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Control Plane Labs

A record

Maps a hostname to a single IPv4 address.

Last updated July 27, 2026

Zone-file example

; type 1, defined in RFC 1035
www.example.com.    300    IN    A    203.0.113.10
example.com.        300    IN    A    203.0.113.10
example.com.        300    IN    A    203.0.113.11    ; round-robin second answer

Typical uses

  • Pointing a hostname at a web server, load balancer, or VIP
  • Publishing several addresses for crude round-robin distribution
  • Zone apex records, where CNAME is not allowed
  • Glue records that give a nameserver's own address

When it breaks, check

  • A stale answer almost always means TTL, not a bad zone — check the remaining TTL in a dig answer
  • Multiple A records are unordered; clients pick their own, so do not use them for failover
  • A missing AAAA alongside A is fine, but a broken AAAA will make dual-stack clients appear to hang
  • Confirm you are querying the authoritative server, not a resolver cache

What the A record does

The A record answers the only question most clients care about: what address do I connect to. The value is a single IPv4 address stored as four bytes, and a name can carry several A records at once. That plurality is where most misunderstandings start. A name with three A records is not a load balancer and is not failover; the resolver returns all three, usually in rotating order, and the client picks one by its own rules. If the address it picks is down, most clients retry another, but the timing is entirely up to the client, and browsers, curl, and Java runtimes each differ.

Zone-file and wire format

In a zone file an A record is name TTL class A address, and the class is almost always IN. On the wire the RDATA is exactly four octets in network byte order, which is why an A record can never hold an IPv6 address or a hostname. A trailing dot makes the name fully qualified; without it the zone’s origin is appended, which is the classic way to create www.example.com.example.com.

; name              TTL   class type  rdata
www.example.com.     300   IN    A     203.0.113.10
api.example.com.     60    IN    A     203.0.113.20
example.com.         300   IN    A     203.0.113.10

; short TTLs are for records you expect to move; 300s is a common floor
; because many resolvers ignore anything lower in practice.

Common uses

Most A records point at a load balancer or a CDN edge rather than a single origin, because changing DNS is slow and changing a load balancer target is instant. The one place you are forced to use an A record is the zone apex: example.com itself cannot be a CNAME, so if your provider only gives you a hostname you either need an A record with an address you control, or a provider-specific ALIAS record. Glue records in a parent zone are also A records, supplying the address of a nameserver so resolution can bootstrap.

Troubleshooting

When a change has not taken effect, measure rather than guess. Query the authoritative nameserver directly for the intended answer, then query a public resolver for what the world sees. A decreasing TTL on the resolver answer tells you exactly how many seconds are left before it refetches.

Dual-stack surprises are the other recurring problem. If a name has both A and AAAA records and the IPv6 path is broken, clients implementing Happy Eyeballs (RFC 8305) will still connect, but with added latency; clients that do not will appear to hang. Check both families, not just the one you expect to be used.

# what the authoritative server intends
dig +norecurse @ns1.example.com www.example.com A

# what a public resolver currently has cached, with the remaining TTL
dig @1.1.1.1 www.example.com A +noall +answer

# check both address families before blaming the application
dig www.example.com A +short
dig www.example.com AAAA +short

The defining RFC

The A record is type 1 and is defined in RFC 1035 §3.4.1, part of the original DNS specification, which remains an Internet Standard. The rules for how names are composed and how a resolver follows them are in the companion document RFC 1034. RFC 1912 collects the common configuration errors and is still accurate three decades on. Query a live zone with the DNS lookup tool.

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