Network Working Group R. Fielding Request for Comments: 2616 ...

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strongly correlated to the membership of a particular ethnic group.User agents which offer the option to configure the contents of anAccept-Language header to be sent in every request are stronglyencouraged to let the configuration process include a message whichmakes the user aware of the loss of privacy involved.An approach that limits the loss of privacy would be for a user agentto omit the sending of Accept-Language headers by default, and to askthe user whether or not to start sending Accept-Language headers to aserver if it detects, by looking for any Vary response-header fieldsgenerated by the server, that such sending could improve the qualityof service.Elaborate user-customized accept header fields sent in every request,in particular if these include quality values, can be used by serversas relatively reliable and long-lived user identifiers. Such useridentifiers would allow content providers to do click-trail tracking,and would allow collaborating content providers to match cross-serverclick-trails or form submissions of individual users. Note that formany users not behind a proxy, the network address of the hostrunning the user agent will also serve as a long-lived useridentifier. In environments where proxies are used to enhanceprivacy, user agents ought to be conservative in offering acceptheader configuration options to end users. As an extreme privacymeasure, proxies could filter the accept headers in relayed requests.General purpose user agents which provide a high degree of headerconfigurability SHOULD warn users about the loss of privacy which canbe involved.15.2 Attacks Based On File and Path NamesImplementations of HTTP origin servers SHOULD be careful to restrictthe documents returned by HTTP requests to be only those that wereintended by the server administrators. If an HTTP server translatesHTTP URIs directly into file system calls, the server MUST takespecial care not to serve files that were not intended to bedelivered to HTTP clients. For example, UNIX, Microsoft Windows, andother operating systems use ".." as a path component to indicate adirectory level above the current one. On such a system, an HTTPserver MUST disallow any such construct in the Request-URI if itwould otherwise allow access to a resource outside those intended tobe accessible via the HTTP server. Similarly, files intended forreference only internally to the server (such as access controlfiles, configuration files, and script code) MUST be protected frominappropriate retrieval, since they might contain sensitiveinformation. Experience has shown that minor bugs in such HTTP serverimplementations have turned into security risks.
Fielding, et al. Standards Track [Page 153]RFC 2616 HTTP/1.1 June 199915.3 DNS SpoofingClients using HTTP rely heavily on the Domain Name Service, and arethus generally prone to security attacks based on the deliberatemis-association of IP addresses and DNS names. Clients need to becautious in assuming the continuing validity of an IP number/DNS nameassociation.In particular, HTTP clients SHOULD rely on their name resolver forconfirmation of an IP number/DNS name association, rather thancaching the result of previous host name lookups. Many platformsalready can cache host name lookups locally when appropriate, andthey SHOULD be configured to do so. It is proper for these lookups tobe cached, however, only when the TTL (Time To Live) informationreported by the name server makes it likely that the cachedinformation will remain useful.If HTTP clients cache the results of host name lookups in order toachieve a performance improvement, they MUST observe the TTLinformation reported by DNS.If HTTP clients do not observe this rule, they could be spoofed whena previously-accessed server's IP address changes. As networkrenumbering is expected to become increasingly common [24], thepossibility of this form of attack will grow. Observing thisrequirement thus reduces this potential security vulnerability.This requirement also improves the load-balancing behavior of clientsfor replicated servers using the same DNS name and reduces thelikelihood of a user's experiencing failure in accessing sites whichuse that strategy.15.4 Location Headers and SpoofingIf a single server supports multiple organizations that do not trustone another, then it MUST check the values of Location and Content-Location headers in responses that are generated under control ofsaid organizations to make sure that they do not attempt toinvalidate resources over which they have no authority.15.5 Content-Disposition IssuesRFC 1806 [35], from which the often implemented Content-Disposition(see section 19.5.1) header in HTTP is derived, has a number of very
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eferred to as "HTTP/1.1", and is an
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12.1 Server-driven Negotiation ....
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14.40 Trailer .....................
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1.2 RequirementsThe key words "MUST
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Fielding, et al. Standards Track [P
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equest chain ----------------------
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Elements separated by a bar ("|") a
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HTTP/1.1 header field values can be
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An application that sends a request
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3.2.3 URI ComparisonWhen comparing
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is not desirable and is discouraged
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CRLFchunk = chunk-size [ chunk-exte
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When in canonical form, media subty
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3.9 Quality ValuesHTTP content nego
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RFC 2616 HTTP/1.1 June 1999HTTP/1.1
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message-body = entity-body| Transfe
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in advance.Messages MUST NOT includ
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equested resource, and 501 (Not Imp
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names) MUST use the following rules
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values for the first digit:- 1xx: I
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how the transfer-length of a messag
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connection-token close.If either th
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idempotent (see section 9.1.2). Non
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compatibility with RFC 2068, a serv
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If at any point an error status is
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optional features implemented by th
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- Extending a database through an a
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intervention (or other means) on th
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10.1.2 101 Switching ProtocolsThe s
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esponse MAY include new or updated
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The response MUST include the follo
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If the client is sending data, a se
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If the response could be unacceptab
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10.4.15 414 Request-URI Too LongThe
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10.5.5 504 Gateway TimeoutThe serve
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Caching would be useless if it did
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Warning (but without removing any e
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If the user has overridden the basi
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in the past. This means that the re
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RFC 2616 HTTP/1.1 June 1999the time
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simple:response_is_fresh = (freshne
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match.In HTTP/1.1, a conditional re
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validators or strong validators. Cl
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- MAY send a weak entity tag instea
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Unless specifically constrained by
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warn-code 1xx, which are deleted ev
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for the associated entity, its enti
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- DELETE- POSTIn order to prevent d
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The Accept request-header field can
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image/jpeg = 0.5text/html;level=2 =
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Note: If the request does not inclu
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Accept-Ranges: bytesbut are not req
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Page 154 and 155: Content) instead of 200 (OK).- The
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Page 182 and 183: Notes:1) Additional CRLFs may prece
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Page 192 and 193: also needed to be a general header,
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Network Working Group R. Fielding Request for Comments: 2616 ...

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