- Class
- C3 Standard
- Weight
- 2
- Severity on fail
- minor
- Applies to
- RFC 6596
Test this rule on a live site with the canonical tag checker. Free, no account needed.
Scope and evidence
Evaluated on pages in P whose head canonical differs from their own URL. The book marks this requirement REQUIRED, STANDARD (Deliverable 5 frames the duplicate-or-superset MUST; the conditions below are the RFC's avoid list).
308 did not exist when RFC 6596 was published; the scanner treats it like 301 by analogy, and the reason says "permanently redirects". The RFC explicitly allows a temporary-redirect source as a target, so 302, 303 and 307 never fail here. Whether the target is truly a duplicate or superset is a manual check (§5).
Fail conditions
Each condition has a stable code and a reason template. The evaluator fills the placeholders from observed values only; no sentence in a report is generated.
target_error{url} names {target} as canonical, but it returned {outcome}. RFC 6596 recommends avoiding a canonical target that returns an error code.
target_permanent_redirect{url} names {target} as canonical, but {target} permanently redirects to {final}. RFC 6596 recommends avoiding the source of a permanent redirect as the canonical target.
target_canonical_chain{url} names {target} as canonical, and {target} names {third} as its own canonical. RFC 6596 recommends avoiding a target that specifies a canonical other than itself.
The documented fix
Point the canonical straight at the final, 200, self-canonical URL.
Sources, quoted
These are the passages the rule rests on, stored once in the knowledge base and emitted exactly as written. A citation marked for specific conditions appears only on findings with those codes.
“Avoid designating the target (canonical) as:”
[RFC6596] RFC 6596: The Canonical Link Relation · 3. The Canonical Link Relation · last updated April 2012
https://www.rfc-editor.org/rfc/rfc6596.html“The source IRI of a permanent redirect (for HTTP, this refers to 300 and 301 response codes, defined in Sections 10.3.1 and 10.3.2 of [RFC2616]).”
[RFC6596] RFC 6596: The Canonical Link Relation · 3. The Canonical Link Relation · last updated April 2012 · for target_permanent_redirect
https://www.rfc-editor.org/rfc/rfc6596.html“An IRI that also specifies a canonical link relation to an IRI other than itself.”
[RFC6596] RFC 6596: The Canonical Link Relation · 3. The Canonical Link Relation · last updated April 2012 · for target_canonical_chain
https://www.rfc-editor.org/rfc/rfc6596.html“An IRI that returns an error code, such as a 4xx response in HTTP (Section 10.4 of [RFC2616]).”
[RFC6596] RFC 6596: The Canonical Link Relation · 3. The Canonical Link Relation · last updated April 2012 · for target_error
https://www.rfc-editor.org/rfc/rfc6596.html