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Text of the page (random words):
in a shared cache of the document being downloaded generally the information of a header field is used by software and not shown to the user a header field line is formatted as a name value pair with a colon separator whitespace is not allowed around the name but leading and trailing whitespace is ignored for the value part unlike a method name that must match exactly case sensitive 28 a header field name is matched ignoring case although often shown with each word capitalized 29 for example the following are header fields for host and accept language host www example com accept language en the standards do not limit the size of a header field or the number of fields in a message however most servers clients and proxy software impose limits for practical and security reasons for example the apache 2 3 server by default limits the size of each field to 8190 bytes and there can be at most 100 header fields in a single request 30 although deprecated by rfc 7230 31 in the past long lines could be split into multiple lines with a continuation line starting with a space or tab character request edit a request is sent by a client to a server the start line includes a method name a request uri and the protocol version with a single space between each field 32 the following request start line specifies method get uri customer 123 and protocol version http 1 1 get customer 123 http 1 1 request header fields allow the client to pass additional information beyond the request line acting as request modifiers similarly to the parameters of a procedure they give information about the client about the target resource or about the expected handling of the request in the http 1 1 protocol all header fields except host are optional a request line containing only the path name is accepted by servers to maintain compatibility with http clients before the http 1 0 specification in rfc 1945 33 resource edit the protocol structures transaction as operating on resources what a resource represents whether pre existing data or data that is generated dynamically depends on the implementation of the server often the resource corresponds to a file or the output of an executable running on the server method edit a request identifies a method sometimes informally called verb to classify the desired action to be performed on a resource the http 1 0 specification 3 8 defined the get head and post methods as well as listing the put delete link and unlink methods under additional methods however the http 1 1 specification 34 9 added five new methods put delete connect options and trace any client can use any method and the server can be configured to support any combination of methods if a method is unknown to an intermediate it will be treated as an unsafe and non idempotent method there is no limit to the number of methods that can be defined which allows for future methods to be specified without breaking existing infrastructure for example webdav defined seven new methods and rfc 5789 specified the patch method a general purpose web server is required to implement at least get and head and all other methods are considered optional by the specification 18 9 1 method names are case sensitive 4 3 18 9 1 this is in contrast to http header field names which are case insensitive 18 6 3 get the request is for a representation of a resource the server should only retrieve data not modify state 1 for retrieving without making changes get is preferred over post as it can be addressed through a url clarification needed this enables bookmarking and sharing and makes get responses eligible for caching which can save bandwidth the w3c has published guidance principles on this distinction saying web application design should be informed by the above principles but also by the relevant limitations 35 head the request is like a get except that the response should not include the representation data in the body this is useful for retrieving the representation metadata in the response header without having to transfer the entire representation uses include checking whether a page is available via the status code and getting the size of a file via header field content length post the request is to process a resource in some way for example it is used for posting a message to an internet forum subscribing to a mailing list or completing an online shopping transaction 18 9 3 3 put the request is to create or update a resource with the state in the request a distinction from post is that the client specifies the target location on the server 18 9 3 4 delete the request is to delete a resource connect requests that the intermediary establish a tcp ip tunnel to the origin server identified by the request target it is often used to secure connections through one or more http proxies with tls 18 9 3 6 36 see http connect method options request is for a report of the http methods that are supported for a resource this can be used to check the functionality of a web server by requesting instead of a specific resource trace requests the server to respond with the received request in the response body that way a client can see what if any changes or additions have been made by intermediaries useful for debugging patch the request is to modify a resource according to its partial state in the request compared to put this can save bandwidth by sending only part of a resource s representation instead of all of it 37 query requests that the server process the enclosed content in a safe and idempotent manner and then respond with the result this is similar to post requests but query requests can be automatically repeated or restarted without concern for partial state changes 38 properties of request methods method rfc request has payload body response has payload body safe idempotent cacheable get rfc 9110 optional yes yes yes yes head rfc 9110 optional no yes yes yes post rfc 9110 yes yes no no yes put rfc 9110 yes yes no yes no delete rfc 9110 optional yes no yes no connect rfc 9110 optional yes no no no options rfc 9110 optional yes yes yes no trace rfc 9110 no yes yes yes no patch rfc 5789 yes yes no no no query rfc 10008 yes yes yes yes yes safe method edit a request method is safe if a request with that method has no intended effect on the server the methods get head options trace and query are defined as safe in other words safe methods are intended to be read only safe methods can still have side effects not seen by the client such as appending request information to a log file or charging an advertising account in contrast the methods post put delete connect and patch are not safe they may modify the state of the server or have other effects such as sending an email such methods are therefore not usually used by conforming web robots or web crawlers some that do not conform tend to make requests without regard to context or consequences despite the prescribed safety of get requests in practice their handling by the server is not technically limited in any way careless or deliberately irregular programming can allow get requests to cause non trivial changes on the server this is discouraged because of the problems which can occur when web caching search engines and other automated agents make unintended changes on the server for example a website might allow deletion of a resource through a url such as https example com article 1234 delete which if arbitrarily fetched even using get would simply delete the article 39 a properly coded website would require a delete or post method for this action which non malicious bots would not make one example of this occurring in practice was during the short lived google web accelerator beta which prefetched arbitrary urls on the page a user was viewing causing records to be automatically altered or deleted en masse the beta was suspended only weeks after its first release following widespread criticism 40 idempotent method edit see also idempotence computer science meaning a request method is idempotent if multiple identical requests with that method have the same effect as a single such request the methods put and delete and safe methods are defined as idempotent safe methods are trivially idempotent since they are intended to have no effect on the server whatsoever the put and delete methods meanwhile are idempotent since successive identical requests will be ignored a website might for instance set up a put endpoint to modify a user s recorded email address if this endpoint is configured correctly any requests which ask to change a user s email address to the same email address which is already recorded e g duplicate requests following a successful request will have no effect similarly a request to delete a certain user will have no effect if that user has already been deleted in contrast the methods post connect and patch are not necessarily idempotent and therefore sending an identical post request multiple times may further modify the state of the server or have further effects such as sending multiple emails in some cases this is the desired effect but in other cases it may occur accidentally a user might for example inadvertently send multiple post requests by clicking a button again if they were not given clear feedback that the first click was being processed while web browsers may show alert dialog boxes to warn users in some cases where reloading a page may re submit a post request it is generally up to the web application to handle cases where a post request should not be submitted more than once note that whether or not a method is idempotent is not enforced by the protocol or web server it is perfectly possible to write a web application in which for example a database insert or other non idempotent action is triggered by a get or other request to do so against recommendations however may result in undesirable consequences if a user agent assumes that repeating the same request is safe when it is not cacheable method edit see also web cache a request method is cacheable if responses to requests with that method may be stored for future reuse the methods get head and post are defined as cacheable in contrast the methods put delete connect options trace and patch are not cacheable response edit a response is sent to the client by the server the start line of a response consists of the protocol version a status code and optionally a reason phrase with fields separated by a single space character 4 2 1 the following response start line specifies protocol version http 1 1 status code 400 and reason phrase bad request http 1 1 400 bad request response header fields allow the server to pass additional information beyond the status line acting as response modifiers they give information about the server or about further access to the target resource or related resources each response header field has a defined meaning which can be further refined by the semantics of the request method or response status code status code edit see also list of http status codes the status code is a three digit decimal integer value that represents the disposition of the server s attempt to satisfy the client s request generally a client handles a response primarily based on the status code and secondarily on response header fields a client may not understand each status code that a server reports but it must understand the class as indicated by the first digit and treat an unrecognized code as equivalent to the x00 code of that class the classes are as follows 1xx informational the request was received continuing process 2xx successful the request was successfully received understood and accepted 3xx redirection further action needs to be taken in order to complete the request 4xx client error the request cannot be fulfilled due to an issue that the client might be able to control 5xx server error the server failed to fulfill an apparently valid request reason phrase edit the standard reason phrases are only recommendations a web server is allowed to use a localized equivalent if a status code indicates a problem the user agent might display the reason phrase to the user to provide further information about the nature of the problem the standard also allows the user agent to attempt to interpret the reason phrase though this might be unwise since the standard explicitly specifies that status codes are machine readable and reason phrases are human readable example edit the following demonstrates an http 1 1 request response transaction for a server at www example com port 80 http 1 0 would use the same messages except for a few missing headers http 2 and http 3 would use the same request response mechanism but with different representations for http headers the following is a request with no body it consists of a start line 6 header fields and a blank line each terminated with a carriage return and line feed sequence the host header field distinguishes between various dns names sharing a single ip address allowing name based virtual hosting while optional in http 1 0 it is mandatory in http 1 1 get http 1 1 host www example com user agent mozilla 5 0 accept text html application xhtml xml application xml q 0 9 image avif image webp q 0 8 accept language en gb en q 0 5 accept encoding gzip deflate br connection keep alive although not clear in the representation above due to limitations of this wiki the blank line at the end results in ending in two line terminator sequences represented as a stream of characters a shorted version of above shows this more clearly with crlf representing a line terminator sequence get http 1 1 crlf host www example com crlf crlf in the following response the etag entity tag header field is used to determine if a cached version of the requested resource is identical to the current version of the resource on the server the content type header field specifies the internet media type of the data conveyed by the http message and content length indicates its length in bytes the http 1 1 webserver publishes its ability to respond to requests for a byte range of the resource by including accept ranges bytes this is useful if the client needs to have only certain portions 41 of a resource sent by the server which is called byte serving when connection close is sent it means that the web server will close the tcp connection immediately after the end of the transfer of this response 4 9 1 most of the header fields are optional but some are mandatory when header content length is missing from a response with a body then this should be considered an error in http 1 0 but it may not be an error in http 1 1 if header transfer encoding chunked is present chunked transfer encoding uses a chunk size of 0 to mark the end of the content some old implementations of http 1 0 omitted the header content length when the length of the body was not known at the beginning of the response and so the transfer of data to client continued until s...
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