What Is an SOA Record

Learn what an SOA (Start of Authority) record is, what each field means, and how it controls zone transfers, caching, and DNS authority for your domain.

An SOA (Start of Authority) record is a mandatory DNS record that exists at the top of every DNS zone. It defines key information about the zone, including which name server is the primary authority, the contact email of the zone administrator, and timing parameters that control how secondary DNS servers synchronize with the primary.

SOA Record Format

Every SOA record contains seven fields:

example.com.  86400  IN  SOA  ns1.dnscale.eu. admin.example.com. (
                              2026030901  ; Serial number
                              3600        ; Refresh (1 hour)
                              900         ; Retry (15 minutes)
                              1209600     ; Expire (2 weeks)
                              300         ; Minimum TTL (5 minutes)
                              )

Field Breakdown

Field	Example	Description
MNAME	`ns1.dnscale.eu.`	Primary (master) name server for the zone
RNAME	`admin.example.com.`	Admin email — the first `.` replaces `@` (i.e., `admin@example.com`)
Serial	`2026030901`	Version number — incremented on every change
Refresh	`3600`	How often (seconds) secondary servers check for updates
Retry	`900`	How long secondaries wait before retrying a failed refresh
Expire	`1209600`	How long secondaries serve stale data if the primary is unreachable
Minimum TTL	`300`	Default TTL for negative responses (NXDOMAIN caching)

How the SOA Record Works

Zone Authority

The SOA record declares which name server is the primary authority for a zone. When DNS resolvers need to find the authoritative answer for a domain, the SOA tells them where to look.

Zone Transfers

Secondary DNS servers use SOA fields to decide when to synchronize:

Secondary checks the primary at the refresh interval
It compares the serial number — if it's higher, the zone has changed
The secondary initiates a zone transfer (AXFR or IXFR) to get the latest data
If the primary is unreachable, the secondary retries at the retry interval
If the primary stays unreachable past the expire time, the secondary stops serving the zone

Negative Caching

The minimum TTL field controls how long resolvers cache negative responses. When someone queries a name that doesn't exist, the NXDOMAIN answer is cached for this duration, reducing unnecessary queries to your authoritative servers.

Serial Number Conventions

The serial number must increase with every zone change. Two common formats:

Date-Based (Recommended)

YYYYMMDDNN
2026030901  →  2026-03-09, change #01
2026030902  →  2026-03-09, change #02

This format is human-readable and supports up to 99 changes per day.

Simple Incrementing

1, 2, 3, 4, ...

Simpler but gives no indication of when changes were made. DNScale automatically manages serial numbers for you when you create or update records.

Recommended SOA Values

Field	Recommended	Use Case
Refresh	3600 (1h)	Standard zones
Refresh	300 (5min)	Frequently changing zones
Retry	900 (15min)	Standard — gives time for transient issues
Expire	1209600 (2w)	Standard — secondaries serve data for up to 2 weeks
Minimum TTL	300 (5min)	Good balance of caching vs. responsiveness
Minimum TTL	60 (1min)	Dynamic environments where names change often

Querying SOA Records

Check a domain's SOA record using dig:

dig SOA example.com
 
# Output:
# example.com.  86400  IN  SOA  ns1.dnscale.eu. admin.example.com. (
#                              2026030901 3600 900 1209600 300 )

Query a specific nameserver:

dig SOA example.com @ns1.dnscale.eu

Compare serial numbers across nameservers to verify synchronization:

dig +short SOA example.com @ns1.dnscale.eu
dig +short SOA example.com @ns2.dnscale.eu

If the serial numbers match, your secondary is in sync.

SOA and DNScale

DNScale automatically manages the SOA record for every zone you create:

Serial numbers are incremented automatically when you add, update, or delete records
MNAME is set to the DNScale primary nameserver
Timing parameters use sensible defaults optimized for reliability
Zone transfers to edge nodes are handled automatically via PostgreSQL replication, ensuring faster propagation than traditional AXFR-based transfers

Common Issues

Serial Number Not Incrementing

If secondaries aren't picking up changes, the serial number may not have been updated. DNScale handles this automatically, but if you manage zones manually, always increment the serial.

Expire Too Short

If your expire value is too low and the primary goes down, secondaries will stop serving your zone quickly. Use at least 1 week (604800 seconds).

Negative TTL Too High

A high minimum TTL means that if you create a new record, users who previously got an NXDOMAIN response will continue to see "not found" until the negative cache expires. Keep this at 300 seconds or lower for active zones.

NS Records — define which nameservers are authoritative for a zone
What Is a DNS Zone — understand zones and their structure
DNS Record Types — overview of all DNS record types
System Records — why SOA and apex NS records are protected from modification

Conclusion

The SOA record is the control center of every DNS zone. Its fields govern zone transfers, caching behavior, and authority delegation. While DNScale manages SOA records automatically, understanding the SOA helps you troubleshoot synchronization issues and optimize your DNS infrastructure.