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part of Hypertext Transfer Protocol -- HTTP/ The Accept request-header field can be used to specify certain media types which are acceptable for the response . . "gzip" and "compress") are preferred; some older clients improperly display The HTTP-date sent in a Date header SHOULD NOT represent a date and. We'll use curl to send requests to n3ws.info which is an HTTP client testing service. Lines prefixed with a greater-than (>) sign show the data curl has sent to the server. . The Expires header is also used for caching and specifies the date Linux Academy provides a large library of in-depth online Linux. The Date general HTTP header contains the date and time at which the message was originated.
Basic and Digest Access Authentication" . If a request is authenticated and a realm specified, the same credentials SHOULD be valid for all other requests within this realm assuming that the authentication scheme itself does not require otherwise, such as credentials that vary according to a challenge value or using synchronized clocks.
When a shared cache see section If the response includes the "s-maxage" cache-control directive, the cache MAY use that response in replying to a subsequent request.
But if the specified maximum age has passed a proxy cache MUST first revalidate it with the origin server, using the request-headers from the new request to allow the origin server to authenticate the new request. This is the defined behavior for s-maxage. If the response includes the "must-revalidate" cache-control directive, the cache MAY use that response in replying to a subsequent request.
But if the response is stale, all caches MUST first revalidate it with the origin server, using the request-headers from the new request to allow the origin server to authenticate the new request. If the response includes the "public" cache-control directive, it MAY be returned in reply to any subsequent request. The directives specify behavior intended to prevent caches from adversely interfering with the request or response.
These directives typically override the default caching algorithms. Cache directives are unidirectional in that the presence of a directive in a request does not imply that the same directive is to be given in the response. It is not possible to specify a cache- directive for a specific cache.
When such a directive appears with a 1 field-name parameter, it applies only to the named field or fields, and not to the rest of the request or response. The cache-control directives can be broken down into these general categories: The following Cache-Control response directives allow an origin server to override the default cacheability of a response: See also Authorization, section This allows an origin server to state that the specified parts of the response are intended for only one user and are not a valid response for requests by other users.
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A private non-shared cache MAY cache the response. This usage of the word private only controls where the response may be cached, and cannot ensure the privacy of the message content.
This allows an origin server to prevent caching even by caches that have been configured to return stale responses to client requests. If the no-cache directive does specify one or more field-names, then a cache MAY use the response to satisfy a subsequent request, subject to any other restrictions on caching.
However, the specified field-name s MUST NOT be sent in the response to a subsequent request without successful revalidation with the origin server.
This allows an origin server to prevent the re-use of certain header fields in a response, while still allowing caching of the rest of the response.
The no-store directive applies to the entire message, and MAY be sent either in a response or in a request. This directive applies to both non- shared and shared caches.
Even when this directive is associated with a response, users might explicitly store such a response outside of the caching system e. History buffers MAY store such responses as part of their normal operation.
The purpose of this directive is to meet the stated requirements of certain users and service authors who are concerned about accidental releases of information via unanticipated accesses to cache data structures. While the use of this directive might improve privacy in some cases, we caution that it is NOT in any way a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping.
Alternatively, it MAY be specified using the max-age directive in a response. When the max-age cache-control directive is present in a cached response, the response is stale if its current age is greater than the age value given in seconds at the time of a new request for that resource.
The max-age directive on a response implies that the response is cacheable i. If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header, even if the Expires header is more restrictive. An origin server might wish to use a relatively new HTTP cache control feature, such as the "private" directive, on a network including older caches that do not understand that feature.
The origin server will need to combine the new feature with an Expires field whose value is less than or equal to the Date value. This will prevent older caches from improperly caching the response. The s-maxage directive also implies the semantics of the proxy-revalidate directive see section The s- maxage directive is always ignored by a private cache.
Note that most older caches, not compliant with this specification, do not implement any cache-control directives. Other directives allow a user agent to modify the basic expiration mechanism. These directives MAY be specified on a request: Unless max- stale directive is also included, the client is not willing to accept a stale response.
That is, the client wants a response that will still be fresh for at least the specified number of seconds. If max-stale is assigned a value, then the client is willing to accept a response that has exceeded its expiration time by no more than the specified number of seconds.
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If no value is assigned to max-stale, then the client is willing to accept a stale response of any age. If a cache returns a stale response, either because of a max-stale directive on a request, or because the cache is configured to override the expiration time of a response, the cache MUST attach a Warning header to the stale response, using Warning Response is stale. A cache MAY be configured to return stale responses without validation, but only if this does not conflict with any "MUST"-level requirements concerning cache validation e.
If both the new request and the cached entry include "max-age" directives, then the lesser of the two values is used for determining the freshness of the cached entry for that request.
End-to-end revalidation might be necessary if either the cache or the origin server has overestimated the expiration time of the cached response. End-to-end reload may be necessary if the cache entry has become corrupted for some reason.
End-to-end revalidation may be requested either when the client does not have its own local cached copy, in which case we call it "unspecified end-to-end revalidation", or when the client does have a local cached copy, in which case we call it "specific end-to-end revalidation. The initial request includes a cache-validating conditional with the client's current validator.
The initial request does not include a cache-validating conditional; the first cache along the path if any that holds a cache entry for this resource includes a cache-validating conditional with its current validator.
In this case, the cache MAY use either validator in making its own request without affecting semantic transparency. However, the choice of validator might affect performance. The best approach is for the intermediate cache to use its own validator when making its request. If the server replies with Not Modifiedthen the cache can return its now validated copy to the client with a OK response.
If the server replies with a new entity and cache validator, however, the intermediate cache can compare the returned validator with the one provided in the client's request, using the strong comparison function.
If the client's validator is equal to the origin server's, then the intermediate cache simply returns Not Modified. Otherwise, it returns the new entity with a OK response. To do this, the client may include the only-if-cached directive in a request.
If it receives this directive, a cache SHOULD either respond using a cached entry that is consistent with the other constraints of the request, or respond with a Gateway Timeout status. However, if a group of caches is being operated as a unified system with good internal connectivity, such a request MAY be forwarded within that group of caches.
When the must-revalidate directive is present in a response received by a cache, that cache MUST NOT use the entry after it becomes stale to respond to a subsequent request without first revalidating it with the origin server. The must-revalidate directive is necessary to support reliable operation for certain protocol features. Servers SHOULD send the must-revalidate directive if and only if failure to revalidate a request on the entity could result in incorrect operation, such as a silently unexecuted financial transaction.Servlets : View HTTP Request and Response Header - web sniffer
Although this is not recommended, user agents operating under severe connectivity constraints MAY violate this directive but, if so, MUST explicitly warn the user that an unvalidated response has been provided. It can be used on a response to an authenticated request to permit the user's cache to store and later return the response without needing to revalidate it since it has already been authenticated once by that userwhile still requiring proxies that service many users to revalidate each time in order to make sure that each user has been authenticated.
Note that such authenticated responses also need the public cache control directive in order to allow them to be cached at all. A non- transparent proxy might, for example, convert between image formats in order to save cache space or to reduce the amount of traffic on a slow link. Serious operational problems occur, however, when these transformations are applied to entity bodies intended for certain kinds of applications.
For example, applications for medical imaging, scientific data analysis and those using end-to-end authentication, all depend on receiving an entity body that is bit for bit identical to the original entity-body. Therefore, if a message includes the no-transform directive, an intermediate cache or proxy MUST NOT change those headers that are listed in section This implies that the cache or proxy MUST NOT change any aspect of the entity-body that is specified by these headers, including the value of the entity-body itself.
Informational extensions those which do not require a change in cache behavior MAY be added without changing the semantics of other directives. Behavioral extensions are designed to work by acting as modifiers to the existing base of cache directives.
Both the new directive and the standard directive are supplied, such that applications which do not understand the new directive will default to the behavior specified by the standard directive, and those that understand the new directive will recognize it as modifying the requirements associated with the standard directive.
In this way, extensions to the cache-control directives can be made without requiring changes to the base protocol. This extension mechanism depends on an HTTP cache obeying all of the cache-control directives defined for its native HTTP-version, obeying certain extensions, and ignoring all directives that it does not understand.
For example, consider a hypothetical new response directive called community which acts as a modifier to the private directive. We define this new directive to mean that, in addition to any non-shared cache, any cache which is shared only by members of the community named within its value may cache the response.
An origin server wishing to allow the UCI community to use an otherwise private response in their shared cache s could do so by including Cache-Control: The Connection header has the following grammar: Connection options are signaled by the presence of a connection-token in the Connection header field, not by any corresponding additional header field ssince the additional header field may not be sent if there are no parameters associated with that connection option.
When present, its value indicates what additional content codings have been applied to the entity-body, and thus what decoding mechanisms must be applied in order to obtain the media-type referenced by the Content-Type header field.
Content-Encoding is primarily used to allow a document to be compressed without losing the identity of its underlying media type. An example of its use is Content-Encoding: Typically, the entity-body is stored with this encoding and is only decoded before rendering or analogous usage. However, a non-transparent proxy MAY modify the content-coding if the new coding is known to be acceptable to the recipient, unless the "no-transform" cache-control directive is present in the message.
If the content-coding of an entity is not "identity", then the response MUST include a Content-Encoding entity-header section If the content-coding of an entity in a request message is not acceptable to the origin server, the server SHOULD respond with a status code of Unsupported Media Type.
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If multiple encodings have been applied to an entity, the content codings MUST be listed in the order in which they were applied. Additional information about the encoding parameters MAY be provided by other entity-header fields not defined by this specification.
Note that this might not be equivalent to all the languages used within the entity-body. The primary purpose of Content-Language is to allow a user to identify and differentiate entities according to the user's own preferred language. The order in which header fields with the same field-name are received is therefore significant to the interpretation of the combined field value, and thus a proxy MUST NOT change the order of these field values when a message is forwarded.
The message-body differs from the entity-body only when a transfer-coding has been applied, as indicated by the Transfer-Encoding header field section The rules for when a message-body is allowed in a message differ for requests and responses.
The presence of a message-body in a request is signaled by the inclusion of a Content-Length or Transfer-Encoding header field in the request's message-headers.
For response messages, whether or not a message-body is included with a message is dependent on both the request method and the response status code section 6. All other responses do include a message-body, although it MAY be of zero length. When a message-body is included with a message, the transfer-length of that body is determined by one of the following in order of precedence: Any response message which "MUST NOT" include a message-body such as the 1xx,and responses and any response to a HEAD request is always terminated by the first empty line after the header fields, regardless of the entity-header fields present in the message.
If a Transfer-Encoding header field section If a Content-Length header field section This media type MUST NOT be used unless the sender knows that the recipient can parse it; the presence in a request of a Range header with multiple byte- range specifiers from a 1.
A range header might be forwarded by a 1. By the server closing the connection. Closing the connection cannot be used to indicate the end of a request body, since that would leave no possibility for the server to send back a response.