RFC 7231
Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content
4. Request Methods
4.1. Overview
The request method token is the primary source of request semantics; it indicates the purpose for which the client has made this request and what is expected by the client as a successful result. The request method's semantics might be further specialized by the semantics of some header fields when present in a request (Section 5) if those additional semantics do not conflict with the method. For example, 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 ([RFC7232]). method = token HTTP was originally designed to be usable as an interface to distributed object systems. The request method was envisioned as applying semantics to a target resource in much the same way as invoking a defined method on an identified object would apply semantics. The method token is case-sensitive because it might be used as a gateway to object-based systems with case-sensitive method names. Unlike distributed objects, the standardized request methods in HTTP are not resource-specific, since uniform interfaces provide for better visibility and reuse in network-based systems [REST]. Once defined, a standardized method ought to have the same semantics when applied to any resource, though each resource determines for itself whether those semantics are implemented or allowed. This specification defines a number of standardized methods that are commonly used in HTTP, as outlined by the following table. By convention, standardized methods are defined in all-uppercase US-ASCII letters.
+---------+-------------------------------------------------+-------+ | Method | Description | Sec. | +---------+-------------------------------------------------+-------+ | GET | Transfer a current representation of the target | 4.3.1 | | | resource. | | | HEAD | Same as GET, but only transfer the status line | 4.3.2 | | | and header section. | | | POST | Perform resource-specific processing on the | 4.3.3 | | | request payload. | | | PUT | Replace all current representations of the | 4.3.4 | | | target resource with the request payload. | | | DELETE | Remove all current representations of the | 4.3.5 | | | target resource. | | | CONNECT | Establish a tunnel to the server identified by | 4.3.6 | | | the target resource. | | | OPTIONS | Describe the communication options for the | 4.3.7 | | | target resource. | | | TRACE | Perform a message loop-back test along the path | 4.3.8 | | | to the target resource. | | +---------+-------------------------------------------------+-------+ All general-purpose servers MUST support the methods GET and HEAD. All other methods are OPTIONAL. Additional methods, outside the scope of this specification, have been standardized for use in HTTP. All such methods ought to be registered within the "Hypertext Transfer Protocol (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 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 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 code.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, that is not entirely read-only, or that 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 owner 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
A request method is considered "idempotent" if the intended effect on the server of multiple identical requests with that method is the same as the effect for a single such request. Of the request methods defined by this specification, 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 the client can establish a new connection and retry the idempotent request. It knows that repeating the request will have the same intended effect, even if the original request succeeded, though the response might differ.4.2.3. Cacheable Methods
Request methods can be defined as "cacheable" to indicate that responses to them are allowed to be stored for future reuse; for specific requirements see [RFC7234]. In general, safe methods that do not depend on a current or authoritative response are defined as cacheable; this specification defines GET, HEAD, and POST as cacheable, although the overwhelming majority of cache implementations only support GET and HEAD.4.3. Method Definitions
4.3.1. GET
The GET method requests transfer of a current selected representation for the target resource. 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 file system 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 file system, 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 ([RFC7233]). 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 5.2 of [RFC7234]).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 section). 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, except that the payload header fields (Section 3.3) MAY be omitted. This method can be used for obtaining metadata about the selected representation without transferring the representation data and 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 5.2 of [RFC7234]). A HEAD response might also have an effect on previously cached responses to GET; see Section 4.3.5 of [RFC7234].4.3.3. POST
The POST method requests that the target resource 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): o Providing a block of data, such as the fields entered into an HTML form, to a data-handling process;
o Posting a message to a bulletin board, newsgroup, mailing list,
blog, or similar group of articles;
o Creating a new resource that has yet to be identified by the
origin server; and
o 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 (Partial Content), 304 (Not Modified), and 416 (Range Not
Satisfiable)).
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) response containing a 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 4.2.1 of [RFC7234]).
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) response containing the result and a
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) response
with the existing resource's identifier in the 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 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) 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) 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 the origin server MUST send either a 200 (OK) or
a 204 (No Content) response 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)
or 415 (Unsupported Media Type) status codes are suggested, with the
latter being specific to constraints on Content-Type values.
For example, if the target resource is configured to always have a
Content-Type of "text/html" and the representation being PUT has a
Content-Type of "image/jpeg", the origin server ought to do one of:
a. reconfigure the target resource to reflect the new media type;
b. transform the PUT representation to a format consistent with that
of the resource before saving it as the new resource state; or,
c. reject the request with a 415 (Unsupported Media Type) 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 being retrieved 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) 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 can 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 that allows PUT on a given target resource MUST send a 400 (Bad Request) response to a PUT request that contains a Content-Range header field (Section 4.2 of [RFC7233]), since the payload is likely to be partial content that has been 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 successful 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 4.4 of [RFC7234]).4.3.5. DELETE
The DELETE method requests that the origin server remove the association between the target resource 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) 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 code if the action will likely succeed but has not yet been enacted, a 204 (No Content) status code if the action has been enacted and no further information is to be supplied, or a 200 (OK) 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 4.4 of [RFC7234]).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 tunnel is closed. Tunnels are commonly used to create an end-to-end virtual connection, through one or more proxies, which can then be secured using TLS (Transport Layer Security, [RFC5246]). 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 (Successful) 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 [RFC7230]); 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) 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 section;
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 tunnel is closed when a tunnel intermediary detects that either
side has closed its connection: the intermediary MUST attempt to send
any outstanding data that came from the closed side to the other
side, close both connections, and then discard any remaining data
left undelivered.
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=
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.
A server MUST NOT send any Transfer-Encoding or Content-Length header
fields in a 2xx (Successful) response to CONNECT. A client MUST
ignore any Content-Length or Transfer-Encoding header fields received
in a successful response to CONNECT.
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 for the target resource, at either the origin
server or an intervening intermediary. 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 [RFC7230]) 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),
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 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 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) response with a
Content-Type of "message/http" (Section 8.3.1 of [RFC7230]). The
final recipient is either the origin server or the first server to
receive a Max-Forwards value of zero (0) in the request
(Section 5.1.2).
A client MUST NOT generate 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 [RFC7235] or cookies [RFC6265] in a TRACE request. The final recipient of the request SHOULD exclude any request header fields that are likely to contain sensitive data when that recipient generates the response body. 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 [RFC7230]) is of particular interest, since it acts as a trace of the request chain. Use of the 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. +-------------------+--------------------------+ | Header Field Name | Defined in... | +-------------------+--------------------------+ | Cache-Control | Section 5.2 of [RFC7234] | | Expect | Section 5.1.1 | | Host | Section 5.4 of [RFC7230] | | Max-Forwards | Section 5.1.2 | | Pragma | Section 5.4 of [RFC7234] | | Range | Section 3.1 of [RFC7233] | | TE | Section 4.3 of [RFC7230] | +-------------------+--------------------------+
5.1.1. Expect
The "Expect" header field in a request indicates a certain set of behaviors (expectations) that need to be supported by the server in order to properly handle this request. The only such expectation defined by this specification is 100-continue. Expect = "100-continue" The Expect field-value is case-insensitive. A server that receives an Expect field-value other than 100-continue MAY respond with a 417 (Expectation Failed) status code to indicate that the unexpected expectation cannot be met. A 100-continue expectation informs recipients that the client is about to send a (presumably large) message body in this request and wishes to receive a 100 (Continue) interim response if the request-line and header fields are not sufficient to cause an immediate success, redirect, or error response. This allows the client to wait for an indication that it is worthwhile to send the message body before actually doing so, which can improve efficiency when the message body is huge or when the client anticipates that an error is likely (e.g., when sending a state-changing method, for the first time, without previously verified authentication credentials). For example, a request that begins with PUT /somewhere/fun HTTP/1.1 Host: origin.example.com Content-Type: video/h264 Content-Length: 1234567890987 Expect: 100-continue allows the origin server to immediately respond with an error message, such as 401 (Unauthorized) or 405 (Method Not Allowed), before the client starts filling the pipes with an unnecessary data transfer. Requirements for clients: o A client MUST NOT generate a 100-continue expectation in a request that does not include a message body. o A client that will wait for a 100 (Continue) response before sending the request message body MUST send an Expect header field containing a 100-continue expectation.
o A client that sends a 100-continue expectation is not required to
wait for any specific length of time; such a client MAY proceed to
send the message body even if it has not yet received a response.
Furthermore, since 100 (Continue) responses cannot be sent through
an HTTP/1.0 intermediary, such a client SHOULD NOT wait for an
indefinite period before sending the message body.
o A client that receives a 417 (Expectation Failed) status code in
response to a request containing a 100-continue expectation SHOULD
repeat that request without a 100-continue expectation, since the
417 response merely indicates that the response chain does not
support expectations (e.g., it passes through an HTTP/1.0 server).
Requirements for servers:
o A server that receives a 100-continue expectation in an HTTP/1.0
request MUST ignore that expectation.
o A server MAY omit sending a 100 (Continue) response if it has
already received some or all of the message body for the
corresponding request, or if the framing indicates that there is
no message body.
o A server that sends a 100 (Continue) response MUST ultimately send
a final status code, once the message body is received and
processed, unless the connection is closed prematurely.
o A server that responds with a final status code before reading the
entire message body SHOULD indicate in that response whether it
intends to close the connection or continue reading and discarding
the request message (see Section 6.6 of [RFC7230]).
An origin server MUST, upon receiving an HTTP/1.1 (or later)
request-line and a complete header section that contains a
100-continue expectation and indicates a request message body will
follow, either send an immediate response with a final status code,
if that status can be determined by examining just the request-line
and header fields, or send an immediate 100 (Continue) response to
encourage the client to send the request's message body. The origin
server MUST NOT wait for the message body before sending the 100
(Continue) response.
A proxy MUST, upon receiving an HTTP/1.1 (or later) request-line and
a complete header section that contains a 100-continue expectation
and indicates a request message body will follow, either send an
immediate response with a final status code, if that status can be
determined by examining just the request-line and header fields, or
begin forwarding the request toward the origin server by sending a
corresponding request-line and header section to the next inbound
server. If the proxy believes (from configuration or past
interaction) that the next inbound server only supports HTTP/1.0, the
proxy MAY generate an immediate 100 (Continue) response to encourage
the client to begin sending the message body.
Note: The Expect header field was added after the original
publication of HTTP/1.1 [RFC2068] as both the means to request an
interim 100 (Continue) response and the general mechanism for
indicating must-understand extensions. However, the extension
mechanism has not been used by clients and the must-understand
requirements have not been implemented by many servers, rendering
the extension mechanism useless. This specification has removed
the extension mechanism in order to simplify the definition and
processing of 100-continue.
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) request 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 intermediary that receives 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
intermediary MUST NOT forward the request; instead, the intermediary
MUST respond as the final recipient. If the received Max-Forwards
value is greater than zero, the intermediary MUST generate an updated
Max-Forwards field in the forwarded message with a field-value that
is the lesser of a) the received value decremented by one (1) or b)
the recipient's maximum supported value for Max-Forwards.
A recipient MAY ignore a Max-Forwards header field received with any
other request methods.
5.2. Conditionals
The HTTP conditional request header fields [RFC7232] 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 (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 [RFC7232]. +---------------------+--------------------------+ | Header Field Name | Defined in... | +---------------------+--------------------------+ | If-Match | Section 3.1 of [RFC7232] | | If-None-Match | Section 3.2 of [RFC7232] | | If-Modified-Since | Section 3.3 of [RFC7232] | | If-Unmodified-Since | Section 3.4 of [RFC7232] | | If-Range | Section 3.2 of [RFC7233] | +---------------------+--------------------------+5.3. Content Negotiation
The following request header fields are sent by a user agent to engage in 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. +-------------------+---------------+ | Header Field Name | Defined in... | +-------------------+---------------+ | Accept | Section 5.3.2 | | Accept-Charset | Section 5.3.3 | | Accept-Encoding | Section 5.3.4 | | Accept-Language | Section 5.3.5 | +-------------------+---------------+5.3.1. Quality Values
Many of the request header fields for 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 [ "=" ( token / quoted-string ) ]
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), 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
is interpreted as "I prefer audio/basic, but send me any audio type
if it is the best available after an 80% markdown 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) 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:
+-------------------+---------------+
| Media Type | Quality Value |
+-------------------+---------------+
| text/html;level=1 | 1 |
| text/html | 0.7 |
| text/plain | 0.3 |
| image/jpeg | 0.5 |
| text/html;level=2 | 0.4 |
| text/html;level=3 | 0.7 |
+-------------------+---------------+
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 9.7). 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) 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, see [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) 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 9.7). Since intelligibility is highly dependent on the individual user, user agents need to allow user control over the linguistic preference (either through configuration of the user agent itself or by defaulting to a user controllable system setting). 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 [RFC7235]. Note that various custom mechanisms for user authentication use the Cookie header field for this purpose, as defined in [RFC6265]. +---------------------+--------------------------+ | Header Field Name | Defined in... | +---------------------+--------------------------+ | Authorization | Section 4.2 of [RFC7235] | | Proxy-Authorization | Section 4.4 of [RFC7235] | +---------------------+--------------------------+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. +-------------------+---------------+ | Header Field Name | Defined in... | +-------------------+---------------+ | From | Section 5.5.1 | | Referer | Section 5.5.2 | | User-Agent | Section 5.5.3 | +-------------------+---------------+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, see [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.
A robotic user agent 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. A server 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 NOT include the fragment and userinfo components of the URI reference [RFC3986], if any, when generating the Referer field value. Referer = absolute-URI / partial-URI The Referer header field 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 the Referer header field 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 it. 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 the Referer field 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 MUST NOT send a Referer header field in an unsecured HTTP request if the referring page was received with a secure protocol. See Section 9.4 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. An intermediary SHOULD NOT modify or delete the Referer header 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 [RFC7230]), 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 A sender SHOULD limit generated product identifiers to what is necessary to identify the product; a sender MUST NOT generate advertising or other nonessential information within the product identifier. A sender SHOULD NOT generate information in product-version that is not a version identifier (i.e., successive versions of the same product name ought to differ only 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.
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