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  4. HTTP response status codes
  5. 501 Not Implemented
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501 Not Implemented

The HTTP 501 Not Implemented server error response status code means that the server does not support the functionality required to fulfill the request.

A response with this status may also include a Retry-After header, telling the client that they can retry the request after the specified time has elapsed. A 501 response is cacheable by default unless caching headers instruct otherwise.

501 is the appropriate response when the server does not recognize the request method and is incapable of supporting it for any resource. Servers are required to support GET and HEAD, and therefore must not return 501 in response to requests with these methods. If the server does recognize the method, but intentionally does not allow it, the appropriate response is 405 Method Not Allowed.

If you have visited a web page and you are seeing 501 errors, these issues require investigation and fixing by server owners or administrators. You can clear your browser cache for the domain, disable proxies if you are using one, or try again later to see if it works as expected.

A 501 response can occur if proxies cannot not handle request methods used in the context of HTTP Extension Framework (RFC 2774) applications. This status can also occur in Web Distributed Authoring and Versioning (WebDAV) when a request method (SEARCH, PROPFIND) does not have a URL handler configured to process it.

In this article

Status

http
501 Not Implemented

Examples

Extension method not supported

In the following HTTP Extension Framework example, a client sends a request with a mandatory extension specified in the C-MAN header. The Connection header specifies that these extensions are to be handled on a hop-by-hop basis. A proxy refuses to forward the M-GET method, and sends a 501 error in response:

http
M-GET /document HTTP/1.1
Host: example.com
C-Man: "http://www.example.org/"
Connection: C-Man
http
HTTP/1.1 501 Not Implemented

Specifications

Specification
HTTP Semantics
# status.501

See also

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  1. HTTP
  2. Guides
  3. Overview of HTTP
  4. Evolution of HTTP
  5. A typical HTTP session
  6. HTTP messages
  7. Media types
    1. Common types
  8. Compression in HTTP
  9. HTTP caching
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  11. Using HTTP cookies
  12. Redirections in HTTP
  13. Conditional requests
  14. Range requests
  15. Client hints
  16. User-Agent reduction
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  18. Network Error Logging
  19. Content negotiation
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  20. Browser detection using the UA string
  21. Connection management in HTTP/1.x
  22. Protocol upgrade mechanism
  23. Proxy servers and tunneling
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  24. Security and privacy
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    7. Cross-Origin Resource Sharing (CORS)
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      2. Reason: CORS header 'Access-Control-Allow-Origin' does not match 'xyz'
      3. Reason: CORS header 'Access-Control-Allow-Origin' missing
      4. Reason: CORS header 'Origin' cannot be added
      5. Reason: CORS preflight channel did not succeed
      6. Reason: CORS request did not succeed
      7. Reason: CORS request external redirect not allowed
      8. Reason: CORS request not HTTP
      9. Reason: Credential is not supported if the CORS header 'Access-Control-Allow-Origin' is '*'
      10. Reason: Did not find method in CORS header 'Access-Control-Allow-Methods'
      11. Reason: expected 'true' in CORS header 'Access-Control-Allow-Credentials'
      12. Reason: invalid token 'xyz' in CORS header 'Access-Control-Allow-Headers'
      13. Reason: invalid token 'xyz' in CORS header 'Access-Control-Allow-Methods'
      14. Reason: missing token 'xyz' in CORS header 'Access-Control-Allow-Headers' from CORS preflight channel
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  25. Reference
  26. HTTP headers
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  27. HTTP request methods
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  28. HTTP response status codes
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    5. 200 OK
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    47. 428 Precondition Required
    48. 429 Too Many Requests
    49. 431 Request Header Fields Too Large
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    52. 501 Not Implemented
    53. 502 Bad Gateway
    54. 503 Service Unavailable
    55. 504 Gateway Timeout
    56. 505 HTTP Version Not Supported
    57. 506 Variant Also Negotiates
    58. 507 Insufficient Storage
    59. 508 Loop Detected
    60. 510 Not Extended
    61. 511 Network Authentication Required
  29. CSP directives
    1. base-uri
    2. block-all-mixed-content
    3. child-src
    4. connect-src
    5. default-src
    6. fenced-frame-src
    7. font-src
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    9. frame-ancestors
    10. frame-src
    11. img-src
    12. manifest-src
    13. media-src
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    16. report-to
    17. report-uri
    18. require-trusted-types-for
    19. sandbox
    20. script-src
    21. script-src-attr
    22. script-src-elem
    23. style-src
    24. style-src-attr
    25. style-src-elem
    26. trusted-types
    27. upgrade-insecure-requests
    28. worker-src
  30. Permissions-Policy directives
    1. accelerometer
    2. ambient-light-sensor
    3. aria-notify
    4. attribution-reporting
    5. autoplay
    6. bluetooth
    7. browsing-topics
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    21. hid
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    23. idle-detection
    24. language-detector
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    26. local-fonts
    27. local-network
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    29. loopback-network
    30. magnetometer
    31. microphone
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    34. otp-credentials
    35. payment
    36. picture-in-picture
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    38. private-state-token-redemption
    39. publickey-credentials-create
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    41. screen-wake-lock
    42. serial
    43. speaker-selection
    44. storage-access
    45. summarizer
    46. translator
    47. usb
    48. web-share
    49. window-management
    50. xr-spatial-tracking
  31. HTTP resources and specifications
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