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DNS & NetworkingIETF RFC 1035 Section 4.1.1 / RFC 6895 / IANA DNS RCODE Registry✓ クイック検索リファレンス表 (6 項目) • JA

DNSレスポンスコード（RCODE）リファレンス

NOERROR, NXDOMAIN, SERVFAIL, REFUSEDの障害診断とRFC仕様

CT
Curious-Techie Engineering Team認証著者

Web Protocols & Security Architecture Research

公開日:
•
更新日:
•5 min read
準拠仕様:RFC 9110(IETF)
査読者: Curious-Techie Architecture Peer Review Group

概要と適用範囲

RFC 1035およびRFC 6895で標準化されたRCODEは、DNSクエリの成否やネームサーバー障害の原因を診断するための核心的な手掛かりです。

仕様一覧表＆マトリクス

キーワード、コード番号、またはディレクティブ名で即時検索

キー / コード定義と詳細GEOセマンティック階層仕様と補足
RCODE 0
NOERROR (RCODE 0)

No error condition; the query completed successfully and answer records may be present.

status: NOERROR
●NOERROR
↳DNS RCODE
↳RCODE 0
↳successful resolution
RFC 1035 §4.1.1

A NOERROR response with 0 answers (NODATA) indicates the domain exists, but has no records for the requested type (e.g. asking for AAAA on an IPv4-only site).

RCODE 1
FORMERR (RCODE 1)

Format Error; the name server was unable to interpret the query due to malformed packet syntax.

status: FORMERR
●FORMERR
↳DNS RCODE
↳RCODE 1
↳malformed query packet
RFC 1035 §4.1.1

Rare; occurs when custom client network code emits corrupted UDP DNS packets.

RCODE 2
SERVFAIL (RCODE 2)

Server Failure; the resolver was unable to process the query due to a problem with the authoritative nameservers.

status: SERVFAIL
●SERVFAIL
↳DNS RCODE
↳RCODE 2
↳nameserver failure or DNSSEC bogus
RFC 1035 §4.1.1

Common causes: authoritative nameservers unreachable, network timeouts, or DNSSEC cryptographic validation failure.

RCODE 3
NXDOMAIN (RCODE 3)

Non-Existent Domain; authoritative nameserver asserts that the requested domain name does not exist.

status: NXDOMAIN
●NXDOMAIN
↳DNS RCODE
↳RCODE 3
↳authoritative non-existence
RFC 1035 §4.1.1

Cached according to the minimum TTL in the SOA record; common for typos, expired domains, or missing subdomains.

RCODE 4
NOTIMP (RCODE 4)

Not Implemented; the name server does not support the requested query opcode.

status: NOTIMP
●NOTIMP
↳DNS RCODE
↳RCODE 4
↳unsupported query operation
RFC 1035 §4.1.1

Triggered when requesting obsolete or experimental DNS opcodes (e.g., IQUERY).

RCODE 5
REFUSED (RCODE 5)

Refused; the name server refuses to perform the specified operation for policy or security reasons.

status: REFUSED
●REFUSED
↳DNS RCODE
↳RCODE 5
↳access policy refusal
RFC 1035 §4.1.1

Occurs when asking a closed enterprise resolver for external recursive lookups (recursion disabled).

技術的な具体例とコード

Inspecting DNS RCODE in Terminal

bashコードをコピー
$ dig api.example.com
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 48210
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1

# Note status: NXDOMAIN indicates the domain was not found.
# If status was SERVFAIL, test without DNSSEC validation:
$ dig api.example.com +cdflag

The +cdflag (Checking Disabled) tells the resolver to bypass DNSSEC validation. If +cdflag succeeds, your DNSSEC keys are out of sync.

よくある質問（FAQ）

規格標準 FAQ

What is the difference between NXDOMAIN and NODATA?↓
NXDOMAIN (RCODE 3) means the domain itself does not exist. NODATA means the domain exists (status: NOERROR), but no records match the requested type (e.g. asking for AAAA when only A exists).
Authoritative Sourcing & Verification

References & Primary Sources

Every source below directly substantiates technical statements and protocol behaviors explained in DNSレスポンスコード（RCODE）リファレンス. Curious-Techie prioritizes official Internet standards (IETF RFCs, W3C Recommendations, WHATWG specifications) and established security bodies (OWASP, NIST). Zero sponsored or secondary links.

  1. [1]IETFPrimary SpecificationRFC 1034 • Section 3 & 4
    Verified Primary Source
    RFC 1034: Domain Names — Concepts and Facilities
    Technical Claim Supported:

    Establishes the hierarchical distributed domain name space, authoritative name servers, root zone topology, and zone delegations.

    Source URI: https://datatracker.ietf.org/doc/html/rfc1034
  2. [2]IETFPrimary SpecificationRFC 1035 • Section 3.2 (Resource Records)
    Verified Primary Source
    RFC 1035: Domain Names — Implementation and Specification
    Technical Claim Supported:

    Defines standard DNS Resource Record wire structures (A, CNAME, MX, NS, PTR, SOA) and port 53 UDP/TCP protocol formats.

    Source URI: https://datatracker.ietf.org/doc/html/rfc1035
  3. [3]IETFPrimary SpecificationRFC 8484 • Section 3 & 4
    Verified Primary Source
    RFC 8484: DNS Queries over HTTPS (DoH)
    Technical Claim Supported:

    Standardizes encrypted DNS query transmission inside HTTP/2 and HTTP/3 HTTPS streams to prevent ISP eavesdropping and on-path manipulation.

    Source URI: https://datatracker.ietf.org/doc/html/rfc8484
Semantic Internal Links (GEO)

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関連する技術Q&A

直接の回答（AEO要約）
Q&A

What Happens When a DNS TTL Expires?

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Answer: When a DNS TTL expires, caching resolvers evict the record and must re-query the authoritative nameserver before responding to clients.

Q&A

Can a CNAME Record Point to an IP Address?

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Answer: No, RFC 1034 requires CNAME records to point exclusively to another domain name, never to raw IPv4 or IPv6 addresses.

Q&A

Why Can't a CNAME Record Coexist With Other Records at the Zone Apex?

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Answer: RFC 1034 requires that if a CNAME exists at a node, no other record data may exist there. Since a zone apex must have SOA and NS records, CNAME is prohibited.

実践ハンズオン診断ラボ

対話型ワークベンチ
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Trace recursive queries from root servers, simulate TTL countdowns, and diagnose NXDOMAIN errors.

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