Module reqwest::header

HTTP header types

The module provides HeaderName, HeaderMap, and a number of types used for interacting with HeaderMap. These types allow representing both HTTP/1 and HTTP/2 headers.

HeaderName

The HeaderName type represents both standard header names as well as custom header names. The type handles the case insensitive nature of header names and is used as the key portion of HeaderMap. Header names are normalized to lower case. In other words, when creating a HeaderName with a string, even if upper case characters are included, when getting a string representation of the HeaderName, it will be all lower case. This allows for faster HeaderMap comparison operations.

The internal representation is optimized to efficiently handle the cases most commonly encountered when working with HTTP. Standard header names are special cased and are represented internally as an enum. Short custom headers will be stored directly in the HeaderName struct and will not incur any allocation overhead, however longer strings will require an allocation for storage.

Limitations

HeaderName has a max length of 32,768 for header names. Attempting to parse longer names will result in a panic.

HeaderMap

HeaderMap is a map structure of header names highly optimized for use cases common with HTTP. It is a multimap structure, where each header name may have multiple associated header values. Given this, some of the APIs diverge from HashMap.

Overview

Just like HashMap in Rust’s stdlib, HeaderMap is based on Robin Hood hashing. This algorithm tends to reduce the worst case search times in the table and enables high load factors without seriously affecting performance. Internally, keys and values are stored in vectors. As such, each insertion will not incur allocation overhead. However, once the underlying vector storage is full, a larger vector must be allocated and all values copied.

Deterministic ordering

Unlike Rust’s HashMap, values in HeaderMap are deterministically ordered. Roughly, values are ordered by insertion. This means that a function that deterministically operates on a header map can rely on the iteration order to remain consistent across processes and platforms.

Adaptive hashing

HeaderMap uses an adaptive hashing strategy in order to efficiently handle most common cases. All standard headers have statically computed hash values which removes the need to perform any hashing of these headers at runtime. The default hash function emphasizes performance over robustness. However, HeaderMap detects high collision rates and switches to a secure hash function in those events. The threshold is set such that only denial of service attacks should trigger it.

Limitations

HeaderMap can store a maximum of 32,768 headers (header name / value pairs). Attempting to insert more will result in a panic.

Structs

Drain	A drain iterator for HeaderMap.
GetAll	A view to all values stored in a single entry.
HeaderMap	A set of HTTP headers
HeaderName	Represents an HTTP header field name
HeaderValue	Represents an HTTP header field value.
IntoIter	An owning iterator over the entries of a HeaderMap.
InvalidHeaderName	A possible error when converting a HeaderName from another type.
InvalidHeaderValue	A possible error when converting a HeaderValue from a string or byte slice.
Iter	HeaderMap entry iterator.
IterMut	HeaderMap mutable entry iterator
Keys	An iterator over HeaderMap keys.
OccupiedEntry	A view into a single occupied location in a HeaderMap.
ToStrError	A possible error when converting a HeaderValue to a string representation.
VacantEntry	A view into a single empty location in a HeaderMap.
ValueDrain	An drain iterator of all values associated with a single header name.
ValueIter	An iterator of all values associated with a single header name.
ValueIterMut	A mutable iterator of all values associated with a single header name.
Values	HeaderMap value iterator.
ValuesMut	HeaderMap mutable value iterator

Enums

Entry

A view into a single location in a HeaderMap, which may be vacant or occupied.

Constants

ACCEPT	Advertises which content types the client is able to understand.
ACCEPT_CHARSET	Advertises which character set the client is able to understand.
ACCEPT_ENCODING	Advertises which content encoding the client is able to understand.
ACCEPT_LANGUAGE	Advertises which languages the client is able to understand.
ACCEPT_RANGES	Marker used by the server to advertise partial request support.
ACCESS_CONTROL_ALLOW_CREDENTIALS	Preflight response indicating if the response to the request can be exposed to the page.
ACCESS_CONTROL_ALLOW_HEADERS	Preflight response indicating permitted HTTP headers.
ACCESS_CONTROL_ALLOW_METHODS	Preflight header response indicating permitted access methods.
ACCESS_CONTROL_ALLOW_ORIGIN	Indicates whether the response can be shared with resources with the given origin.
ACCESS_CONTROL_EXPOSE_HEADERS	Indicates which headers can be exposed as part of the response by listing their names.
ACCESS_CONTROL_MAX_AGE	Indicates how long the results of a preflight request can be cached.
ACCESS_CONTROL_REQUEST_HEADERS	Informs the server which HTTP headers will be used when an actual request is made.
ACCESS_CONTROL_REQUEST_METHOD	Informs the server know which HTTP method will be used when the actual request is made.
AGE	Indicates the time in seconds the object has been in a proxy cache.
ALLOW	Lists the set of methods support by a resource.
ALT_SVC	Advertises the availability of alternate services to clients.
AUTHORIZATION	Contains the credentials to authenticate a user agent with a server.
CACHE_CONTROL	Specifies directives for caching mechanisms in both requests and responses.
CONNECTION	Controls whether or not the network connection stays open after the current transaction finishes.
CONTENT_DISPOSITION	Indicates if the content is expected to be displayed inline.
CONTENT_ENCODING	Used to compress the media-type.
CONTENT_LANGUAGE	Used to describe the languages intended for the audience.
CONTENT_LENGTH	Indicates the size fo the entity-body.
CONTENT_LOCATION	Indicates an alternate location for the returned data.
CONTENT_RANGE	Indicates where in a full body message a partial message belongs.
CONTENT_SECURITY_POLICY	Allows controlling resources the user agent is allowed to load for a given page.
CONTENT_SECURITY_POLICY_REPORT_ONLY	Allows experimenting with policies by monitoring their effects.
CONTENT_TYPE	Used to indicate the media type of the resource.
COOKIE	Contains stored HTTP cookies previously sent by the server with the Set-Cookie header.
DATE	Contains the date and time at which the message was originated.
DNT	Indicates the client’s tracking preference.
ETAG	Identifier for a specific version of a resource.
EXPECT	Indicates expectations that need to be fulfilled by the server in order to properly handle the request.
EXPIRES	Contains the date/time after which the response is considered stale.
FORWARDED	Contains information from the client-facing side of proxy servers that is altered or lost when a proxy is involved in the path of the request.
FROM	Contains an Internet email address for a human user who controls the requesting user agent.
HOST	Specifies the domain name of the server and (optionally) the TCP port number on which the server is listening.
IF_MATCH	Makes a request conditional based on the E-Tag.
IF_MODIFIED_SINCE	Makes a request conditional based on the modification date.
IF_NONE_MATCH	Makes a request conditional based on the E-Tag.
IF_RANGE	Makes a request conditional based on range.
IF_UNMODIFIED_SINCE	Makes the request conditional based on the last modification date.
LAST_MODIFIED	Content-Types that are acceptable for the response.
LINK	Allows the server to point an interested client to another resource containing metadata about the requested resource.
LOCATION	Indicates the URL to redirect a page to.
MAX_FORWARDS	Indicates the max number of intermediaries the request should be sent through.
ORIGIN	Indicates where a fetch originates from.
PRAGMA	HTTP/1.0 header usually used for backwards compatibility.
PROXY_AUTHENTICATE	Defines the authentication method that should be used to gain access to a proxy.
PROXY_AUTHORIZATION	Contains the credentials to authenticate a user agent to a proxy server.
PUBLIC_KEY_PINS	Associates a specific cryptographic public key with a certain server.
PUBLIC_KEY_PINS_REPORT_ONLY	Sends reports of pinning violation to the report-uri specified in the header.
RANGE	Indicates the part of a document that the server should return.
REFERER	Contains the address of the previous web page from which a link to the currently requested page was followed.
REFERRER_POLICY	Governs which referrer information should be included with requests made.
REFRESH	Informs the web browser that the current page or frame should be refreshed.
RETRY_AFTER	The Retry-After response HTTP header indicates how long the user agent should wait before making a follow-up request. There are two main cases this header is used:
SEC_WEBSOCKET_ACCEPT	The |Sec-WebSocket-Accept| header field is used in the WebSocket opening handshake. It is sent from the server to the client to confirm that the server is willing to initiate the WebSocket connection.
SEC_WEBSOCKET_EXTENSIONS	The |Sec-WebSocket-Extensions| header field is used in the WebSocket opening handshake. It is initially sent from the client to the server, and then subsequently sent from the server to the client, to agree on a set of protocol-level extensions to use for the duration of the connection.
SEC_WEBSOCKET_KEY	The |Sec-WebSocket-Key| header field is used in the WebSocket opening handshake. It is sent from the client to the server to provide part of the information used by the server to prove that it received a valid WebSocket opening handshake. This helps ensure that the server does not accept connections from non-WebSocket clients (e.g., HTTP clients) that are being abused to send data to unsuspecting WebSocket servers.
SEC_WEBSOCKET_PROTOCOL	The |Sec-WebSocket-Protocol| header field is used in the WebSocket opening handshake. It is sent from the client to the server and back from the server to the client to confirm the subprotocol of the connection. This enables scripts to both select a subprotocol and be sure that the server agreed to serve that subprotocol.
SEC_WEBSOCKET_VERSION	The |Sec-WebSocket-Version| header field is used in the WebSocket opening handshake. It is sent from the client to the server to indicate the protocol version of the connection. This enables servers to correctly interpret the opening handshake and subsequent data being sent from the data, and close the connection if the server cannot interpret that data in a safe manner.
SERVER	Contains information about the software used by the origin server to handle the request.
SET_COOKIE	Used to send cookies from the server to the user agent.
STRICT_TRANSPORT_SECURITY	Tells the client to communicate with HTTPS instead of using HTTP.
TE	Informs the server of transfer encodings willing to be accepted as part of the response.
TRAILER	Allows the sender to include additional fields at the end of chunked messages.
TRANSFER_ENCODING	Specifies the form of encoding used to safely transfer the entity to the client.
UPGRADE	Used as part of the exchange to upgrade the protocol.
UPGRADE_INSECURE_REQUESTS	Sends a signal to the server expressing the client’s preference for an encrypted and authenticated response.
USER_AGENT	Contains a string that allows identifying the requesting client’s software.
VARY	Determines how to match future requests with cached responses.
VIA	Added by proxies to track routing.
WARNING	General HTTP header contains information about possible problems with the status of the message.
WWW_AUTHENTICATE	Defines the authentication method that should be used to gain access to a resource.
X_CONTENT_TYPE_OPTIONS	Marker used by the server to indicate that the MIME types advertised in the content-type headers should not be changed and be followed.
X_DNS_PREFETCH_CONTROL	Controls DNS prefetching.
X_FRAME_OPTIONS	Indicates whether or not a browser should be allowed to render a page in a frame.
X_XSS_PROTECTION	Stop pages from loading when an XSS attack is detected.

Traits

AsHeaderName	A marker trait used to identify values that can be used as search keys to a HeaderMap.
IntoHeaderName	A marker trait used to identify values that can be used as insert keys to a HeaderMap.