All general-purpose servers MUST support the methods GET and HEAD. All other methods are OPTIONAL; when implemented, a server MUST implement the above methods according to the semantics defined for them in Section 4.3.

Additional methods, outside the scope of this specification, have been standardized for use in HTTP. All such methods ought to be registered within the HTTP Method Registry maintained by IANA, as defined in Section 8.1.

The set of methods allowed by a target resource can be listed in an Allow [header.allow] header field (Section 7.4.1). However, the set of allowed methods can change dynamically. When a request method is received that is unrecognized or not implemented by an origin server, the origin server SHOULD respond with the 501 (Not Implemented) [status.501] status code. When a request method is received that is known by an origin server but not allowed for the target resource, the origin server SHOULD respond with the 405 (Method Not Allowed) [status.405] status code.

A client can send conditional request header fields (Section 5.2) to make the requested action conditional on the current state of the target resource ([Part4]).

4.2. Common Method Properties

4.2.1. Safe Methods

Request methods are considered "safe" if their defined semantics are essentially read-only; i.e., the client does not request, and does not expect, any state change on the origin server as a result of applying a safe method to a target resource. Likewise, reasonable use of a safe method is not expected to cause any harm, loss of property, or unusual burden on the origin server.

This definition of safe methods does not prevent an implementation from including behavior that is potentially harmful, not entirely read-only, or which causes side-effects while invoking a safe method. What is important, however, is that the client did not request that additional behavior and cannot be held accountable for it. For example, most servers append request information to access log files at the completion of every response, regardless of the method, and that is considered safe even though the log storage might become full and crash the server. Likewise, a safe request initiated by selecting an advertisement on the Web will often have the side-effect of charging an advertising account.

Of the request methods defined by this specification, the GET, HEAD, OPTIONS, and TRACE methods are defined to be safe.

The purpose of distinguishing between safe and unsafe methods is to allow automated retrieval processes (spiders) and cache performance optimization (pre-fetching) to work without fear of causing harm. In addition, it allows a user agent to apply appropriate constraints on the automated use of unsafe methods when processing potentially untrusted content.

A user agent SHOULD distinguish between safe and unsafe methods when presenting potential actions to a user, such that the user can be made aware of an unsafe action before it is requested.

When a resource is constructed such that parameters within the effective request URI have the effect of selecting an action, it is the resource owner's responsibility to ensure that the action is consistent with the request method semantics. For example, it is common for Web-based content editing software to use actions within query parameters, such as "page?do=delete". If the purpose of such a resource is to perform an unsafe action, then the resource MUST disable or disallow that action when it is accessed using a safe request method. Failure to do so will result in unfortunate side-effects when automated processes perform a GET on every URI reference for the sake of link maintenance, pre-fetching, building a search index, etc.

4.2.2. Idempotent Methods

Request methods are considered "idempotent" if the intended effect of multiple identical requests is the same as for a single request. Of the request methods defined by this specification, the PUT, DELETE, and safe request methods are idempotent.

Like the definition of safe, the idempotent property only applies to what has been requested by the user; a server is free to log each request separately, retain a revision control history, or implement other non-idempotent side-effects for each idempotent request.

Idempotent methods are distinguished because the request can be repeated automatically if a communication failure occurs before the client is able to read the server's response. For example, if a client sends a PUT request and the underlying connection is closed before any response is received, then it can establish a new connection and retry the idempotent request because it knows that repeating the request will have the same effect even if the original request succeeded. Note, however, that repeated failures would indicate a problem within the server.

4.2.3. Cacheable Methods

Request methods are considered "cacheable" if it is possible and useful to answer a current client request with a stored response from a prior request. GET and HEAD are defined to be cacheable. In general, safe methods that do not depend on a current or authoritative response are cacheable, though the overwhelming majority of caches only support GET and HEAD. HTTP requirements for cache behavior and cacheable responses are defined in [Part6].

4.3. Method Definitions

4.3.1. GET

The GET method requests transfer of a current selected representation for the target resource [resources]. GET is the primary mechanism of information retrieval and the focus of almost all performance optimizations. Hence, when people speak of retrieving some identifiable information via HTTP, they are generally referring to making a GET request.

It is tempting to think of resource identifiers as remote filesystem pathnames, and of representations as being a copy of the contents of such files. In fact, that is how many resources are implemented (see Section 9.1 for related security considerations). However, there are no such limitations in practice. The HTTP interface for a resource is just as likely to be implemented as a tree of content objects, a programmatic view on various database records, or a gateway to other information systems. Even when the URI mapping mechanism is tied to a filesystem, an origin server might be configured to execute the files with the request as input and send the output as the representation, rather than transfer the files directly. Regardless, only the origin server needs to know how each of its resource identifiers corresponds to an implementation, and how each implementation manages to select and send a current representation of the target resource in a response to GET.

A client can alter the semantics of GET to be a "range request", requesting transfer of only some part(s) of the selected representation, by sending a Range header field in the request ([Part5]).

A payload within a GET request message has no defined semantics; sending a payload body on a GET request might cause some existing implementations to reject the request.

The response to a GET request is cacheable; a cache MAY use it to satisfy subsequent GET and HEAD requests unless otherwise indicated by the Cache-Control header field (Section 7.2 of [Part6]).

4.3.2. HEAD

The HEAD method is identical to GET except that the server MUST NOT send a message body in the response (i.e., the response terminates at the end of the header block). Aside from the payload header fields (Section 3.3), the server SHOULD send the same header fields in response to a HEAD request as it would have sent if the request had been a GET. This method can be used for obtaining metadata about the selected representation without transferring the representation data. This method is often used for testing hypertext links for validity, accessibility, and recent modification.

A payload within a HEAD request message has no defined semantics; sending a payload body on a HEAD request might cause some existing implementations to reject the request.

The response to a HEAD request is cacheable; a cache MAY use it to satisfy subsequent HEAD requests unless otherwise indicated by the Cache-Control header field (Section 7.2 of [Part6]). A HEAD response might also have an effect on previously cached responses to GET; see Section 5 of [Part6].

4.3.3. POST

The POST method requests that the target resource [resources] process the representation enclosed in the request according to the resource's own specific semantics. For example, POST is used for the following functions (among others):

• Providing a block of data, such as the fields entered into an HTML form, to a data-handling process;
• Posting a message to a bulletin board, newsgroup, mailing list, blog, or similar group of articles;
• Creating a new resource that has yet to be identified by the origin server; and
• Appending data to a resource's existing representation(s).

An origin server indicates response semantics by choosing an appropriate status code depending on the result of processing the POST request; almost all of the status codes defined by this specification might be received in a response to POST (the exceptions being 206, 304, and 416).

If one or more resources has been created on the origin server as a result of successfully processing a POST request, the origin server SHOULD send a 201 (Created) [status.201] response containing a Location [header.location] header field that provides an identifier for the primary resource created (Section 7.1.2) and a representation that describes the status of the request while referring to the new resource(s).

Responses to POST requests are only cacheable when they include explicit freshness information (see Section 4.1.1 of [Part6]). However, POST caching is not widely implemented. For cases where an origin server wishes the client to be able to cache the result of a POST in a way that can be reused by a later GET, the origin server MAY send a 200 (OK) [status.200] response containing the result and a Content-Location [header.content-location] header field that has the same value as the POST's effective request URI (Section 3.1.4.2).

If the result of processing a POST would be equivalent to a representation of an existing resource, an origin server MAY redirect the user agent to that resource by sending a 303 (See Other) [status.303] response with the existing resource's identifier in the Location [header.location] field. This has the benefits of providing the user agent a resource identifier and transferring the representation via a method more amenable to shared caching, though at the cost of an extra request if the user agent does not already have the representation cached.

4.3.4. PUT

The PUT method requests that the state of the target resource [resources] be created or replaced with the state defined by the representation enclosed in the request message payload. A successful PUT of a given representation would suggest that a subsequent GET on that same target resource will result in an equivalent representation being sent in a 200 (OK) [status.200] response. However, there is no guarantee that such a state change will be observable, since the target resource might be acted upon by other user agents in parallel, or might be subject to dynamic processing by the origin server, before any subsequent GET is received. A successful response only implies that the user agent's intent was achieved at the time of its processing by the origin server.

If the target resource does not have a current representation and the PUT successfully creates one, then the origin server MUST inform the user agent by sending a 201 (Created) [status.201] response. If the target resource does have a current representation and that representation is successfully modified in accordance with the state of the enclosed representation, then either a 200 (OK) [status.200] or 204 (No Content) [status.204] response SHOULD be sent to indicate successful completion of the request.

An origin server SHOULD ignore unrecognized header fields received in a PUT request (i.e., do not save them as part of the resource state).

An origin server SHOULD verify that the PUT representation is consistent with any constraints the server has for the target resource that cannot or will not be changed by the PUT. This is particularly important when the origin server uses internal configuration information related to the URI in order to set the values for representation metadata on GET responses. When a PUT representation is inconsistent with the target resource, the origin server SHOULD either make them consistent, by transforming the representation or changing the resource configuration, or respond with an appropriate error message containing sufficient information to explain why the representation is unsuitable. The 409 (Conflict) [status.409] or 415 (Unsupported Media Type) [status.415] status codes are suggested, with the latter being specific to constraints on Content-Type [header.content-type] values.

For example, if the target resource is configured to always have a Content-Type [header.content-type] of "text/html" and the representation being PUT has a Content-Type of "image/jpeg", then the origin server SHOULD do one of:

1. reconfigure the target resource to reflect the new media type;
2. transform the PUT representation to a format consistent with that of the resource before saving it as the new resource state; or,
3. reject the request with a 415 (Unsupported Media Type) [status.415] response indicating that the target resource is limited to "text/html", perhaps including a link to a different resource that would be a suitable target for the new representation.

HTTP does not define exactly how a PUT method affects the state of an origin server beyond what can be expressed by the intent of the user agent request and the semantics of the origin server response. It does not define what a resource might be, in any sense of that word, beyond the interface provided via HTTP. It does not define how resource state is "stored", nor how such storage might change as a result of a change in resource state, nor how the origin server translates resource state into representations. Generally speaking, all implementation details behind the resource interface are intentionally hidden by the server.

An origin server MUST NOT send a validator header field (Section 7.2), such as an ETag or Last-Modified field, in a successful response to PUT unless the request's representation data was saved without any transformation applied to the body (i.e., the resource's new representation data is identical to the representation data received in the PUT request) and the validator field value reflects the new representation. This requirement allows a user agent to know when the representation body it has in memory remains current as a result of the PUT, thus not in need of retrieving again from the origin server, and that the new validator(s) received in the response can be used for future conditional requests in order to prevent accidental overwrites (Section 5.2).

The fundamental difference between the POST and PUT methods is highlighted by the different intent for the enclosed representation. The target resource in a POST request is intended to handle the enclosed representation according to the resource's own semantics, whereas the enclosed representation in a PUT request is defined as replacing the state of the target resource. Hence, the intent of PUT is idempotent and visible to intermediaries, even though the exact effect is only known by the origin server.

Proper interpretation of a PUT request presumes that the user agent knows which target resource is desired. A service that selects a proper URI on behalf of the client, after receiving a state-changing request, SHOULD be implemented using the POST method rather than PUT. If the origin server will not make the requested PUT state change to the target resource and instead wishes to have it applied to a different resource, such as when the resource has been moved to a different URI, then the origin server MUST send an appropriate 3xx (Redirection) [status.3xx] response; the user agent MAY then make its own decision regarding whether or not to redirect the request.

A PUT request applied to the target resource MAY have side-effects on other resources. For example, an article might have a URI for identifying "the current version" (a resource) that is separate from the URIs identifying each particular version (different resources that at one point shared the same state as the current version resource). A successful PUT request on "the current version" URI might therefore create a new version resource in addition to changing the state of the target resource, and might also cause links to be added between the related resources.

An origin server SHOULD reject any PUT request that contains a Content-Range header field (Section 4.2 of [Part5]), since it might be misinterpreted as partial content (or might be partial content that is being mistakenly PUT as a full representation). Partial content updates are possible by targeting a separately identified resource with state that overlaps a portion of the larger resource, or by using a different method that has been specifically defined for partial updates (for example, the PATCH method defined in [RFC5789]).

Responses to the PUT method are not cacheable. If a PUT request passes through a cache that has one or more stored responses for the effective request URI, those stored responses will be invalidated (see Section 6 of [Part6]).

4.3.5. DELETE

The DELETE method requests that the origin server remove the association between the target resource [resources] and its current functionality. In effect, this method is similar to the rm command in UNIX: it expresses a deletion operation on the URI mapping of the origin server, rather than an expectation that the previously associated information be deleted.

If the target resource has one or more current representations, they might or might not be destroyed by the origin server, and the associated storage might or might not be reclaimed, depending entirely on the nature of the resource and its implementation by the origin server (which are beyond the scope of this specification). Likewise, other implementation aspects of a resource might need to be deactivated or archived as a result of a DELETE, such as database or gateway connections. In general, it is assumed that the origin server will only allow DELETE on resources for which it has a prescribed mechanism for accomplishing the deletion.

Relatively few resources allow the DELETE method — its primary use is for remote authoring environments, where the user has some direction regarding its effect. For example, a resource that was previously created using a PUT request, or identified via the Location header field after a 201 (Created) [status.201] response to a POST request, might allow a corresponding DELETE request to undo those actions. Similarly, custom user agent implementations that implement an authoring function, such as revision control clients using HTTP for remote operations, might use DELETE based on an assumption that the server's URI space has been crafted to correspond to a version repository.

If a DELETE method is successfully applied, the origin server SHOULD send a 202 (Accepted) [status.202] status code if the action seems okay but has not yet been enacted, a 204 (No Content) [status.204] status code if the action has been enacted and no further information is to be supplied, or a 200 (OK) [status.200] status code if the action has been enacted and the response message includes a representation describing the status.

A payload within a DELETE request message has no defined semantics; sending a payload body on a DELETE request might cause some existing implementations to reject the request.

Responses to the DELETE method are not cacheable. If a DELETE request passes through a cache that has one or more stored responses for the effective request URI, those stored responses will be invalidated (see Section 6 of [Part6]).

4.3.6. CONNECT

The CONNECT method requests that the recipient establish a tunnel to the destination origin server identified by the request-target and, if successful, thereafter restrict its behavior to blind forwarding of packets, in both directions, until the connection is closed.

CONNECT is intended only for use in requests to a proxy. An origin server that receives a CONNECT request for itself MAY respond with a 2xx [status.2xx] status code to indicate that a connection is established. However, most origin servers do not implement CONNECT.

A client sending a CONNECT request MUST send the authority form of request-target (Section 5.3 of [Part1]); i.e., the request-target consists of only the host name and port number of the tunnel destination, separated by a colon. For example,

  CONNECT server.example.com:80 HTTP/1.1
  Host: server.example.com:80
  
  

The recipient proxy can establish a tunnel either by directly connecting to the request-target or, if configured to use another proxy, by forwarding the CONNECT request to the next inbound proxy. Any 2xx (Successful) [status.2xx] response indicates that the sender (and all inbound proxies) will switch to tunnel mode immediately after the blank line that concludes the successful response's header block; data received after that blank line is from the server identified by the request-target. Any response other than a successful response indicates that the tunnel has not yet been formed and that the connection remains governed by HTTP.

A server SHOULD NOT send any Transfer-Encoding or Content-Length header fields in a successful response. A client MUST ignore any Content-Length or Transfer-Encoding header fields received in a successful response.

There are significant risks in establishing a tunnel to arbitrary servers, particularly when the destination is a well-known or reserved TCP port that is not intended for Web traffic. For example, a CONNECT to a request-target of "example.com:25" would suggest that the proxy connect to the reserved port for SMTP traffic; if allowed, that could trick the proxy into relaying spam email. Proxies that support CONNECT SHOULD restrict its use to a limited set of known ports or a configurable whitelist of safe request targets.

Proxy authentication might be used to establish the authority to create a tunnel. For example,

  CONNECT server.example.com:80 HTTP/1.1
  Host: server.example.com:80
  Proxy-Authorization: basic aGVsbG86d29ybGQ=
  
  

When a tunnel intermediary detects that either side has closed its connection, any outstanding data that came from that side will first be sent to the other side and then the intermediary will close both connections. If there is outstanding data left undelivered, that data will be discarded.

A payload within a CONNECT request message has no defined semantics; sending a payload body on a CONNECT request might cause some existing implementations to reject the request.

Responses to the CONNECT method are not cacheable.

4.3.7. OPTIONS

The OPTIONS method requests information about the communication options available on the request/response chain identified by the effective request URI. This method allows a client to determine the options and/or requirements associated with a resource, or the capabilities of a server, without implying a resource action.

An OPTIONS request with an asterisk ("*") as the request-target (Section 5.3 of [Part1]) applies to the server in general rather than to a specific resource. Since a server's communication options typically depend on the resource, the "*" request is only useful as a "ping" or "no-op" type of method; it does nothing beyond allowing the client to test the capabilities of the server. For example, this can be used to test a proxy for HTTP/1.1 conformance (or lack thereof).

If the request-target is not an asterisk, the OPTIONS request applies to the options that are available when communicating with the target resource.

A server generating a successful response to OPTIONS SHOULD send any header fields that might indicate optional features implemented by the server and applicable to the target resource (e.g., Allow [header.allow]), including potential extensions not defined by this specification. The response payload, if any, might also describe the communication options in a machine or human-readable representation. A standard format for such a representation is not defined by this specification, but might be defined by future extensions to HTTP. A server MUST generate a Content-Length field with a value of "0" if no payload body is to be sent in the response.

A client MAY send a Max-Forwards [header.max-forwards] header field in an OPTIONS request to target a specific recipient in the request chain (see Section 5.1.2). A proxy MUST NOT generate a Max-Forwards header field while forwarding a request unless that request was received with a Max-Forwards field.

A client that generates an OPTIONS request containing a payload body MUST send a valid Content-Type [header.content-type] header field describing the representation media type. Although this specification does not define any use for such a payload, future extensions to HTTP might use the OPTIONS body to make more detailed queries about the target resource.

Responses to the OPTIONS method are not cacheable.

4.3.8. TRACE

The TRACE method requests a remote, application-level loop-back of the request message. The final recipient of the request SHOULD reflect the message received, excluding some fields described below, back to the client as the message body of a 200 (OK) [status.200] response with a Content-Type [header.content-type] of "message/http" (Section 7.3.1 of [Part1]). The final recipient is either the origin server or the first server to receive a Max-Forwards [header.max-forwards] value of zero (0) in the request (Section 5.1.2).

A client MUST NOT send header fields in a TRACE request containing sensitive data that might be disclosed by the response. For example, it would be foolish for a user agent to send stored user credentials [Part7] or cookies [RFC6265] in a TRACE request. The final recipient SHOULD exclude any request header fields from the response body that are likely to contain sensitive data.

TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing or diagnostic information. The value of the Via header field (Section 5.7.1 of [Part1]) is of particular interest, since it acts as a trace of the request chain. Use of the Max-Forwards [header.max-forwards] header field allows the client to limit the length of the request chain, which is useful for testing a chain of proxies forwarding messages in an infinite loop.

A client MUST NOT send a message body in a TRACE request.

Responses to the TRACE method are not cacheable.

5. Request Header Fields

A client sends request header fields to provide more information about the request context, make the request conditional based on the target resource state, suggest preferred formats for the response, supply authentication credentials, or modify the expected request processing. These fields act as request modifiers, similar to the parameters on a programming language method invocation.

5.1. Controls

Controls are request header fields that direct specific handling of the request.

5.1.1. Expect

The "Expect" header field is used to indicate that particular server behaviors are required by the client.

  Expect       = 1#expectation
  
  expectation  = expect-name [ BWS "=" BWS expect-value ]
                             *( OWS ";" [ OWS expect-param ] )
  expect-param = expect-name [ BWS "=" BWS expect-value ]
  
  expect-name  = token
  expect-value = token / quoted-string

If all received Expect header field(s) are syntactically valid but contain an expectation that the recipient does not understand or cannot comply with, the recipient MUST respond with a 417 (Expectation Failed) [status.417] status code. A recipient of a syntactically invalid Expectation header field MUST respond with a 4xx [status.4xx] status code other than 417.

Comparison is case-insensitive for names (expect-name), and case-sensitive for values (expect-value).

The Expect header field MUST be forwarded if the request is forwarded.

Many older HTTP/1.0 and HTTP/1.1 servers do not understand the Expect header field.

5.1.1.1. Use of the 100 (Continue) Status

The only expectation defined by this specification is:

• The request includes a payload body and the client will wait for a 100 (Continue) [status.100] response after sending the request header section but before sending the payload body. The 100-continue expectation does not use any expect-params.

The primary purpose of the 100 (Continue) [status.100] status code (Section 6.2.1) is to allow a client that is sending a request message with a payload to determine if the origin server is willing to accept the request (based on the request header fields) before the client sends the payload body. In some cases, it might either be inappropriate or highly inefficient for the client to send the payload body if the server will reject the message without looking at the body.

Requirements for HTTP/1.1 clients:

• If a client will wait for a 100 (Continue) [status.100] response before sending the payload body, it MUST send an Expect [header.expect] header field with the "100-continue" expectation.
• A client MUST NOT send an Expect [header.expect] header field with the "100-continue" expectation if it does not intend to send a payload body.

Because of the presence of older implementations, the protocol allows ambiguous situations in which a client might send "Expect: 100-continue" without receiving either a 417 (Expectation Failed) [status.417] or a 100 (Continue) [status.100] status code. Therefore, when a client sends this header field to an origin server (possibly via a proxy) from which it has never seen a 100 (Continue) [status.100] status code, the client SHOULD NOT wait for an indefinite period before sending the payload body.

Requirements for HTTP/1.1 origin servers:

• Upon receiving a request that includes an Expect [header.expect] header field with the "100-continue" expectation, an origin server MUST either respond with 100 (Continue) [status.100] status code and continue to read from the input stream, or respond with a final status code. The origin server MUST NOT wait for the payload body before sending the 100 (Continue) [status.100] response. If the origin server responds with a final status code, it MUST NOT have performed the request method and MAY either close the connection or continue to read and discard the rest of the request.
• An origin server SHOULD NOT send a 100 (Continue) [status.100] response if the request message does not include an Expect [header.expect] header field with the "100-continue" expectation, and MUST NOT send a 100 (Continue) [status.100] response if such a request comes from an HTTP/1.0 (or earlier) client. There is an exception to this rule: for compatibility with [RFC2068], a server MAY send a 100 (Continue) [status.100] status code in response to an HTTP/1.1 PUT or POST request that does not include an Expect header field with the "100-continue" expectation. This exception, the purpose of which is to minimize any client processing delays associated with an undeclared wait for 100 (Continue) [status.100] status code, applies only to HTTP/1.1 requests, and not to requests with any other HTTP-version value.
• An origin server MAY omit a 100 (Continue) [status.100] response if it has already received some or all of the payload body for the corresponding request.
• An origin server that sends a 100 (Continue) [status.100] response MUST ultimately send a final status code, once the payload body is received and processed, unless it terminates the transport connection prematurely.
• If an origin server receives a request that does not include an Expect [header.expect] header field with the "100-continue" expectation, the request includes a payload body, and the server responds with a final status code before reading the entire payload body from the transport connection, then the server SHOULD NOT close the transport connection until it has read the entire request, or until the client closes the connection. Otherwise, the client might not reliably receive the response message. However, this requirement ought not be construed as preventing a server from defending itself against denial-of-service attacks, or from badly broken client implementations.

Requirements for HTTP/1.1 proxies:

• If a proxy receives a request that includes an Expect [header.expect] header field with the "100-continue" expectation, and the proxy either knows that the next-hop server complies with HTTP/1.1 or higher, or does not know the HTTP version of the next-hop server, it MUST forward the request, including the Expect header field.
• If the proxy knows that the version of the next-hop server is HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST respond with a 417 (Expectation Failed) [status.417] status code.
• Proxies SHOULD maintain a record of the HTTP version numbers received from recently-referenced next-hop servers.
• A proxy MUST NOT forward a 100 (Continue) [status.100] response if the request message was received from an HTTP/1.0 (or earlier) client and did not include an Expect [header.expect] header field with the "100-continue" expectation. This requirement overrides the general rule for forwarding of 1xx [status.1xx] responses (see Section 6.2.1).

5.1.2. Max-Forwards

The "Max-Forwards" header field provides a mechanism with the TRACE (Section 4.3.8) and OPTIONS (Section 4.3.7) methods to limit the number of times that the request is forwarded by proxies. This can be useful when the client is attempting to trace a request that appears to be failing or looping mid-chain.

  Max-Forwards = 1*DIGIT

The Max-Forwards value is a decimal integer indicating the remaining number of times this request message can be forwarded.

Each recipient of a TRACE or OPTIONS request containing a Max-Forwards header field MUST check and update its value prior to forwarding the request. If the received value is zero (0), the recipient MUST NOT forward the request; instead, it MUST respond as the final recipient. If the received Max-Forwards value is greater than zero, then the forwarded message MUST contain an updated Max-Forwards field with a value decremented by one (1).

The Max-Forwards header field MAY be ignored for all other request methods.

5.2. Conditionals

The HTTP conditional request header fields [Part4] allow a client to place a precondition on the state of the target resource, so that the action corresponding to the method semantics will not be applied if the precondition evaluates to false. Each precondition defined by this specification consists of a comparison between a set of validators obtained from prior representations of the target resource to the current state of validators for the selected representation [representations] (Section 7.2). Hence, these preconditions evaluate whether the state of the target resource has changed since a given state known by the client. The effect of such an evaluation depends on the method semantics and choice of conditional, as defined in Section 5 of [Part4].

5.3. Content Negotiation

The following request header fields are sent by a user agent to engage in proactive negotiation [proactive.negotiation] of the response content, as defined in Section 3.4.1. The preferences sent in these fields apply to any content in the response, including representations of the target resource, representations of error or processing status, and potentially even the miscellaneous text strings that might appear within the protocol.

5.3.1. Quality Values

Many of the request header fields for proactive negotiation [proactive.negotiation] use a common parameter, named "q" (case-insensitive), to assign a relative "weight" to the preference for that associated kind of content. This weight is referred to as a "quality value" (or "qvalue") because the same parameter name is often used within server configurations to assign a weight to the relative quality of the various representations that can be selected for a resource.

The weight is normalized to a real number in the range 0 through 1, where 0.001 is the least preferred and 1 is the most preferred; a value of 0 means "not acceptable". If no "q" parameter is present, the default weight is 1.

  weight = OWS ";" OWS "q=" qvalue
  qvalue = ( "0" [ "." 0*3DIGIT ] )
         / ( "1" [ "." 0*3("0") ] )

A sender of qvalue MUST NOT generate more than three digits after the decimal point. User configuration of these values ought to be limited in the same fashion.

5.3.2. Accept

The "Accept" header field can be used by user agents to specify response media types that are acceptable. Accept header fields can be used to indicate that the request is specifically limited to a small set of desired types, as in the case of a request for an in-line image.

  Accept = #( media-range [ accept-params ] )
  
  media-range    = ( "*/*"
                   / ( type "/" "*" )
                   / ( type "/" subtype )
                   ) *( OWS ";" OWS parameter )
  accept-params  = weight *( accept-ext )
  accept-ext     = OWS ";" OWS token [ "=" word ]

The asterisk "*" character is used to group media types into ranges, with "*/*" indicating all media types and "type/*" indicating all subtypes of that type. The media-range can include media type parameters that are applicable to that range.

Each media-range might be followed by zero or more applicable media type parameters (e.g., charset [charset]), an optional "q" parameter for indicating a relative weight (Section 5.3.1), and then zero or more extension parameters. The "q" parameter is necessary if any extensions (accept-ext) are present, since it acts as a separator between the two parameter sets.

• Note: Use of the "q" parameter name to separate media type parameters from Accept extension parameters is due to historical practice. Although this prevents any media type parameter named "q" from being used with a media range, such an event is believed to be unlikely given the lack of any "q" parameters in the IANA media type registry and the rare usage of any media type parameters in Accept. Future media types are discouraged from registering any parameter named "q".

The example

  Accept: audio/*; q=0.2, audio/basic

SHOULD be interpreted as "I prefer audio/basic, but send me any audio type if it is the best available after an 80% mark-down in quality".

A request without any Accept header field implies that the user agent will accept any media type in response. If the header field is present in a request and none of the available representations for the response have a media type that is listed as acceptable, the origin server can either honor the header field by sending a 406 (Not Acceptable) [status.406] response or disregard the header field by treating the response as if it is not subject to content negotiation.

A more elaborate example is

  Accept: text/plain; q=0.5, text/html,
          text/x-dvi; q=0.8, text/x-c

Verbally, this would be interpreted as "text/html and text/x-c are the equally preferred media types, but if they do not exist, then send the text/x-dvi representation, and if that does not exist, send the text/plain representation".

Media ranges can be overridden by more specific media ranges or specific media types. If more than one media range applies to a given type, the most specific reference has precedence. For example,

  Accept: text/*, text/plain, text/plain;format=flowed, */*

have the following precedence:

1. text/plain;format=flowed
2. text/plain
3. text/*
4. */*

The media type quality factor associated with a given type is determined by finding the media range with the highest precedence that matches the type. For example,

  Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
          text/html;level=2;q=0.4, */*;q=0.5

would cause the following values to be associated:

Note: A user agent might be provided with a default set of quality values for certain media ranges. However, unless the user agent is a closed system that cannot interact with other rendering agents, this default set ought to be configurable by the user.

5.3.3. Accept-Charset

The "Accept-Charset" header field can be sent by a user agent to indicate what charsets are acceptable in textual response content. This field allows user agents capable of understanding more comprehensive or special-purpose charsets to signal that capability to an origin server that is capable of representing information in those charsets.

  Accept-Charset = 1#( ( charset / "*" ) [ weight ] )

Charset names are defined in Section 3.1.1.2. A user agent MAY associate a quality value with each charset to indicate the user's relative preference for that charset, as defined in Section 5.3.1. An example is

  Accept-Charset: iso-8859-5, unicode-1-1;q=0.8

The special value "*", if present in the Accept-Charset field, matches every charset that is not mentioned elsewhere in the Accept-Charset field. If no "*" is present in an Accept-Charset field, then any charsets not explicitly mentioned in the field are considered "not acceptable" to the client.

A request without any Accept-Charset header field implies that the user agent will accept any charset in response. Most general-purpose user agents do not send Accept-Charset, unless specifically configured to do so, because a detailed list of supported charsets makes it easier for a server to identify an individual by virtue of the user agent's request characteristics (Section 9.6).

If an Accept-Charset header field is present in a request and none of the available representations for the response has a charset that is listed as acceptable, the origin server can either honor the header field, by sending a 406 (Not Acceptable) [status.406] response, or disregard the header field by treating the resource as if it is not subject to content negotiation.

5.3.4. Accept-Encoding

The "Accept-Encoding" header field can be used by user agents to indicate what response content-codings (Section 3.1.2.1) are acceptable in the response. An "identity" token is used as a synonym for "no encoding" in order to communicate when no encoding is preferred.

  Accept-Encoding  = #( codings [ weight ] )
  codings          = content-coding / "identity" / "*"

Each codings value MAY be given an associated quality value representing the preference for that encoding, as defined in Section 5.3.1. The asterisk "*" symbol in an Accept-Encoding field matches any available content-coding not explicitly listed in the header field.

For example,

  Accept-Encoding: compress, gzip
  Accept-Encoding:
  Accept-Encoding: *
  Accept-Encoding: compress;q=0.5, gzip;q=1.0
  Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0

A request without an Accept-Encoding header field implies that the user agent has no preferences regarding content-codings. Although this allows the server to use any content-coding in a response, it does not imply that the user agent will be able to correctly process all encodings.

A server tests whether a content-coding for a given representation is acceptable using these rules:

1. If no Accept-Encoding field is in the request, any content-coding is considered acceptable by the user agent.
2. If the representation has no content-coding, then it is acceptable by default unless specifically excluded by the Accept-Encoding field stating either "identity;q=0" or "*;q=0" without a more specific entry for "identity".
3. If the representation's content-coding is one of the content-codings listed in the Accept-Encoding field, then it is acceptable unless it is accompanied by a qvalue of 0. (As defined in Section 5.3.1, a qvalue of 0 means "not acceptable".)
4. If multiple content-codings are acceptable, then the acceptable content-coding with the highest non-zero qvalue is preferred.

An Accept-Encoding header field with a combined field-value that is empty implies that the user agent does not want any content-coding in response. If an Accept-Encoding header field is present in a request and none of the available representations for the response have a content-coding that is listed as acceptable, the origin server SHOULD send a response without any content-coding.

• Note: Most HTTP/1.0 applications do not recognize or obey qvalues associated with content-codings. This means that qvalues might not work and are not permitted with x-gzip or x-compress.

5.3.5. Accept-Language

The "Accept-Language" header field can be used by user agents to indicate the set of natural languages that are preferred in the response. Language tags are defined in Section 3.1.3.1.

  Accept-Language = 1#( language-range [ weight ] )
  language-range  = 
            <language-range, defined in [RFC4647], Section 2.1>

Each language-range can be given an associated quality value representing an estimate of the user's preference for the languages specified by that range, as defined in Section 5.3.1. For example,

  Accept-Language: da, en-gb;q=0.8, en;q=0.7

would mean: "I prefer Danish, but will accept British English and other types of English".

A request without any Accept-Language header field implies that the user agent will accept any language in response. If the header field is present in a request and none of the available representations for the response have a matching language tag, the origin server can either disregard the header field by treating the response as if it is not subject to content negotiation, or honor the header field by sending a 406 (Not Acceptable) [status.406] response. However, the latter is not encouraged, as doing so can prevent users from accessing content that they might be able to use (with translation software, for example).

Note that some recipients treat the order in which language tags are listed as an indication of descending priority, particularly for tags that are assigned equal quality values (no value is the same as q=1). However, this behavior cannot be relied upon. For consistency and to maximize interoperability, many user agents assign each language tag a unique quality value while also listing them in order of decreasing quality. Additional discussion of language priority lists can be found in Section 2.3 of [RFC4647].

For matching, Section 3 of [RFC4647] defines several matching schemes. Implementations can offer the most appropriate matching scheme for their requirements. The "Basic Filtering" scheme ([RFC4647], Section 3.3.1) is identical to the matching scheme that was previously defined for HTTP in Section 14.4 of [RFC2616].

It might be contrary to the privacy expectations of the user to send an Accept-Language header field with the complete linguistic preferences of the user in every request (Section 9.6).

Since intelligibility is highly dependent on the individual user, user agents need to allow user control over the linguistic preference. A user agent that does not provide such control to the user MUST NOT send an Accept-Language header field.

• Note: User agents ought to provide guidance to users when setting a preference, since users are rarely familiar with the details of language matching as described above. For example, users might assume that on selecting "en-gb", they will be served any kind of English document if British English is not available. A user agent might suggest, in such a case, to add "en" to the list for better matching behavior.

5.4. Authentication Credentials

Two header fields are used for carrying authentication credentials, as defined in [Part7]. Note that various custom mechanisms for user authentication use the Cookie header field for this purpose, as defined in [RFC6265].

5.5. Request Context

The following request header fields provide additional information about the request context, including information about the user, user agent, and resource behind the request.

5.5.1. From

The "From" header field contains an Internet email address for a human user who controls the requesting user agent. The address ought to be machine-usable, as defined by "mailbox" in Section 3.4 of [RFC5322]:

  From    = mailbox
  
  mailbox = <mailbox, defined in [RFC5322], Section 3.4>

An example is:

  From: webmaster@example.org

The From header field is rarely sent by non-robotic user agents. A user agent SHOULD NOT send a From header field without explicit configuration by the user, since that might conflict with the user's privacy interests or their site's security policy.

Robotic user agents SHOULD send a valid From header field so that the person responsible for running the robot can be contacted if problems occur on servers, such as if the robot is sending excessive, unwanted, or invalid requests.

Servers SHOULD NOT use the From header field for access control or authentication, since most recipients will assume that the field value is public information.

5.5.2. Referer

The "Referer" [sic] header field allows the user agent to specify a URI reference for the resource from which the target URI was obtained (i.e., the "referrer", though the field name is misspelled). A user agent MUST exclude any fragment or userinfo components [RFC3986] when generating the Referer field value.

  Referer = absolute-URI / partial-URI

Referer allows servers to generate back-links to other resources for simple analytics, logging, optimized caching, etc. It also allows obsolete or mistyped links to be found for maintenance. Some servers use Referer as a means of denying links from other sites (so-called "deep linking") or restricting cross-site request forgery (CSRF), but not all requests contain a Referer header field.

Example:

  Referer: http://www.example.org/hypertext/Overview.html

If the target URI was obtained from a source that does not have its own URI (e.g., input from the user keyboard, or an entry within the user's bookmarks/favorites), the user agent MUST either exclude Referer or send it with a value of "about:blank".

The Referer field has the potential to reveal information about the request context or browsing history of the user, which is a privacy concern if the referring resource's identifier reveals personal information (such as an account name) or a resource that is supposed to be confidential (such as behind a firewall or internal to a secured service). Most general-purpose user agents do not send the Referer header field when the referring resource is a local "file" or "data" URI. A user agent SHOULD NOT send a Referer [header.referer] header field in an unsecured HTTP request if the referring page was received with a secure protocol. See Section 9.3 for additional security considerations.

Some intermediaries have been known to indiscriminately remove Referer header fields from outgoing requests. This has the unfortunate side-effect of interfering with protection against CSRF attacks, which can be far more harmful to their users. Intermediaries and user agent extensions that wish to limit information disclosure in Referer ought to restrict their changes to specific edits, such as replacing internal domain names with pseudonyms or truncating the query and/or path components. Intermediaries SHOULD NOT modify or delete the Referer field when the field value shares the same scheme and host as the request target.

5.5.3. User-Agent

The "User-Agent" header field contains information about the user agent originating the request, which is often used by servers to help identify the scope of reported interoperability problems, to work around or tailor responses to avoid particular user agent limitations, and for analytics regarding browser or operating system use. A user agent SHOULD send a User-Agent field in each request unless specifically configured not to do so.

  User-Agent = product *( RWS ( product / comment ) )

The User-Agent field-value consists of one or more product identifiers, each followed by zero or more comments (Section 3.2 of [Part1]), which together identify the user agent software and its significant subproducts. By convention, the product identifiers are listed in decreasing order of their significance for identifying the user agent software. Each product identifier consists of a name and optional version.

  product         = token ["/" product-version]
  product-version = token

Senders SHOULD limit generated product identifiers to what is necessary to identify the product; senders MUST NOT generate advertising or other non-essential information within the product identifier. Senders SHOULD NOT generate information in product-version [header.user-agent] that is not a version identifier (i.e., successive versions of the same product name ought to only differ in the product-version portion of the product identifier).

Example:

  User-Agent: CERN-LineMode/2.15 libwww/2.17b3

A user agent SHOULD NOT generate a User-Agent field containing needlessly fine-grained detail and SHOULD limit the addition of subproducts by third parties. Overly long and detailed User-Agent field values increase request latency and the risk of a user being identified against their wishes ("fingerprinting").

Likewise, implementations are encouraged not to use the product tokens of other implementations in order to declare compatibility with them, as this circumvents the purpose of the field. If a user agent masquerades as a different user agent, recipients can assume that the user intentionally desires to see responses tailored for that identified user agent, even if they might not work as well for the actual user agent being used.

6. Response Status Codes

The status-code element is a 3-digit integer code giving the result of the attempt to understand and satisfy the request.

HTTP status codes are extensible. HTTP clients are not required to understand the meaning of all registered status codes, though such understanding is obviously desirable. However, clients MUST understand the class of any status code, as indicated by the first digit, and treat an unrecognized status code as being equivalent to the x00 status code of that class, with the exception that a response with an unrecognized status code MUST NOT be cached.

For example, if an unrecognized status code of 471 is received by a client, the client can assume that there was something wrong with its request and treat the response as if it had received a 400 status code. The response message will usually contain a representation that explains the status.

The first digit of the status-code defines the class of response. The last two digits do not have any categorization role. There are 5 values for the first digit:

• 1xx (Informational) [status.1xx]: The request was received, continuing process
• 2xx (Successful) [status.2xx]: The request was successfully received, understood, and accepted
• 3xx (Redirection) [status.3xx]: Further action needs to be taken in order to complete the request
• 4xx (Client Error) [status.4xx]: The request contains bad syntax or cannot be fulfilled
• 5xx (Server Error) [status.5xx]: The server failed to fulfill an apparently valid request

6.1. Overview of Status Codes

The status codes listed below are defined in this specification, Section 4 of [Part4], Section 4 of [Part5], and Section 3 of [Part7]. The reason phrases listed here are only recommendations — they can be replaced by local equivalents without affecting the protocol.

Note that this list is not exhaustive — it does not include extension status codes defined in other specifications.

Responses with status codes that are defined as cacheable by default (e.g., 200, 203, 206, 300, 301, and 410 in this specification) can be reused by a cache with heuristic expiration unless otherwise indicated by the method definition or explicit cache controls [Part6]; all other status codes are not cacheable by default.

6.2. Informational 1xx

The 1xx (Informational) class of status code indicates an interim response for communicating connection status or request progress prior to completing the requested action and sending a final response. All 1xx responses consist of only the status-line and optional header fields, and thus are terminated by the empty line at the end of the header block. Since HTTP/1.0 did not define any 1xx status codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client except under experimental conditions.

A client MUST be prepared to accept one or more 1xx status responses prior to a final response, even if the client does not expect one. A user agent MAY ignore unexpected 1xx status responses.

Proxies MUST forward 1xx responses, unless the connection between the proxy and its client has been closed, or unless the proxy itself requested the generation of the 1xx response. For example, if a proxy adds an "Expect: 100-continue" field when it forwards a request, then it need not forward the corresponding 100 (Continue) [status.100] response(s).

6.2.1. 100 Continue

The 100 (Continue) status code indicates that the initial part of a request has been received and has not yet been rejected by the server. The server intends to send a final response after the request has been fully received and acted upon.

When the request contains an Expect [header.expect] header field that includes a 100-continue [use.of.the.100.status] expectation, the 100 response indicates that the server wishes to receive the request payload body, as described in Section 5.1.1.1. The client ought to continue sending the request and discard the 100 response.

If the request did not contain an Expect [header.expect] header field containing the 100-continue [use.of.the.100.status] expectation, the client can simply discard this interim response.

6.2.2. 101 Switching Protocols

The 101 (Switching Protocols) status code indicates that the server understands and is willing to comply with the client's request, via the Upgrade header field (Section 6.7 of [Part1]), for a change in the application protocol being used on this connection. The server MUST generate an Upgrade header field in the response that indicates which protocol(s) will be switched to immediately after the empty line that terminates the 101 response.

It is assumed that the server will only agree to switch protocols when it is advantageous to do so. For example, switching to a newer version of HTTP might be advantageous over older versions, and switching to a real-time, synchronous protocol might be advantageous when delivering resources that use such features.

6.3. Successful 2xx

The 2xx (Successful) class of status code indicates that the client's request was successfully received, understood, and accepted.

6.3.1. 200 OK

The

GET

a representation of the target resource [resources];

HEAD

the same representation as GET, but without the representation data;

POST

a representation of the status of, or results obtained from, the action;

PUT, DELETE

a representation of the status of the action;

OPTIONS

a representation of the communications options;

TRACE

a representation of the request message as received by the end server.

Aside from responses to CONNECT, a 200 response always has a payload, though an origin server MAY generate a payload body of zero length. If no payload is desired, an origin server ought to send 204 (No Content) instead. For CONNECT, no payload is allowed because the successful result is a tunnel, which begins immediately after the 200 response header block.

A 200 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.3.2. 201 Created

The 201 (Created) status code indicates that the request has been fulfilled and has resulted in one or more new resources being created. The primary resource created by the request is identified by either a Location [header.location] header field in the response or, if no Location [header.location] field is received, by the effective request URI.

The 201 response payload typically describes and links to the resource(s) created. See Section 7.2 for a discussion of the meaning and purpose of validator header fields, such as ETag and Last-Modified, in a 201 response.

6.3.3. 202 Accepted

The 202 (Accepted) status code indicates that the request has been accepted for processing, but the processing has not been completed. The request might or might not eventually be acted upon, as it might be disallowed when processing actually takes place. There is no facility in HTTP for re-sending a status code from an asynchronous operation.

The 202 response is intentionally non-committal. Its purpose is to allow a server to accept a request for some other process (perhaps a batch-oriented process that is only run once per day) without requiring that the user agent's connection to the server persist until the process is completed. The representation sent with this response ought to describe the request's current status and point to (or embed) a status monitor that can provide the user with an estimate of when the request will be fulfilled.

6.3.4. 203 Non-Authoritative Information

The 203 (Non-Authoritative Information) status code indicates that the request was successful but the enclosed payload has been modified from that of the origin server's 200 (OK) [status.200] response by a transforming proxy (Section 5.7.2 of [Part1]). This status code allows the proxy to notify recipients when a transformation has been applied, since that knowledge might impact later decisions regarding the content. For example, future cache validation requests for the content might only be applicable along the same request path (through the same proxies).

The 203 response is similar to the Warning code of 214 Transformation Applied (Section 7.5 of [Part6]), which has the advantage of being applicable to responses with any status code.

A 203 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.3.5. 204 No Content

The 204 (No Content) status code indicates that the server has successfully fulfilled the request and that there is no additional content to send in the response payload body. Metadata in the response header fields refer to the target resource [resources] and its selected representation [representations] after the requested action was applied.

For example, if a 204 status code is received in response to a PUT request and the response contains an ETag header field, then the PUT was successful and the ETag field-value contains the entity-tag for the new representation of that target resource.

The 204 response allows a server to indicate that the action has been successfully applied to the target resource, while implying that the user agent does not need to traverse away from its current "document view" (if any). The server assumes that the user agent will provide some indication of the success to its user, in accord with its own interface, and apply any new or updated metadata in the response to its active representation.

For example, a 204 status code is commonly used with document editing interfaces corresponding to a "save" action, such that the document being saved remains available to the user for editing. It is also frequently used with interfaces that expect automated data transfers to be prevalent, such as within distributed version control systems.

A 204 response is terminated by the first empty line after the header fields because it cannot contain a message body.

A 204 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.3.6. 205 Reset Content

The 205 (Reset Content) status code indicates that the server has fulfilled the request and desires that the user agent reset the "document view", which caused the request to be sent, to its original state as received from the origin server.

This response is intended to support a common data entry use case where the user receives content that supports data entry (a form, notepad, canvas, etc.), enters or manipulates data in that space, causes the entered data to be submitted in a request, and then the data entry mechanism is reset for the next entry so that the user can easily initiate another input action.

Since the 205 status code implies that no additional content will be provided in the payload, the server MUST send a message body of zero length. In other words, the server MUST send a "Content-Length: 0" field in a 205 response or close the connection immediately after sending the blank line terminating the header section.

6.4. Redirection 3xx

The 3xx (Redirection) class of status code indicates that further action needs to be taken by the user agent in order to fulfill the request. If a Location [header.location] header field (Section 7.1.2) is provided, the user agent MAY automatically redirect its request to the URI referenced by the Location field value, even if the specific status code is not understood. Automatic redirection needs to done with care for methods not known to be safe [safe.methods], as defined in Section 4.2.1, since the user might not wish to redirect an unsafe request.

There are several types of redirects:

1. Redirects that indicate the resource might be available at a different URI, as provided by the Location [header.location] field, as in the status codes 301 (Moved Permanently) [status.301], 302 (Found) [status.302], and 307 (Temporary Redirect) [status.307].
2. Redirection that offers a choice of matching resources, each capable of representing the original request target, as in the 300 (Multiple Choices) [status.300] status code.
3. Redirection to a different resource, identified by the Location [header.location] field, that can represent an indirect response to the request, as in the 303 (See Other) [status.303] status code.
4. Redirection to a previously cached result, as in the 304 (Not Modified) status code.

• Note: In HTTP/1.0, the status codes 301 (Moved Permanently) [status.301] and 302 (Found) [status.302] were defined for the first type of redirect ([RFC1945], Section 9.3). Early user agents split on whether the method applied to the redirect target would be the same as the original request or would be rewritten as GET. Although HTTP originally defined the former semantics for 301 [status.301] and 302 [status.302] (to match its original implementation at CERN), and defined 303 (See Other) [status.303] to match the latter semantics, prevailing practice gradually converged on the latter semantics for 301 [status.301] and 302 [status.302] as well. The first revision of HTTP/1.1 added 307 (Temporary Redirect) [status.307] to indicate the former semantics without being impacted by divergent practice. Over 10 years later, most user agents still do method rewriting for 301 [status.301] and 302 [status.302]; therefore, this specification makes that behavior conformant when the original request is POST.

Clients SHOULD detect and intervene in cyclical redirections (i.e., "infinite" redirection loops).

• Note: An earlier version of this specification recommended a maximum of five redirections ([RFC2068], Section 10.3). Content developers need to be aware that some clients might implement such a fixed limitation.

6.4.1. 300 Multiple Choices

The 300 (Multiple Choices) status code indicates that the target resource [resources] has more than one representation, each with its own more specific identifier, and information about the alternatives is being provided so that the user (or user agent) can select a preferred representation by redirecting its request to one or more of those identifiers. In other words, the server desires that the user agent engage in reactive negotiation to select the most appropriate representation(s) for its needs (Section 3.4).

If the server has a preferred choice, the server SHOULD generate a Location [header.location] header field containing a preferred choice's URI reference. The user agent MAY use the Location field value for automatic redirection.

For request methods other than HEAD, the server SHOULD generate a payload in the 300 response containing a list of representation metadata and URI reference(s) from which the user or user agent can choose the one most preferred. The user agent MAY make a selection from that list automatically, depending upon the list format, but this specification does not define a standard for such automatic selection.

A 300 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see Section 4.1.2 of [Part6]).

• Note: The original proposal for 300 defined the URI header field as providing a list of alternative representations, such that it would be usable for 200, 300, and 406 responses and be transferred in responses to the HEAD method. However, lack of deployment and disagreement over syntax led to both URI and Alternates (a subsequent proposal) being dropped from this specification. It is possible to communicate the list using a set of Link header fields [RFC5988], each with a relationship of "alternate", though deployment is a chicken-and-egg problem.

6.4.2. 301 Moved Permanently

The 301 (Moved Permanently) status code indicates that the target resource [resources] has been assigned a new permanent URI and any future references to this resource ought to use one of the enclosed URIs. Clients with link editing capabilities ought to automatically re-link references to the effective request URI to one or more of the new references sent by the server, where possible.

The server SHOULD generate a Location [header.location] header field in the response containing a preferred URI reference for the new permanent URI. The user agent MAY use the Location field value for automatic redirection. The server's response payload usually contains a short hypertext note with a hyperlink to the new URI(s).

• Note: For historic reasons, user agents MAY change the request method from POST to GET for the subsequent request. If this behavior is undesired, status code 307 (Temporary Redirect) [status.307] can be used instead.

A 301 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.4.3. 302 Found

The 302 (Found) status code indicates that the target resource resides temporarily under a different URI. Since the redirection might be altered on occasion, the client ought to continue to use the effective request URI for future requests.

The server SHOULD generate a Location [header.location] header field in the response containing a URI reference for the different URI. The user agent MAY use the Location field value for automatic redirection. The server's response payload usually contains a short hypertext note with a hyperlink to the different URI(s).

• Note: For historic reasons, user agents MAY change the request method from POST to GET for the subsequent request. If this behavior is undesired, status code 307 (Temporary Redirect) [status.307] can be used instead.

6.4.4. 303 See Other

The 303 (See Other) status code indicates that the server is redirecting the user agent to a different resource, as indicated by a URI in the Location [header.location] header field, that is intended to provide an indirect response to the original request. In order to satisfy the original request, a user agent ought to perform a retrieval request using the Location URI (a GET or HEAD request if using HTTP), which can itself be redirected further, and present the eventual result as an answer to the original request. Note that the new URI in the Location header field is not considered equivalent to the effective request URI.

This status code is applicable to any HTTP method. It is primarily used to allow the output of a POST action to redirect the user agent to a selected resource, since doing so provides the information corresponding to the POST response in a form that can be separately identified, bookmarked, and cached independent of the original request.

A 303 response to a GET request indicates that the origin server does not have a representation of the target resource [resources] that can be transferred by the server over HTTP. However, the Location [header.location] field value refers to a resource that is descriptive of the target resource, such that making a retrieval request on that other resource might result in a representation that is useful to recipients without implying that it represents the original target resource. Note that answers to the questions of what can be represented, what representations are adequate, and what might be a useful description are outside the scope of HTTP.

Except for responses to a HEAD request, the representation of a 303 response ought to contain a short hypertext note with a hyperlink to the same URI reference provided in the Location [header.location] header field.

6.4.5. 305 Use Proxy

The 305 (Use Proxy) status code was defined in a previous version of this specification and is now deprecated (Appendix B).

6.4.6. 306 (Unused)

The 306 status code was defined in a previous version of this specification, is no longer used, and the code is reserved.

6.4.7. 307 Temporary Redirect

The 307 (Temporary Redirect) status code indicates that the target resource [resources] resides temporarily under a different URI and the user agent MUST NOT change the request method if it performs an automatic redirection to that URI. Since the redirection can change over time, the client ought to continue using the original effective request URI for future requests.

The server SHOULD generate a Location [header.location] header field in the response containing a URI reference for the different URI. The user agent MAY use the Location field value for automatic redirection. The server's response payload usually contains a short hypertext note with a hyperlink to the different URI(s).

• Note: This status code is similar to 302 (Found) [status.302], except that it does not allow changing the request method from POST to GET. This specification defines no equivalent counterpart for 301 (Moved Permanently) [status.301] ([status-308], however, defines the status code 308 (Permanent Redirect) for this purpose).

6.5. Client Error 4xx

The 4xx (Client Error) class of status code indicates that the client seems to have erred. Except when responding to a HEAD request, the server SHOULD send a representation containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. User agents SHOULD display any included representation to the user.

6.5.1. 400 Bad Request

The 400 (Bad Request) status code indicates that the server cannot or will not process the request because the received syntax is invalid, nonsensical, or exceeds some limitation on what the server is willing to process.

6.5.2. 402 Payment Required

The 402 (Payment Required) status code is reserved for future use.

6.5.3. 403 Forbidden

The 403 (Forbidden) status code indicates that the server understood the request but refuses to authorize it. A server that wishes to make public why the request has been forbidden can describe that reason in the response payload (if any).

If authentication credentials were provided in the request, the server considers them insufficient to grant access. The client SHOULD NOT repeat the request with the same credentials. The client MAY repeat the request with new or different credentials. However, a request might be forbidden for reasons unrelated to the credentials.

An origin server that wishes to "hide" the current existence of a forbidden target resource [resources] MAY instead respond with a status code of 404 (Not Found) [status.404].

6.5.4. 404 Not Found

The 404 (Not Found) status code indicates that the origin server did not find a current representation for the target resource [resources] or is not willing to disclose that one exists. A 404 status does not indicate whether this lack of representation is temporary or permanent; the 410 (Gone) [status.410] status code is preferred over 404 if the origin server knows, presumably through some configurable means, that the condition is likely to be permanent.

A 404 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.5.5. 405 Method Not Allowed

The 405 (Method Not Allowed) status code indicates that the method specified in the request-line is known by the origin server but not supported by the target resource [resources]. The origin server MUST generate an Allow [header.allow] header field in a 405 response containing a list of the target resource's currently supported methods.

A 405 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.5.6. 406 Not Acceptable

The 406 (Not Acceptable) status code indicates that the target resource [resources] does not have a current representation that would be acceptable to the user agent, according to the proactive negotiation [proactive.negotiation] header fields received in the request (Section 5.3), and the server is unwilling to supply a default representation.

The server SHOULD generate a payload containing a list of available representation characteristics and corresponding resource identifiers from which the user or user agent can choose the one most appropriate. A user agent MAY automatically select the most appropriate choice from that list. However, this specification does not define any standard for such automatic selection, as described in Section 6.4.1.

6.5.7. 408 Request Timeout

The 408 (Request Timeout) status code indicates that the server did not receive a complete request message within the time that it was prepared to wait. A server SHOULD send the close connection option (Section 6.1 of [Part1]) in the response, since 408 implies that the server has decided to close the connection rather than continue waiting. If the client has an outstanding request in transit, the client MAY repeat that request on a new connection.

6.5.8. 409 Conflict

The 409 (Conflict) status code indicates that the request could not be completed due to a conflict with the current state of the resource. This code is used in situations where the user might be able to resolve the conflict and resubmit the request. The server SHOULD generate a payload that includes enough information for a user to recognize the source of the conflict.

Conflicts are most likely to occur in response to a PUT request. For example, if versioning were being used and the representation being PUT included changes to a resource that conflict with those made by an earlier (third-party) request, the origin server might use a 409 response to indicate that it can't complete the request. In this case, the response representation would likely contain information useful for merging the differences based on the revision history.

6.5.9. 410 Gone

The 410 (Gone) status code indicates that access to the target resource [resources] is no longer available at the origin server and that this condition is likely to be permanent. If the origin server does not know, or has no facility to determine, whether or not the condition is permanent, the status code 404 (Not Found) [status.404] ought to be used instead.

The 410 response is primarily intended to assist the task of web maintenance by notifying the recipient that the resource is intentionally unavailable and that the server owners desire that remote links to that resource be removed. Such an event is common for limited-time, promotional services and for resources belonging to individuals no longer associated with the origin server's site. It is not necessary to mark all permanently unavailable resources as "gone" or to keep the mark for any length of time — that is left to the discretion of the server owner.

A 410 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.5.10. 411 Length Required

The 411 (Length Required) status code indicates that the server refuses to accept the request without a defined Content-Length (Section 3.3.2 of [Part1]). The client MAY repeat the request if it adds a valid Content-Length header field containing the length of the message body in the request message.

6.5.11. 413 Payload Too Large

The 413 (Payload Too Large) status code indicates that the server is refusing to process a request because the request payload is larger than the server is willing or able to process. The server MAY close the connection to prevent the client from continuing the request.

If the condition is temporary, the server SHOULD generate a Retry-After [header.retry-after] header field to indicate that it is temporary and after what time the client MAY try again.

6.5.12. 414 URI Too Long

The 414 (URI Too Long) status code indicates that the server is refusing to service the request because the request-target (Section 5.3 of [Part1]) is longer than the server is willing to interpret. This rare condition is only likely to occur when a client has improperly converted a POST request to a GET request with long query information, when the client has descended into a "black hole" of redirection (e.g., a redirected URI prefix that points to a suffix of itself), or when the server is under attack by a client attempting to exploit potential security holes.

A 414 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.5.13. 415 Unsupported Media Type

The 415 (Unsupported Media Type) status code indicates that the origin server is refusing to service the request because the payload is in a format not supported by the target resource [resources] for this method. The format problem might be due to the request's indicated Content-Type [header.content-type] or Content-Encoding [header.content-encoding], or as a result of inspecting the data directly.

6.5.14. 417 Expectation Failed

The 417 (Expectation Failed) status code indicates that the expectation given in the request's Expect [header.expect] header field (Section 5.1.1) could not be met by at least one of the inbound servers.

6.5.15. 426 Upgrade Required

The 426 (Upgrade Required) status code indicates that the server refuses to perform the request using the current protocol but might be willing to do so after the client upgrades to a different protocol. The server MUST send an Upgrade header field in a 426 response to indicate the required protocol(s) (Section 6.7 of [Part1]).

Example:

  HTTP/1.1 426 Upgrade Required
  Upgrade: HTTP/3.0
  Connection: Upgrade
  Content-Length: 53
  Content-Type: text/plain
  
  This service requires use of the HTTP/3.0 protocol.
  

6.6. Server Error 5xx

The 5xx (Server Error) class of status code indicates that the server is aware that it has erred or is incapable of performing the requested method. Except when responding to a HEAD request, the server SHOULD send a representation containing an explanation of the error situation, and whether it is a temporary or permanent condition. User agents SHOULD display any included representation to the user. These response codes are applicable to any request method.

6.6.1. 500 Internal Server Error

The 500 (Internal Server Error) status code indicates that the server encountered an unexpected condition that prevented it from fulfilling the request.

6.6.2. 501 Not Implemented

The 501 (Not Implemented) status code indicates that the server does not support the functionality required to fulfill the request. This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.

A 501 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.6.3. 502 Bad Gateway

The 502 (Bad Gateway) status code indicates that the server, while acting as a gateway or proxy, received an invalid response from an inbound server it accessed while attempting to fulfill the request.

6.6.4. 503 Service Unavailable

The 503 (Service Unavailable) status code indicates that the server is currently unable to handle the request due to a temporary overload or scheduled maintenance, which will likely be alleviated after some delay. The server MAY send a Retry-After [header.retry-after] header field (Section 7.1.3) to suggest an appropriate amount of time for the client to wait before retrying the request.

• Note: The existence of the 503 status code does not imply that a server has to use it when becoming overloaded. Some servers might simply refuse the connection.

6.6.5. 504 Gateway Timeout

The 504 (Gateway Timeout) status code indicates that the server, while acting as a gateway or proxy, did not receive a timely response from an upstream server it needed to access in order to complete the request.

6.6.6. 505 HTTP Version Not Supported

The 505 (HTTP Version Not Supported) status code indicates that the server does not support, or refuses to support, the major version of HTTP that was used in the request message. The server is indicating that it is unable or unwilling to complete the request using the same major version as the client, as described in Section 2.6 of [Part1], other than with this error message. The server SHOULD generate a representation for the 505 response that describes why that version is not supported and what other protocols are supported by that server.

7. Response Header Fields

The response header fields allow the server to pass additional information about the response beyond what is placed in the status-line. These header fields give information about the server, about further access to the target resource [resources], or about related resources.

Although each response header field has a defined meaning, in general, the precise semantics might be further refined by the semantics of the request method and/or response status code.

7.1. Control Data

Response header fields can supply control data that supplements the status code, directs caching, or instructs the client where to go next.

7.1.1. Origination Date

7.1.1.1. Date/Time Formats

Prior to 1995, there were three different formats commonly used by servers to communicate timestamps. For compatibility with old implementations, all three are defined here. The preferred format is a fixed-length and single-zone subset of the date and time specification used by the Internet Message Format [RFC5322].

  HTTP-date    = IMF-fixdate / obs-date

An example of the preferred format is

  Sun, 06 Nov 1994 08:49:37 GMT    ; IMF-fixdate
  

Examples of the two obsolete formats are

  Sunday, 06-Nov-94 08:49:37 GMT   ; obsolete RFC 850 format
  Sun Nov  6 08:49:37 1994         ; ANSI C's asctime() format
  

A recipient that parses a timestamp value in an HTTP header field MUST accept all three formats. A sender MUST generate the IMF-fixdate format when sending an HTTP-date value in a header field.

An HTTP-date value represents time as an instance of Coordinated Universal Time (UTC). The first two formats indicate UTC by the three-letter abbreviation for Greenwich Mean Time, "GMT", a predecessor of the UTC name; values in the asctime format are assumed to be in UTC. A sender that generates HTTP-date values from a local clock ought to use NTP ([RFC1305]) or some similar protocol to synchronize its clock to UTC.

Preferred format:

  IMF-fixdate  = day-name "," SP date1 SP time-of-day SP GMT
  ; fixed length/zone subset of the format defined in
  ; Section 3.3 of [RFC5322]
  
  day-name     = %x4D.6F.6E ; "Mon", case-sensitive
               / %x54.75.65 ; "Tue", case-sensitive
               / %x57.65.64 ; "Wed", case-sensitive
               / %x54.68.75 ; "Thu", case-sensitive
               / %x46.72.69 ; "Fri", case-sensitive
               / %x53.61.74 ; "Sat", case-sensitive
               / %x53.75.6E ; "Sun", case-sensitive
               
  date1        = day SP month SP year
               ; e.g., 02 Jun 1982

  day          = 2DIGIT
  month        = %x4A.61.6E ; "Jan", case-sensitive
               / %x46.65.62 ; "Feb", case-sensitive
               / %x4D.61.72 ; "Mar", case-sensitive
               / %x41.70.72 ; "Apr", case-sensitive
               / %x4D.61.79 ; "May", case-sensitive
               / %x4A.75.6E ; "Jun", case-sensitive
               / %x4A.75.6C ; "Jul", case-sensitive
               / %x41.75.67 ; "Aug", case-sensitive
               / %x53.65.70 ; "Sep", case-sensitive
               / %x4F.63.74 ; "Oct", case-sensitive
               / %x4E.6F.76 ; "Nov", case-sensitive
               / %x44.65.63 ; "Dec", case-sensitive
  year         = 4DIGIT

  GMT          = %x47.4D.54 ; "GMT", case-sensitive

  time-of-day  = hour ":" minute ":" second
               ; 00:00:00 - 23:59:60 (leap second)
                 
  hour         = 2DIGIT               
  minute       = 2DIGIT               
  second       = 2DIGIT               

Obsolete formats:

  obs-date     = rfc850-date / asctime-date 

  rfc850-date  = day-name-l "," SP date2 SP time-of-day SP GMT
  date2        = day "-" month "-" 2DIGIT
               ; e.g., 02-Jun-82

  day-name-l   = %x4D.6F.6E.64.61.79    ; "Monday", case-sensitive
         / %x54.75.65.73.64.61.79       ; "Tuesday", case-sensitive
         / %x57.65.64.6E.65.73.64.61.79 ; "Wednesday", case-sensitive
         / %x54.68.75.72.73.64.61.79    ; "Thursday", case-sensitive
         / %x46.72.69.64.61.79          ; "Friday", case-sensitive
         / %x53.61.74.75.72.64.61.79    ; "Saturday", case-sensitive
         / %x53.75.6E.64.61.79          ; "Sunday", case-sensitive

  asctime-date = day-name SP date3 SP time-of-day SP year
  date3        = month SP ( 2DIGIT / ( SP 1DIGIT ))
               ; e.g., Jun  2

HTTP-date is case sensitive. A sender MUST NOT generate additional whitespace in an HTTP-date beyond that specifically included as SP in the grammar. The semantics of day-name [preferred.date.format], day [preferred.date.format], month [preferred.date.format], year [preferred.date.format], and time-of-day [preferred.date.format] are the same as those defined for the Internet Message Format constructs with the corresponding name ([RFC5322], Section 3.3).

Recipients of a timestamp value in rfc850-date format, which uses a two-digit year, SHOULD interpret a timestamp that appears to be more than 50 years in the future as representing the most recent year in the past that had the same last two digits.

Recipients of timestamp values are encouraged to be robust in parsing timestamps unless otherwise restricted by the field definition. For example, messages are occasionally forwarded over HTTP from a non-HTTP source that might generate any of the date and time specifications defined by the Internet Message Format.

• Note: HTTP requirements for the date/time stamp format apply only to their usage within the protocol stream. Implementations are not required to use these formats for user presentation, request logging, etc.

7.1.1.2. Date

The "Date" header field represents the date and time at which the message was originated, having the same semantics as the Origination Date Field (orig-date) defined in Section 3.6.1 of [RFC5322]. The field value is an HTTP-date, as defined in Section 7.1.1.1.

  Date = HTTP-date

An example is

  Date: Tue, 15 Nov 1994 08:12:31 GMT

When a Date header field is generated, the sender SHOULD generate its field value as the best available approximation of the date and time of message generation. In theory, the date ought to represent the moment just before the payload is generated. In practice, the date can be generated at any time during message origination.

An origin server MUST NOT send a Date header field if it does not have a clock capable of providing a reasonable approximation of the current instance in Coordinated Universal Time. An origin server MAY send a Date header field if the response is in the 1xx (Informational) [status.1xx] or 5xx (Server Error) [status.5xx] class of status codes. An origin server MUST send a Date header field in all other cases.

A recipient with a clock that receives a response message without a Date header field MUST record the time it was received and append a corresponding Date header field to the message's header block if it is cached or forwarded downstream.

A user agent MAY send a Date header field in a request, though generally will not do so unless it is believed to convey useful information to the server. For example, custom applications of HTTP might convey a Date if the server is expected to adjust its interpretation of the user's request based on differences between the user agent and server clocks.

7.1.2. Location

The "Location" header field is used in some responses to refer to a specific resource in relation to the response. The type of relationship is defined by the combination of request method and status code semantics.

  Location = URI-reference

The field value consists of a single URI-reference. When it has the form of a relative reference ([RFC3986], Section 4.2), the final value is computed by resolving it against the effective request URI ([RFC3986], Section 5).

For 201 (Created) [status.201] responses, the Location value refers to the primary resource created by the request. For 3xx (Redirection) [status.3xx] responses, the Location value refers to the preferred target resource for automatically redirecting the request.

When Location is provided in a 3xx (Redirection) [status.3xx] response and the URI reference that the user agent used to generate the request target contains a fragment identifier, the user agent SHOULD process the redirection as if the Location field value inherits the original fragment. In other words, if the Location does not have a fragment component, the user agent SHOULD interpret the Location reference as if it had the original reference's fragment.

For example, a GET request generated for the URI reference "http://www.example.org/~tim" might result in a 303 (See Other) [status.303] response containing the header field:

  Location: /People.html#tim

which suggests that the user agent redirect to "http://www.example.org/People.html#tim"

Likewise, a GET request generated for the URI reference "http://www.example.org/index.html#larry" might result in a 301 (Moved Permanently) [status.301] response containing the header field:

  Location: http://www.example.net/index.html

which suggests that the user agent redirect to "http://www.example.net/index.html#larry", preserving the original fragment identifier.

There are circumstances in which a fragment identifier in a Location value would not be appropriate. For example, the Location header field in a 201 (Created) [status.201] response is supposed to provide a URI that is specific to the created resource.

• Note: Some recipients attempt to recover from Location fields that are not valid URI references. This specification does not mandate or define such processing, but does allow it for the sake of robustness.

• Note: The Content-Location [header.content-location] header field (Section 3.1.4.2) differs from Location in that the Content-Location refers to the most specific resource corresponding to the enclosed representation. It is therefore possible for a response to contain both the Location and Content-Location header fields.

7.1.3. Retry-After

Servers send the "Retry-After" header field to indicate how long the user agent ought to wait before making a follow-up request. When sent with a 503 (Service Unavailable) [status.503] response, Retry-After indicates how long the service is expected to be unavailable to the client. When sent with any 3xx (Redirection) [status.3xx] response, Retry-After indicates the minimum time that the user agent is asked to wait before issuing the redirected request.

The value of this field can be either an HTTP-date or an integer number of seconds (in decimal) after the time of the response.

  Retry-After = HTTP-date / delta-seconds

Time spans are non-negative decimal integers, representing time in seconds.

  delta-seconds  = 1*DIGIT

  Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
  Retry-After: 120

7.1.4. Vary

The "Vary" header field describes what parts of a request message, aside from the method, the Host header field and the request target, might influence the origin server's process for selecting and representing the response. The value consists of either a single asterisk ("*") or a list of header field names (case-insensitive).

  Vary = "*" / 1#field-name

A Vary field value of "*" signals that anything about the request might play a role in selecting the response representation, possibly including elements outside the message syntax (e.g., the client's network address), and thus a recipient will not be able to determine whether this response is appropriate for a later request without forwarding the request to the origin server. A proxy MUST NOT generate a Vary field with a "*" value.

A Vary field value consisting of a comma-separated list of names indicates that the named request header fields, known as the selecting header fields, might have a role in selecting the representation. The potential selecting header fields are not limited to those defined by this specification.

For example, a response that contains

  Vary: accept-encoding, accept-language

indicates that the origin server might have used the request's Accept-Encoding [header.accept-encoding] and Accept-Language [header.accept-language] fields (or lack thereof) as determining factors while choosing the content for this response.

An origin server might send Vary with a list of fields for two purposes:

1. To inform cache recipients that they MUST NOT use this response to satisfy a later request unless the later request has the same values for the listed fields as the original request (Section 4.3 of [Part6]). In other words, Vary expands the cache key required to match a new request to the stored cache entry.
2. To inform user agent recipients that this response is subject to content negotiation (Section 5.3) and that a different representation might be sent in a subsequent request if additional parameters are provided in the listed header fields (proactive negotiation [proactive.negotiation]).

An origin server SHOULD send a Vary header field when its algorithm for selecting a representation varies based on aspects of the request message other than the method and request target, unless the variance cannot be crossed or the origin server has been deliberately configured to prevent cache transparency. For example, there is no need to send the Authorization field name in Vary because reuse across users is constrained by the field definition (Section 4.1 of [Part7]). Likewise, an origin server might use Cache-Control directives (Section 7.2 of [Part6]) to supplant Vary if it considers the variance less significant than the performance cost of Vary's impact on caching.

7.2. Validator Header Fields

Validator header fields convey metadata about the selected representation [representations] (Section 3). In responses to safe requests, validator fields describe the selected representation chosen by the origin server while handling the response. Note that, depending on the status code semantics, the selected representation [representations] for a given response is not necessarily the same as the representation enclosed as response payload.

In a successful response to a state-changing request, validator fields describe the new representation that has replaced the prior selected representation [representations] as a result of processing the request.

For example, an ETag header field in a 201 response communicates the entity-tag of the newly created resource's representation, so that it can be used in later conditional requests to prevent the "lost update" problem [Part4].

7.3. Authentication Challenges

Authentication challenges indicate what mechanisms are available for the client to provide authentication credentials in future requests.

7.4. Response Context

The remaining response header fields provide more information about the target resource [resources] for potential use in later requests.

7.4.1. Allow

The "Allow" header field lists the set of methods advertised as supported by the target resource [resources]. The purpose of this field is strictly to inform the recipient of valid request methods associated with the resource.

  Allow = #method

Example of use:

  Allow: GET, HEAD, PUT

The actual set of allowed methods is defined by the origin server at the time of each request. An origin server MUST generate an Allow field in a 405 (Method Not Allowed) [status.405] response and MAY do so in any other response. An empty Allow field value indicates that the resource allows no methods, which might occur in a 405 response if the resource has been temporarily disabled by configuration.

A proxy MUST NOT modify the Allow header field — it does not need to understand all of the indicated methods in order to handle them according to the generic message handling rules.

7.4.2. Server

The "Server" header field contains information about the software used by the origin server to handle the request, which is often used by clients to help identify the scope of reported interoperability problems, to work around or tailor requests to avoid particular server limitations, and for analytics regarding server or operating system use. An origin server MAY generate a Server field in its responses.

  Server = product *( RWS ( product / comment ) )

The Server field-value consists of one or more product identifiers, each followed by zero or more comments (Section 3.2 of [Part1]), which together identify the origin server software and its significant subproducts. By convention, the product identifiers are listed in decreasing order of their significance for identifying the origin server software. Each product identifier consists of a name and optional version, as defined in Section 5.5.3.

Example:

  Server: CERN/3.0 libwww/2.17

An origin server SHOULD NOT generate a Server field containing needlessly fine-grained detail and SHOULD limit the addition of subproducts by third parties. Overly long and detailed Server field values increase response latency and potentially reveal internal implementation details that might make it (slightly) easier for attackers to find and exploit known security holes.

8. IANA Considerations

8.1. Method Registry

The HTTP Method Registry defines the name space for the request method token (Section 4). The method registry will be created and maintained at http://www.iana.org/assignments/http-methods.

8.1.1. Procedure

HTTP method registrations MUST include the following fields:

• Method Name (see Section 4)
• Safe ("yes" or "no", see Section 4.2.1)
• Idempotent ("yes" or "no", see Section 4.2.2)
• Pointer to specification text

Values to be added to this name space require IETF Review (see [RFC5226], Section 4.1).

8.1.2. Considerations for New Methods

Standardized methods are generic; that is, they are potentially applicable to any resource, not just one particular media type, kind of resource, or application. As such, it is preferred that new methods be registered in a document that isn't specific to a single application or data format, since orthogonal technologies deserve orthogonal specification.

Since message parsing (Section 3.3 of [Part1]) needs to be independent of method semantics (aside from responses to HEAD), definitions of new methods cannot change the parsing algorithm or prohibit the presence of a message body on either the request or the response message. Definitions of new methods can specify that only a zero-length message body is allowed by requiring a Content-Length header field with a value of "0".

A new method definition needs to indicate whether it is safe (Section 4.2.1), idempotent (Section 4.2.2), cacheable (Section 4.2.3), what semantics are to be associated with the payload body if any is present in the request, and what refinements the method makes to header field or status code semantics. If the new method is cacheable, its definition ought to describe how, and under what conditions, a cache can store a response and use it to satisfy a subsequent request. The new method ought to describe whether it can be made conditional (Section 5.2) and, if so, how a server responds when the condition is false. Likewise, if the new method might have some use for partial response semantics ([Part5]), it ought to document this too.

• Note: Avoid defining a method name that starts with "M-", since that prefix might be misinterpreted as having the semantics assigned to it by [RFC2774].

8.1.3. Registrations

The HTTP Method Registry shall be populated with the registrations below:

  Example-URI-Field: "http://example.com/a.html,foo",
                     "http://without-a-comma.example.com/"
  Example-Date-Field: "Sat, 04 May 1996", "Wed, 14 Sep 2005"

  BWS           = <BWS, defined in [Part1], Section 3.2.3>
  OWS           = <OWS, defined in [Part1], Section 3.2.3>
  RWS           = <RWS, defined in [Part1], Section 3.2.3>
  URI-reference = <URI-reference, defined in [Part1], Section 2.7>
  absolute-URI  = <absolute-URI, defined in [Part1], Section 2.7>
  comment       = <comment, defined in [Part1], Section 3.2.6>
  field-name    = <comment, defined in [Part1], Section 3.2>
  partial-URI   = <partial-URI, defined in [Part1], Section 2.7>
  quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
  token         = <token, defined in [Part1], Section 3.2.6>
  word          = <word, defined in [Part1], Section 3.2.6>

Accept = [ ( "," / ( media-range [ accept-params ] ) ) *( OWS "," [
 OWS ( media-range [ accept-params ] ) ] ) ]
Accept-Charset = *( "," OWS ) ( ( charset / "*" ) [ weight ] ) *( OWS
 "," [ OWS ( ( charset / "*" ) [ weight ] ) ] )
Accept-Encoding = [ ( "," / ( codings [ weight ] ) ) *( OWS "," [ OWS
 ( codings [ weight ] ) ] ) ]
Accept-Language = *( "," OWS ) ( language-range [ weight ] ) *( OWS
 "," [ OWS ( language-range [ weight ] ) ] )
Allow = [ ( "," / method ) *( OWS "," [ OWS method ] ) ]

BWS = <BWS, defined in [Part1], Section 3.2.3>

Content-Encoding = *( "," OWS ) content-coding *( OWS "," [ OWS
 content-coding ] )
Content-Language = *( "," OWS ) language-tag *( OWS "," [ OWS
 language-tag ] )
Content-Location = absolute-URI / partial-URI
Content-Type = media-type

Date = HTTP-date

Expect = *( "," OWS ) expectation *( OWS "," [ OWS expectation ] )

From = mailbox

GMT = %x47.4D.54 ; GMT

HTTP-date = IMF-fixdate / obs-date

IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT

Location = URI-reference

Max-Forwards = 1*DIGIT

OWS = <OWS, defined in [Part1], Section 3.2.3>

RWS = <RWS, defined in [Part1], Section 3.2.3>
Referer = absolute-URI / partial-URI
Retry-After = HTTP-date / delta-seconds

Server = product *( RWS ( product / comment ) )

URI-reference = <URI-reference, defined in [Part1], Section 2.7>
User-Agent = product *( RWS ( product / comment ) )

Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
 ) )

absolute-URI = <absolute-URI, defined in [Part1], Section 2.7>
accept-ext = OWS ";" OWS token [ "=" word ]
accept-params = weight *accept-ext
asctime-date = day-name SP date3 SP time-of-day SP year
attribute = token

charset = token
codings = content-coding / "identity" / "*"
comment = <comment, defined in [Part1], Section 3.2.6>
content-coding = token

date1 = day SP month SP year
date2 = day "-" month "-" 2DIGIT
date3 = month SP ( 2DIGIT / ( SP DIGIT ) )
day = 2DIGIT
day-name = %x4D.6F.6E ; Mon
 / %x54.75.65 ; Tue
 / %x57.65.64 ; Wed
 / %x54.68.75 ; Thu
 / %x46.72.69 ; Fri
 / %x53.61.74 ; Sat
 / %x53.75.6E ; Sun
day-name-l = %x4D.6F.6E.64.61.79 ; Monday
 / %x54.75.65.73.64.61.79 ; Tuesday
 / %x57.65.64.6E.65.73.64.61.79 ; Wednesday
 / %x54.68.75.72.73.64.61.79 ; Thursday
 / %x46.72.69.64.61.79 ; Friday
 / %x53.61.74.75.72.64.61.79 ; Saturday
 / %x53.75.6E.64.61.79 ; Sunday
delta-seconds = 1*DIGIT

expect-name = token
expect-param = expect-name [ BWS "=" BWS expect-value ]
expect-value = token / quoted-string
expectation = expect-name [ BWS "=" BWS expect-value ] *( OWS ";" [
 OWS expect-param ] )

field-name = <comment, defined in [Part1], Section 3.2>

hour = 2DIGIT

language-range = <language-range, defined in [RFC4647], Section 2.1>
language-tag = <Language-Tag, defined in [RFC5646], Section 2.1>

mailbox = <mailbox, defined in [RFC5322], Section 3.4>
media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS
 ";" OWS parameter )
media-type = type "/" subtype *( OWS ";" OWS parameter )
method = token
minute = 2DIGIT
month = %x4A.61.6E ; Jan
 / %x46.65.62 ; Feb
 / %x4D.61.72 ; Mar
 / %x41.70.72 ; Apr
 / %x4D.61.79 ; May
 / %x4A.75.6E ; Jun
 / %x4A.75.6C ; Jul
 / %x41.75.67 ; Aug
 / %x53.65.70 ; Sep
 / %x4F.63.74 ; Oct
 / %x4E.6F.76 ; Nov
 / %x44.65.63 ; Dec

obs-date = rfc850-date / asctime-date

parameter = attribute "=" value
partial-URI = <partial-URI, defined in [Part1], Section 2.7>
product = token [ "/" product-version ]
product-version = token

quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
qvalue = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )

rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT

second = 2DIGIT
subtype = token

time-of-day = hour ":" minute ":" second
token = <token, defined in [Part1], Section 3.2.6>
type = token

value = word

weight = OWS ";" OWS "q=" qvalue
word = <word, defined in [Part1], Section 3.2.6>

year = 4DIGIT