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FormErr

Format Error

Core RCODERFC 1035

FormErr (1) means the DNS server could not interpret the query. The query message is malformed – the format does not comply with the DNS message format specification. This indicates a client-side bug in the DNS query construction, not a server or name issue.

RCODE

1

Name

FormErr

Standard

RFC 1035

Description

FORMERR is returned when the DNS server cannot parse the query message. This can be caused by: a malformed DNS message header, invalid question section, unsupported or malformed EDNS0 option, or a query that violates the DNS wire format. FORMERR is rare with well-implemented DNS clients but can appear when custom DNS implementations have bugs, or when EDNS0 options are not correctly encoded. Stub resolvers rarely see FORMERR because the OS DNS library handles query construction.

Common Causes

  • –DNS query message violates wire format specification
  • –Invalid or malformed EDNS0 option in query
  • –Question section malformed (invalid QTYPE or QCLASS)
  • –Custom DNS client with encoding bug
RFC 1035 – DNS RCODE specification →