Skip to content
Control Plane Labs
Search

CSYNC record

Signals that a child zone's parent may synchronize selected NS, A, and AAAA delegation data from the child.

Written and maintained by Ben Ennis

Last reviewed July 27, 2026 · How we verify this

Zone-file example

example.com. 3600 IN CSYNC 66 3 A NS AAAA

Typical uses

  • Synchronizing child nameserver records with a parent zone
  • Updating in-bailiwick A and AAAA glue after a controlled nameserver change
  • Giving a parental agent a versioned, DNSSEC-validated delegation update request
  • Replacing slow manual coordination with an auditable child-published signal

When it breaks, check

  • CSYNC belongs at the child zone apex and its SOA serial must reflect the requested data when soaminimum is set
  • The operation is all-or-nothing; an inconsistent SOA or failed required query must stop the parent update
  • CSYNC does not synchronize DS, DNSKEY, CDS, CDNSKEY, or CSYNC records
  • A parent may require out-of-band approval even when the immediate flag is present

What the CSYNC record does

CSYNC is a child-to-parent coordination record. It is published at the child apex and tells a parental agent which delegation-related types to process. Flag 1, immediate, permits immediate processing; flag 2, soaminimum, requires the child SOA serial to meet the serial carried in the record. The type bitmap identifies the requested NS, A, or AAAA data.

The protocol is deliberately conservative. The parent must query a consistent authoritative view, validate DNSSEC, and either process the complete set or make no change. It cannot be used to synchronize DS, DNSKEY, CDS, CDNSKEY, or CSYNC itself. For a live record check, use the DNS lookup tool and compare the child’s SOA serial.

Zone-file and wire format

CSYNC RDATA is SOA-serial flags type-bitmap. The serial is a 32-bit value, flags are a 16-bit integer, and the type bitmap uses the encoding defined for NSEC. In zone-file presentation the bitmap is written with RRType mnemonics, such as A NS AAAA. Undefined flags must be zero, and an unknown set flag makes the parental agent ignore the request.

; immediate | soaminimum, synchronizing nameservers and glue
example.com. 3600 IN CSYNC 66 3 A NS AAAA

Common uses

CSYNC is most useful to registries, registrars, and DNS operators that can run a parental agent and need controlled nameserver or glue updates. It does not replace DNSSEC validation, provider-specific approval, or transfer authentication. If the problem is zone content drift rather than delegation state, read about ZONEMD instead.

Troubleshooting

Start by querying the child SOA, CSYNC, and every type named by its bitmap from one authoritative server. Repeat the SOA checks after collecting the data; if the serial changed mid-transaction, the parent should abort. Confirm the zone is DNSSEC-signed and that the parent has a validated answer.

For nameserver changes, compare the child NS set with the parent and make sure in-bailiwick glue still leaves at least one usable address. A missing update may be an approval or polling delay rather than a malformed record; consult the parental agent’s out-of-band logs.

dig example.com SOA +dnssec +noall +answer
dig example.com CSYNC +dnssec +noall +answer

# Inspect the child data named by the bitmap.
dig @ns1.example.net example.com NS +norecurse +dnssec +noall +answer
dig @ns1.example.net example.com A +norecurse +dnssec +noall +answer
dig @ns1.example.net example.com AAAA +norecurse +dnssec +noall +answer

The defining RFC

CSYNC is DNS type 62, defined by RFC 7477. The RFC defines the serial, flags, and NSEC-style type bitmap, requires DNSSEC validation, and limits processing to delegation-related NS, A, and AAAA data. The IANA DNS Parameters registry records the assigned type.

Reference and tooling

Every record type and its assigned numeric value is listed in theIANA DNS Parameters registry. To query a live zone from the browser, use theDNS lookup tool. If the record you are chasing is TLS-related, the TLS inspector andCertificate Transparency lookup are usually the next two stops.

Other record types