Content and representation

Accept-Ranges HTTP Header

Indicates whether the resource supports range requests and the unit used for those ranges.

Applicability

This field applies to HTTP responses.

Syntax

Accept-Ranges: bytes | none | <range-unit>

Examples

Accept-Ranges: bytes
Accept-Ranges: none

Common use cases

  • Advertise byte-range support for large media, archives, or resumable downloads.
  • State that a resource does not support range processing even if a client might attempt it.

Common mistakes

  • Advertising bytes while ignoring Range or returning incorrect Content-Range boundaries.
  • Assuming the field is required before a client is allowed to send a Range request.

Security considerations

Range processing must validate bounds and limit multipart amplification to avoid excessive CPU, memory, bandwidth, or cache inconsistency.

Detailed guidance

Meaning and behavior

Accept-Ranges tells a recipient which range unit an origin supports for the selected representation. bytes advertises byte ranges, enabling clients to request only part of a resource for resume, seeking, or parallel retrieval. none communicates that range requests are not supported. The field is advisory: a client can send Range without previously seeing Accept-Ranges, and the server’s actual response determines whether the request was honored.

A successful single range generally produces status 206 and Content-Range metadata, while an unsatisfiable range produces 416 with appropriate information. A server can ignore Range and return the complete representation with 200. Validators matter because If-Range lets a client request a partial response only if its stored representation is still current. Compression and transformations complicate which byte sequence ranges address.

Implementation notes

Advertise bytes only when every serving layer implements consistent range semantics for the transmitted representation. Validate start and end positions, integer overflow, overlapping ranges, and excessive multipart requests. Set Content-Length and Content-Range accurately for partial responses. Test files at boundaries, empty resources, stale ETags, Last-Modified validators, compression, CDN caching, and resumed downloads. Apply limits to prevent many tiny or overlapping ranges from amplifying work. If a proxy handles ranges, ensure origin and edge agree; inconsistent support can corrupt downloads or poison cached variants. Use none or omit advertising when correct support is unavailable.

Sources