Glossary · HTTP, edge and rendering
canonical URL
The one address a site declares as the authoritative copy of a page, in a link rel=canonical tag or an HTTP header.
What canonical URL means
The one address a site declares as the authoritative copy of a page, in a link rel=canonical tag or an HTTP header. It is a hint that engines usually follow, not a redirect: the duplicate stays reachable, and a canonical that points at a page returning 404 or a different host is discarded rather than obeyed.
What canonical URL can and cannot support
| It can support | It cannot support |
|---|---|
| The one address a site declares as the authoritative copy of a page, in a link rel=canonical tag or an HTTP header. | It is a hint that engines usually follow, not a redirect: the duplicate stays reachable, and a canonical that points at a page returning 404 or a different host is discarded rather than obeyed. |
How CrawlCheck measures canonical URL
2 finding codes in the scanner read this. The share of all scans carrying each is computed from the live corpus when this page is served, not typed into it.
| Finding code | Share of scans | Guide |
|---|---|---|
HOST_DUPLICATE_200 | 3.3% | Canonical URL audit: one page, one address, and the three ways a site publishes it twice |
DOMAIN_IS_ALIAS | 0.8% | Canonical URL audit: one page, one address, and the three ways a site publishes it twice |
Every code, its rule and its current rate: the dataset page. Scan a site to see which apply to it.
Where canonical URL comes up on this site
- Canonical URL audit: one page, one address, and the three ways a site publishes it twiceA canonical URL is the one address a site asks crawlers to treat as the page.
- How to write an llms.txt, and how to check it is actually being servedIf you are generating the file, generate it from a source that already knows the canonical URL of each page — your sitemap, not your navigation.
- How to find broken internal links and dead anchor linksWhen you remove a page that was linked, redirect it to its closest replacement rather than letting it 404; the canonical URL guide covers keeping one address per page.
- Staging site leak: how a staging host ends up in live pagesThe scanner reports that as HOST_DUPLICATE_200 ( of scans) when the www host stays on www and returns a page within 15 percent of the apex's size, and it is covered in full in the canonical URL guide , with the case where it was found on a site whose…
Terms this definition uses
Related terms in HTTP, edge and rendering
The delivery layer between an origin and a client: proxies, caches, redirects, status codes and the headers that decide what a crawler actually received.
Origin server · Reverse proxy · CDN · Edge node · WAF · Bot score · TLS handshake · DNS resolution · DNS timeout · Connection timeout · Read timeout · Redirect chain · Redirect loop · 301 redirect · 302 redirect · 307 redirect · 308 redirect · 304 response · 404 response · 410 response · 429 response · 500 response · 502 response · 503 response · 504 response · HEAD request · GET request · Content type · MIME sniffing · Content encoding · Compression · Brotli · ETag · Last-Modified · If-None-Match · Cache hit · Cache miss · Cache key · Vary header · Stale response · Stale-while-revalidate · Edge redirect · Origin bypass · Response parity · Header parity · hreflang · HSTS · HSTS preload list · Content-Security-Policy · CSP nonce · edge cache · cache split · cache-buster · Cache-Control · Age header · render-blocking resource · payload text share · lazy loading · 404 and 410 status codes
Questions about canonical URL
What is canonical URL?
The one address a site declares as the authoritative copy of a page, in a link rel=canonical tag or an HTTP header.
What does canonical URL not show or guarantee?
It is a hint that engines usually follow, not a redirect: the duplicate stays reachable, and a canonical that points at a page returning 404 or a different host is discarded rather than obeyed.
How does CrawlCheck measure canonical URL?
Through the finding codes HOST_DUPLICATE_200, DOMAIN_IS_ALIAS, each with a stated rule, evidence bytes and a live share of scans on the dataset page.
Which area of the glossary does canonical URL belong to?
HTTP, edge and rendering: The delivery layer between an origin and a client: proxies, caches, redirects, status codes and the headers that decide what a crawler actually received.
See it in the full glossary · 668 terms across 19 areas.