Contents
General format
The header fields are transmitted after the request or response line, which is the first line of a message. Header fields are colon-separated name-value pairs in clear-text string format, terminated by a carriage return (CR) and line feed (LF) character sequence. The end of the header section is indicated by an empty field, resulting in the transmission of two consecutive CR-LF pairs. In the past, long lines could be folded into multiple lines; continuation lines are indicated by the presence of a space (SP) or horizontal tab (HT) as the first character on the next line. This folding is now deprecated.[1]Field names
A core set of fields is standardized by the Internet Engineering Task Force (IETF) in RFCs 7230, 7231, 7232, 7233, 7234, and 7235. The permanent registry of header fields and repository of provisional registrations are maintained by the IANA. Additional field names and permissible values may be defined by each application.HTTP/2 makes some restrictions on specific header fields (see below).
Non-standard header fields were conventionally marked by prefixing the field name with
X-
but this convention was deprecated in June 2012 because of the
inconveniences it caused when non-standard fields became standard.[2] An earlier restriction on use of Downgraded- was lifted in March 2013.[3]Field values
A few fields can contain comments (i.e. in User-Agent, Server, Via fields), which can be ignored by software.[4]Many field values may contain a quality (q) key-value pair, specifying a weight to use in content negotiation.[5]
Size limits
The standard imposes no limits to the size of each header field name or value, or to the number of fields. However, most servers, clients, and proxy software impose some limits for practical and security reasons. For example, the Apache 2.3 server by default limits the size of each field to 8,190 bytes, and there can be at most 100 header fields in a single request.[6]Request fields
| Header field name | Description | Example | Status |
|---|---|---|---|
| Accept | Content-Types that are acceptable for the response. See Content negotiation. | Accept: text/plain |
Permanent |
| Accept-Charset | Character sets that are acceptable. | Accept-Charset: utf-8 |
Permanent |
| Accept-Encoding | List of acceptable encodings. See HTTP compression. | Accept-Encoding: gzip, deflate |
Permanent |
| Accept-Language | List of acceptable human languages for response. See Content negotiation. | Accept-Language: en-US |
Permanent |
| Accept-Datetime | Acceptable version in time. | Accept-Datetime: Thu, 31 May 2007 20:35:00 GMT |
Provisional |
| Access-Control-Request-Method, Access-Control-Request-Headers[7] |
Initiates a request for cross-origin resource sharing with Origin (below). | Access-Control-Request-Method: GET |
Permanent: standard |
| Authorization | Authentication credentials for HTTP authentication. | Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ== |
Permanent |
| Cache-Control | Used to specify directives that must be obeyed by all caching mechanisms along the request-response chain. | Cache-Control: no-cache |
Permanent |
| Connection | Control options for the current connection and list of hop-by-hop request fields.[8]
Must not be used with HTTP/2.[9] |
Connection: keep-alive
Connection: Upgrade |
Permanent |
| Cookie | An HTTP cookie previously sent by the server with Set-Cookie (below). | Cookie: $Version=1; Skin=new; |
Permanent: standard |
| Content-Length | The length of the request body in octets (8-bit bytes). | Content-Length: 348 |
Permanent |
| Content-MD5 | A Base64-encoded binary MD5 sum of the content of the request body. | Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ== |
Obsolete[10] |
| Content-Type | The MIME type of the body of the request (used with POST and PUT requests). | Content-Type: application/x-www-form-urlencoded |
Permanent |
| Date | The date and time that the message was originated (in "HTTP-date" format as defined by RFC 7231 Date/Time Formats). | Date: Tue, 15 Nov 1994 08:12:31 GMT |
Permanent |
| Expect | Indicates that particular server behaviors are required by the client. | Expect: 100-continue |
Permanent |
| Forwarded | Disclose original information of a client connecting to a web server through an HTTP proxy.[11] | Forwarded: for=192.0.2.60;proto=http;by=203.0.113.43 Forwarded: for=192.0.2.43, for=198.51.100.17 |
Permanent |
| From | The email address of the user making the request. | From: user@example.com |
Permanent |
| Host | The domain name of the server (for virtual hosting), and the TCP port number on which the server is listening. The port number may be omitted if the port is the standard port for the service requested.
Mandatory since HTTP/1.1.[12] If the request is generated directly in HTTP/2, it should not be used.[13] |
Host: en.wikipedia.org:8080
Host: en.wikipedia.org |
Permanent |
| If-Match | Only perform the action if the client supplied entity matches the same entity on the server. This is mainly for methods like PUT to only update a resource if it has not been modified since the user last updated it. | If-Match: "737060cd8c284d8af7ad3082f209582d" |
Permanent |
| If-Modified-Since | Allows a 304 Not Modified to be returned if content is unchanged. | If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT |
Permanent |
| If-None-Match | Allows a 304 Not Modified to be returned if content is unchanged, see HTTP ETag. | If-None-Match: "737060cd8c284d8af7ad3082f209582d" |
Permanent |
| If-Range | If the entity is unchanged, send me the part(s) that I am missing; otherwise, send me the entire new entity. | If-Range: "737060cd8c284d8af7ad3082f209582d" |
Permanent |
| If-Unmodified-Since | Only send the response if the entity has not been modified since a specific time. | If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT |
Permanent |
| Max-Forwards | Limit the number of times the message can be forwarded through proxies or gateways. | Max-Forwards: 10 |
Permanent |
| Origin[7] | Initiates a request for cross-origin resource sharing (asks server for Access-Control-* response fields). | Origin: http://www.example-social-network.com |
Permanent: standard |
| Pragma | Implementation-specific fields that may have various effects anywhere along the request-response chain. | Pragma: no-cache |
Permanent |
| Proxy-Authorization | Authorization credentials for connecting to a proxy. | Proxy-Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ== |
Permanent |
| Range | Request only part of an entity. Bytes are numbered from 0. See Byte serving. | Range: bytes=500-999 |
Permanent |
| Referer [sic] | This is the address of the previous web page from which a link to the currently requested page was followed. (The word “referrer” has been misspelled in the RFC as well as in most implementations to the point that it has become standard usage and is considered correct terminology) | Referer: http://en.wikipedia.org/wiki/Main_Page |
Permanent |
| TE | The transfer encodings the user agent is willing to accept: the same
values as for the response header field Transfer-Encoding can be used,
plus the "trailers" value (related to the "chunked"
transfer method) to notify the server it expects to receive additional
fields in the trailer after the last, zero-sized, chunk.
Only trailers is supported in HTTP/2.[9] |
TE: trailers, deflate |
Permanent |
| User-Agent | The user agent string of the user agent. | User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20100101 Firefox/12.0 |
Permanent |
| Upgrade | Ask the server to upgrade to another protocol.
Must not be used to upgrade to HTTP/2.[9] |
Upgrade: HTTPS/1.3, IRC/6.9, RTA/x11, websocket |
Permanent |
| Via | Informs the server of proxies through which the request was sent. | Via: 1.0 fred, 1.1 example.com (Apache/1.1) |
Permanent |
| Warning | A general warning about possible problems with the entity body. | Warning: 199 Miscellaneous warning |
Permanent |
Common non-standard request fields
| Field name | Description | Example |
|---|---|---|
| X-Requested-With | Mainly used to identify Ajax requests. Most JavaScript frameworks send this field with value of XMLHttpRequest |
X-Requested-With: XMLHttpRequest |
| DNT[14] | Requests a web application to disable their tracking of a user. This is Mozilla's version of the X-Do-Not-Track header field (since Firefox 4.0 Beta 11). Safari and IE9 also have support for this field.[15] On March 7, 2011, a draft proposal was submitted to IETF.[16] The W3C Tracking Protection Working Group is producing a specification.[17] | DNT: 1 (Do Not Track Enabled)
DNT: 0 (Do Not Track Disabled) |
| X-Forwarded-For[18] | A de facto standard for identifying the originating IP address of a client connecting to a web server through an HTTP proxy or load balancer. Superseded by Forwarded header. | X-Forwarded-For: client1, proxy1, proxy2
X-Forwarded-For: 129.78.138.66, 129.78.64.103 |
| X-Forwarded-Host[19] | A de facto standard for identifying the original host requested by the client in the Host
HTTP request header, since the host name and/or port of the reverse
proxy (load balancer) may differ from the origin server handling the
request. Superseded by Forwarded header. |
X-Forwarded-Host: en.wikipedia.org:8080
X-Forwarded-Host: en.wikipedia.org |
| X-Forwarded-Proto[20] | A de facto standard for identifying the originating protocol of an HTTP request, since a reverse proxy (or a load balancer) may communicate with a web server using HTTP even if the request to the reverse proxy is HTTPS. An alternative form of the header (X-ProxyUser-Ip) is used by Google clients talking to Google servers. Superseded by Forwarded header. | X-Forwarded-Proto: https |
| Front-End-Https[21] | Non-standard header field used by Microsoft applications and load-balancers | Front-End-Https: on |
| X-Http-Method-Override[22] | Requests a web application to override the method specified in the request (typically POST) with the method given in the header field (typically PUT or DELETE). This can be used when a user agent or firewall prevents PUT or DELETE methods from being sent directly (note that this is either a bug in the software component, which ought to be fixed, or an intentional configuration, in which case bypassing it may be the wrong thing to do). | X-HTTP-Method-Override: DELETE |
| X-ATT-DeviceId[23] | Allows easier parsing of the MakeModel/Firmware that is usually found in the User-Agent String of AT&T Devices | X-Att-Deviceid: GT-P7320/P7320XXLPG |
| X-Wap-Profile[24] | Links to an XML file on the Internet with a full description and details about the device currently connecting. In the example to the right is an XML file for an AT&T Samsung Galaxy S2. | x-wap-profile: http://wap.samsungmobile.com/uaprof/SGH-I777.xml |
| Proxy-Connection[25] | Implemented as a misunderstanding of the HTTP specifications. Common
because of mistakes in implementations of early HTTP versions. Has
exactly the same functionality as standard Connection field.
Must not be used with HTTP/2.[9] |
Proxy-Connection: keep-alive |
| X-UIDH[26][27][28] | Server-side deep packet insertion of a unique ID identifying customers of Verizon Wireless; also known as "perma-cookie" or "supercookie" | X-UIDH: ... |
| X-Csrf-Token[29] | Used to prevent cross-site request forgery. Alternative header names are: X-CSRFToken[30] and X-XSRF-TOKEN[31] |
X-Csrf-Token: i8XNjC4b8KVok4uw5RftR38Wgp2BFwql |
| X-Request-ID,
X-Correlation-ID[32] |
Correlates HTTP requests between a client and server. | X-Request-ID: f058ebd6-02f7-4d3f-942e-904344e8cde5 |
Response fields
| Field name | Description | Example | Status |
|---|---|---|---|
| Access-Control-Allow-Origin, Access-Control-Allow-Credentials, Access-Control-Expose-Headers, Access-Control-Max-Age, Access-Control-Allow-Methods, Access-Control-Allow-Headers[7] |
Specifying which web sites can participate in cross-origin resource sharing | Access-Control-Allow-Origin: * |
Permanent: standard |
| Accept-Patch[33] | Specifies which patch document formats this server supports | Accept-Patch: text/example;charset=utf-8 |
Permanent |
| Accept-Ranges | What partial content range types this server supports via byte serving | Accept-Ranges: bytes |
Permanent |
| Age | The age the object has been in a proxy cache in seconds | Age: 12 |
Permanent |
| Allow | Valid methods for a specified resource. To be used for a 405 Method not allowed | Allow: GET, HEAD |
Permanent |
| Alt-Svc[34] | A server uses "Alt-Svc" header (meaning Alternative Services) to
indicate that its resources can also be accessed at a different network
location (host or port) or using a different protocol
When using HTTP/2, servers should instead send an ALTSVC frame. [35] |
Alt-Svc: http/1.1="http2.example.com:8001"; ma=7200 |
Permanent |
| Cache-Control | Tells all caching mechanisms from server to client whether they may cache this object. It is measured in seconds | Cache-Control: max-age=3600 |
Permanent |
| Connection | Control options for the current connection and list of hop-by-hop response fields.[8]
Must not be used with HTTP/2.[9] |
Connection: close |
Permanent |
| Content-Disposition[36] | An opportunity to raise a "File Download" dialogue box for a known MIME type with binary format or suggest a filename for dynamic content. Quotes are necessary with special characters. | Content-Disposition: attachment; filename="fname.ext" |
Permanent |
| Content-Encoding | The type of encoding used on the data. See HTTP compression. | Content-Encoding: gzip |
Permanent |
| Content-Language | The natural language or languages of the intended audience for the enclosed content[37] | Content-Language: da |
Permanent |
| Content-Length | The length of the response body in octets (8-bit bytes) | Content-Length: 348 |
Permanent |
| Content-Location | An alternate location for the returned data | Content-Location: /index.htm |
Permanent |
| Content-MD5 | A Base64-encoded binary MD5 sum of the content of the response | Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ== |
Obsolete[10] |
| Content-Range | Where in a full body message this partial message belongs | Content-Range: bytes 21010-47021/47022 |
Permanent |
| Content-Type | The MIME type of this content | Content-Type: text/html; charset=utf-8 |
Permanent |
| Date | The date and time that the message was sent (in "HTTP-date" format as defined by RFC 7231) [38] | Date: Tue, 15 Nov 1994 08:12:31 GMT |
Permanent |
| ETag | An identifier for a specific version of a resource, often a message digest | ETag: "737060cd8c284d8af7ad3082f209582d" |
Permanent |
| Expires | Gives the date/time after which the response is considered stale (in "HTTP-date" format as defined by RFC 7231) | Expires: Thu, 01 Dec 1994 16:00:00 GMT |
Permanent: standard |
| Last-Modified | The last modified date for the requested object (in "HTTP-date" format as defined by RFC 7231) | Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT |
Permanent |
| Link | Used to express a typed relationship with another resource, where the relation type is defined by RFC 5988 | Link: </feed>; rel="alternate"[39] |
Permanent |
| Location | Used in redirection, or when a new resource has been created. |
|
|
| P3P | This field is supposed to set P3P policy, in the form of P3P:CP="your_compact_policy". However, P3P did not take off,[40]
most browsers have never fully implemented it, a lot of websites set
this field with fake policy text, that was enough to fool browsers the
existence of P3P policy and grant permissions for third party cookies. |
P3P: CP="This is not a P3P policy! See
http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=151657
for more info." |
Permanent |
| Pragma | Implementation-specific fields that may have various effects anywhere along the request-response chain. | Pragma: no-cache |
Permanent |
| Proxy-Authenticate | Request authentication to access the proxy. | Proxy-Authenticate: Basic |
Permanent |
| Public-Key-Pins[41] | HTTP Public Key Pinning, announces hash of website's authentic TLS certificate | Public-Key-Pins: max-age=2592000; pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; |
Permanent |
| Retry-After | If an entity is temporarily unavailable, this instructs the client to try again later. Value could be a specified period of time (in seconds) or a HTTP-date.[42] |
|
Permanent |
| Server | A name for the server | Server: Apache/2.4.1 (Unix) |
Permanent |
| Set-Cookie | An HTTP cookie | Set-Cookie: UserID=JohnDoe; Max-Age=3600; Version=1 |
Permanent: standard |
| Strict-Transport-Security | A HSTS Policy informing the HTTP client how long to cache the HTTPS only policy and whether this applies to subdomains. | Strict-Transport-Security: max-age=16070400; includeSubDomains |
Permanent: standard |
| Trailer | The Trailer general field value indicates that the given set of header fields is present in the trailer of a message encoded with chunked transfer coding. | Trailer: Max-Forwards |
Permanent |
| Transfer-Encoding | The form of encoding used to safely transfer the entity to the user. Currently defined methods are: chunked, compress, deflate, gzip, identity.
Must not be used with HTTP/2.[9] |
Transfer-Encoding: chunked |
Permanent |
| Tk | Tracking Status header, value suggested to be sent in response to a DNT(do-not-track), possible values:
"!" — under construction "?" — dynamic "G" — gateway to multiple parties "N" — not tracking "T" — tracking "C" — tracking with consent "P" — tracking only if consented "D" — disregarding DNT "U" — updated |
Tk: ? |
Permanent |
| Upgrade | Ask the client to upgrade to another protocol.
Must not be used to upgrade to HTTP/2[9] |
Upgrade: HTTPS/1.3, IRC/6.9, RTA/x11, websocket |
Permanent |
| Vary | Tells downstream proxies how to match future request headers to decide whether the cached response can be used rather than requesting a fresh one from the origin server. |
|
Permanent |
| Via | Informs the client of proxies through which the response was sent. | Via: 1.0 fred, 1.1 example.com (Apache/1.1) |
Permanent |
| Warning | A general warning about possible problems with the entity body. | Warning: 199 Miscellaneous warning |
Permanent |
| WWW-Authenticate | Indicates the authentication scheme that should be used to access the requested entity. | WWW-Authenticate: Basic |
Permanent |
| X-Frame-Options[43] | Clickjacking protection: deny - no rendering within a frame, sameorigin - no rendering if origin mismatch, allow-from - allow from specified location, allowall - non-standard, allow from any location | X-Frame-Options: deny |
Obsolete[44] |
Common non-standard response fields
| Field name | Description | Example |
|---|---|---|
| Content-Security-Policy, X-Content-Security-Policy, X-WebKit-CSP[45] |
Content Security Policy definition. | X-WebKit-CSP: default-src 'self' |
| Refresh | Used in redirection, or when a new resource has been created. This refresh redirects after 5 seconds. Header extension introduced by Netscape and supported by most web browsers. | Refresh: 5; url=http://www.w3.org/pub/WWW/People.html |
| Status | CGI header field specifying the status of the HTTP response. Normal HTTP responses use a separate "Status-Line" instead, defined by RFC 7230.[46] | Status: 200 OK |
| Upgrade-Insecure-Requests[47] | Tells a server which (presumably in the middle of a HTTP -> HTTPS
migration) hosts mixed content that the client would prefer redirection
to HTTPS and can handle Content-Security-Policy: upgrade-insecure-requests
Must not be used with HTTP/2[9] |
Upgrade-Insecure-Requests: 1 |
| X-Content-Duration[48] | Provide the duration of the audio or video in seconds; only supported by Gecko browsers | X-Content-Duration: 42.666 |
| X-Content-Type-Options[49] | The only defined value, "nosniff", prevents Internet Explorer from MIME-sniffing a response away from the declared content-type. This also applies to Google Chrome, when downloading extensions.[50] | X-Content-Type-Options: nosniff[51] |
| X-Powered-By[52] | Specifies the technology (e.g. ASP.NET, PHP, JBoss) supporting the web application (version details are often in X-Runtime, X-Version, or X-AspNet-Version) |
X-Powered-By: PHP/5.4.0 |
| X-Request-ID, X-Correlation-ID[32] |
Correlates HTTP requests between a client and server. | X-Request-ID: f058ebd6-02f7-4d3f-942e-904344e8cde5 |
| X-UA-Compatible[53] | Recommends the preferred rendering engine (often a backward-compatibility mode) to use to display the content. Also used to activate Chrome Frame in Internet Explorer. | X-UA-Compatible: IE=EmulateIE7X-UA-Compatible: IE=edgeX-UA-Compatible: Chrome=1 |
| X-XSS-Protection[54] | Cross-site scripting (XSS) filter | X-XSS-Protection: 1; mode=block |
Effects of selected fields
Avoiding caching
If a web server responds withCache-Control: no-cache then a web browser or other caching system
(intermediate proxies) must not use the response to satisfy subsequent
responses without first checking with the originating server (this
process is called validation). This header field is part of HTTP version
1.1, and is ignored by some caches and browsers. It may be simulated by
setting the Expires HTTP version 1.0 header field value to
a time earlier than the response time. Notice that no-cache is not
instructing the browser or proxies about whether or not to cache the
content. It just tells the browser and proxies to validate the cache
content with the server before using it (this is done by using
If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match
attributes mentioned above). Sending a no-cache value thus instructs a
browser or proxy to not use the cache contents merely based on
"freshness criteria" of the cache content. Another common way to prevent
old content from being shown to the user without validation is Cache-Control: max-age=0. This instructs the user agent that the content is stale and should be validated before use.The header field
Cache-Control: no-store is intended to instruct a browser application to make a best effort not to write it to disk (i.e not to cache it).The request that a resource should not be cached is no guarantee that it will not be written to disk. In particular, the HTTP/1.1 definition draws a distinction between history stores and caches. If the user navigates back to a previous page a browser may still show you a page that has been stored on disk in the history store. This is correct behavior according to the specification. Many user agents show different behavior in loading pages from the history store or cache depending on whether the protocol is HTTP or HTTPS.
The
Cache-Control: no-cache HTTP/1.1 header field is
also intended for use in requests made by the client. It is a means for
the browser to tell the server and any intermediate caches that it wants
a fresh version of the resource. The Pragma: no-cache
header field, defined in the HTTP/1.0 spec, has the same purpose. It,
however, is only defined for the request header. Its meaning in a
response header is not specified.[55] The behavior of Pragma: no-cache in a response is implementation specific. While some user agents do pay attention to this field in responses,[56] the HTTP/1.1 RFC specifically warns against relying on this behavior.
