC7477],
number greater than the Section 22.214.171.124
7.1. Normative References
[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
August 1996, <http://www.rfc-editor.org/info/rfc1982>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, September 2003,
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005,
A thank you goes out to Warren Kumari and Olafur Gudmundsson, whose
work on the CDS record type helped inspire the work in this document,
as well as the definition for the "parental agent" definition and
significant contributions to the text. A thank you also goes out to
Ed Lewis, with whom the author held many conversations about the
issues surrounding parent/child relationships and synchronization.
Much of the work in this document is derived from the careful
existing analysis of these three esteemed colleagues. Thank you to
the following people who have contributed text or detailed reviews to
the document (in no particular order): Matthijs Mekking, Petr Spacek,
JINMEI Tatuya, Pete Resnick, Joel Jaeggli, Brian Haberman, Warren
Kumari, Adrian Farrel, Alia Atlas, Barry Leiba, Richard Barnes,
Stephen Farrell, and Ted Lemon. Lastly, the DNSOP WG chairs Tim
Wicinski and Suzanne Woolf have been a tremendous help in getting
this document moving forward to publication.
A special thanks goes to Roy Arends, for taking the "bite out of that
hamburger" challenge while discussing this document.
A similar project, independently designed and developed, was
conducted by ep.net called "Child Activated DNS Refresh".
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