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5. Message Routing

HTTP request message routing is determined by each client based on the target resource, the client's proxy configuration, and establishment or reuse of an inbound connection. The corresponding response routing follows the same connection chain back to the client.

5.1. Identifying a Target Resource​

HTTP is used in a wide variety of applications, ranging from general-purpose computers to home appliances. In some cases, communication options are hard-coded in a client's configuration. However, most HTTP clients rely on the same resource identification mechanism and configuration techniques as general-purpose Web browsers.

HTTP communication is initiated by a user agent for some purpose. The purpose is a combination of request semantics, which are defined in [RFC7231], and a target resource upon which to apply those semantics. A URI reference (Section 2.7) is typically used as an identifier for the "target resource", which a user agent would resolve to its absolute form in order to obtain the "target URI". The target URI excludes the reference's fragment component, if any, since fragment identifiers are reserved for client-side processing ([RFC3986], Section 3.5).

5.2. Connecting Inbound​

Once the target URI is determined, a client needs to decide whether a network request is necessary to accomplish the desired semantics and, if so, where that request is to be directed.

If the client has a cache [RFC7234] and the request can be satisfied by it, then the request is usually directed there first.

If the request is not satisfied by a cache, then a typical client will check its configuration to determine whether a proxy is to be used to satisfy the request. Proxy configuration is implementation-dependent, but is often based on URI prefix matching, selective authority matching, or both, and the proxy itself is usually identified by an "http" or "https" URI. If a proxy is applicable, the client connects inbound by establishing (or reusing) a connection to that proxy.

If no proxy is applicable, a typical client will invoke a handler routine, usually specific to the target URI's scheme, to connect directly to an authority for the target resource. How that is accomplished is dependent on the target URI scheme and defined by its associated specification, similar to how this specification defines origin server access for resolution of the "http" (Section 2.7.1) and "https" (Section 2.7.2) schemes.

HTTP requirements regarding connection management are defined in Section 6.

5.3. Request Target​

Once an inbound connection is obtained, the client sends an HTTP request message (Section 3) with a request-target derived from the target URI. There are four distinct formats for the request-target, depending on both the method being requested and whether the request is to a proxy.

request-target = origin-form
               / absolute-form
               / authority-form
               / asterisk-form

5.3.1. origin-form​

The most common form of request-target is the origin-form.

origin-form    = absolute-path [ "?" query ]

When making a request directly to an origin server, other than a CONNECT or server-wide OPTIONS request (as detailed below), a client MUST send only the absolute path and query components of the target URI as the request-target. If the target URI's path component is empty, the client MUST send "/" as the path within the origin-form of request-target. A Host header field is also sent, as defined in Section 5.4.

For example, a client wishing to retrieve a representation of the resource identified as

http://www.example.org/where?q=now

directly from the origin server would open (or reuse) a TCP connection to port 80 of the host "www.example.org" and send the lines:

GET /where?q=now HTTP/1.1
Host: www.example.org

followed by the remainder of the request message.

5.3.2. absolute-form​

When making a request to a proxy, other than a CONNECT or server-wide OPTIONS request (as detailed below), a client MUST send the target URI in absolute-form as the request-target.

absolute-form  = absolute-URI

The proxy is requested to either service that request from a valid cache, if possible, or make the same request on the client's behalf to either the next inbound proxy server or directly to the origin server indicated by the request-target. Requirements on such "forwarding" of messages are defined in Section 5.7.

An example absolute-form of request-line would be:

GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1

To allow for transition to the absolute-form for all requests in some future version of HTTP, a server MUST accept the absolute-form in requests, even though HTTP/1.1 clients will only send them in requests to proxies.

5.3.3. authority-form​

The authority-form of request-target is only used for CONNECT requests (Section 4.3.6 of [RFC7231]).

authority-form = authority

When making a CONNECT request to establish a tunnel through one or more proxies, a client MUST send only the target URI's authority component (excluding any userinfo and its "@" delimiter) as the request-target. For example,

CONNECT www.example.com:80 HTTP/1.1

5.3.4. asterisk-form​

The asterisk-form of request-target is only used for a server-wide OPTIONS request (Section 4.3.7 of [RFC7231]).

asterisk-form  = "*"

When a client wishes to request OPTIONS for the server as a whole, as opposed to a specific named resource of that server, the client MUST send only "*" (%x2A) as the request-target. For example,

OPTIONS * HTTP/1.1

If a proxy receives an OPTIONS request with an absolute-form of request-target in which the URI has an empty path and no query component, then the last proxy on the request chain MUST send a request-target of "*" when it forwards the request to the indicated origin server.

For example, the request

OPTIONS http://www.example.org:8001 HTTP/1.1

would be forwarded by the final proxy as

OPTIONS * HTTP/1.1
Host: www.example.org:8001

after connecting to port 8001 of host "www.example.org".

5.4. Host​

The "Host" header field in a request provides the host and port information from the target URI, enabling the origin server to distinguish among resources while servicing requests for multiple host names on a single IP address.

Host = uri-host [ ":" port ] ; Section 2.7.1

A client MUST send a Host header field in all HTTP/1.1 request messages. If the target URI includes an authority component, then a client MUST send a field-value for Host that is identical to that authority component, excluding any userinfo subcomponent and its "@" delimiter (Section 2.7.1). If the authority component is missing or undefined for the target URI, then a client MUST send a Host header field with an empty field-value.

Since the Host field-value is critical information for handling a request, a user agent SHOULD generate Host as the first header field following the request-line.

For example, a GET request to the origin server for http://www.example.org/pub/WWW/ would begin with:

GET /pub/WWW/ HTTP/1.1
Host: www.example.org

A client MUST send a Host header field in an HTTP/1.1 request even if the request-target is in the absolute-form, since this allows the Host information to be forwarded through ancient HTTP/1.0 proxies that might not have implemented Host.

When a proxy receives a request with an absolute-form of request-target, the proxy MUST ignore the received Host header field (if any) and instead replace it with the host information of the request-target. A proxy that forwards such a request MUST generate a new Host field-value based on the received request-target rather than forward the received Host field-value.

Since the Host header field acts as an application-level routing mechanism, it is a frequent target for malware seeking to poison a shared cache or redirect a request to an unintended server. An interception proxy is particularly vulnerable if it relies on the Host field-value for redirecting requests to internal servers, or for use as a cache key in a shared cache, without first verifying that the intercepted connection is targeting a valid IP address for that host.

A server MUST respond with a 400 (Bad Request) status code to any HTTP/1.1 request message that lacks a Host header field and to any request message that contains more than one Host header field or a Host header field with an invalid field-value.

5.5. Effective Request URI​

Since the request-target often contains only part of the user agent's target URI, a server reconstructs the intended target as an "effective request URI" to properly service the request. This reconstruction involves both the server's local configuration and information communicated in the request-target, Host header field, and connection context.

For a user agent, the effective request URI is the target URI.

If the request-target is in absolute-form, the effective request URI is the same as the request-target. Otherwise, the effective request URI is constructed as follows:

If the server's configuration (or outbound gateway) provides a fixed URI scheme, that scheme is used for the effective request URI. Otherwise, if the request is received over a TLS-secured TCP connection, the effective request URI's scheme is "https"; if not, the scheme is "http".

If the server's configuration (or outbound gateway) provides a fixed URI authority component, that authority is used for the effective request URI. If not, then if the request-target is in authority-form, the effective request URI's authority component is the same as the request-target. If not, then if a Host header field is supplied with a non-empty field-value, the authority component is the same as the Host field-value. Otherwise, the authority component is assigned the default name configured for the server and, if the connection's incoming TCP port number differs from the default port for the effective request URI's scheme, then a colon (":") and the incoming port number (in decimal form) are appended to the authority component.

If the request-target is in authority-form or asterisk-form, the effective request URI's combined path and query component is empty. Otherwise, the combined path and query component is the same as the request-target.

The components of the effective request URI, once determined as above, can be combined into absolute-URI form by concatenating the scheme, "://", authority, and combined path and query component.

Example 1: the following message received over an insecure TCP connection

GET /pub/WWW/TheProject.html HTTP/1.1
Host: www.example.org:8080

has an effective request URI of

http://www.example.org:8080/pub/WWW/TheProject.html

Example 2: the following message received over a TLS-secured TCP connection

OPTIONS * HTTP/1.1
Host: www.example.org

has an effective request URI of

https://www.example.org

Recipients of an HTTP/1.0 request that lacks a Host header field might need to use heuristics (e.g., examination of the URI path for something unique to a particular host) in order to guess the effective request URI's authority component.

Once the effective request URI has been constructed, an origin server needs to decide whether or not to provide service for that URI via the connection in which the request was received. For example, the request might have been misdirected, deliberately or accidentally, such that the information within a received request-target or Host header field differs from the host or port upon which the connection has been made. If the connection is from a trusted gateway, that inconsistency might be expected; otherwise, it might indicate an attempt to bypass security filters, trick the server into delivering non-public content, or poison a cache. See Section 9 for security considerations regarding message routing.

5.6. Associating a Response to a Request​

HTTP does not include a request identifier for associating a given request message with its corresponding one or more response messages. Hence, it relies on the order of response arrival to correspond exactly to the order in which requests are made on the same connection. More than one response message per request only occurs when one or more informational responses (1xx, see Section 6.2 of [RFC7231]) precede a final response to the same request.

A client that has more than one outstanding request on a connection MUST maintain a list of outstanding requests in the order sent and MUST associate each received response message on that connection to the highest ordered request that has not yet received a final (non-1xx) response.

5.7. Message Forwarding​

As described in Section 2.3, intermediaries can serve a variety of roles in the processing of HTTP requests and responses. Some intermediaries are used to improve performance or availability. Others are used for access control or to filter content. Since an HTTP stream has characteristics similar to a pipe-and-filter architecture, there are no inherent limits to the extent an intermediary can enhance (or interfere) with either direction of the stream.

An intermediary not acting as a tunnel MUST implement the Connection header field, as specified in Section 6.1, and exclude fields from being forwarded that are only intended for the incoming connection.

An intermediary MUST NOT forward a message to itself unless it is protected from an infinite request loop. In general, an intermediary ought to recognize its own server names, including any aliases, local variations, or literal IP addresses, and respond to such requests directly.

5.7.1. Via​

The "Via" header field indicates the presence of intermediate protocols and recipients between the user agent and the server (on requests) or between the origin server and the client (on responses), similar to the "Received" header field in email (Section 3.6.7 of [RFC5322]). Via can be used for tracking message forwards, avoiding request loops, and identifying the protocol capabilities of senders along the request/response chain.

Via = 1#( received-protocol RWS received-by [ RWS comment ] )

received-protocol = [ protocol-name "/" ] protocol-version
                    ; see Section 6.7
received-by       = ( uri-host [ ":" port ] ) / pseudonym
pseudonym         = token

Multiple Via field values represent each proxy or gateway that has forwarded the message. Each intermediary appends its own information about how the message was received, such that the end result is ordered according to the sequence of forwarding recipients.

A proxy MUST send an appropriate Via header field, as described below, in each message that it forwards. An HTTP-to-HTTP gateway MUST send an appropriate Via header field in each inbound request message and MAY send a Via header field in forwarded response messages.

For each intermediary, the received-protocol indicates the protocol and protocol version used by the upstream sender of the message. Hence, the Via field value records the advertised protocol capabilities of the request/response chain such that they remain visible to downstream recipients; this can be useful for determining what backwards-incompatible features might be safe to use in response, or within a later request, as described in Section 2.6. For brevity, the protocol-name is omitted when the received protocol is HTTP.

The received-by portion of the field value is normally the host and optional port number of a recipient server or client that subsequently forwarded the message. However, if the real host is considered to be sensitive information, a sender MAY replace it with a pseudonym. If a port is not provided, a recipient MAY interpret that as meaning it was received on the default TCP port, if any, for the received-protocol.

A sender MAY generate comments in the Via header field to identify the software of each recipient, analogous to the User-Agent and Server header fields. However, all comments in the Via field are optional, and a recipient MAY remove them prior to forwarding the message.

For example, a request message could be sent from an HTTP/1.0 user agent to an internal proxy code-named "fred", which uses HTTP/1.1 to forward the request to a public proxy at p.example.net, which completes the request by forwarding it to the origin server at www.example.com. The request received by www.example.com would then have the following Via header field:

Via: 1.0 fred, 1.1 p.example.net

An intermediary used as a portal through a network firewall SHOULD NOT forward the names and ports of hosts within the firewall region unless it is explicitly enabled to do so. If not enabled, such an intermediary SHOULD replace each received-by host of any host behind the firewall by an appropriate pseudonym for that host.

An intermediary MAY combine an ordered subsequence of Via header field entries into a single such entry if the entries have identical received-protocol values. For example,

Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy

could be collapsed to

Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

A sender SHOULD NOT combine multiple entries unless they are all under the same organizational control and the hosts have already been replaced by pseudonyms. A sender MUST NOT combine entries that have different received-protocol values.

5.7.2. Transformations​

Some intermediaries include features for transforming messages and their payloads. A 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. However, operational problems might occur when these transformations are applied to payloads intended for critical applications, such as medical imaging or scientific data analysis, particularly when integrity checks or digital signatures are used to ensure that the payload received is identical to the original.

An HTTP-to-HTTP proxy is called a "transforming proxy" if it is designed or configured to modify messages in a semantically meaningful way (i.e., modifications, beyond those required by normal HTTP processing, that change the message in a way that would be significant to the original sender or potentially significant to downstream recipients). For example, a transforming proxy might be acting as a shared annotation server (modifying responses to include references to a local annotation database), a malware filter, a format transcoder, or a privacy filter. Such transformations are presumed to be desired by whichever client (or client organization) selected the proxy.

If a proxy receives a request-target with a host name that is not a fully qualified domain name, it MAY add its own domain to the host name it received when forwarding the request. A proxy MUST NOT change the host name if the request-target contains a fully qualified domain name.

A proxy MUST NOT modify the "absolute-path" and "query" parts of the received request-target when forwarding it to the next inbound server, except as noted above to replace an empty path with "/" or "*".

A proxy MAY modify the message body through application or removal of a transfer coding (Section 4).

A proxy MUST NOT transform the payload (Section 3.3 of [RFC7231]) of a message that contains a no-transform cache-control directive (Section 5.2 of [RFC7234]).

A proxy MAY transform the payload of a message that does not contain a no-transform cache-control directive. A proxy that transforms a payload MUST add a Warning header field with the warn-code of 214 ("Transformation Applied") if one is not already in the message (see Section 5.5 of [RFC7234]). A proxy that transforms the payload of a 200 (OK) response can further inform downstream recipients that a transformation has been applied by changing the response status code to 203 (Non-Authoritative Information) (Section 6.3.4 of [RFC7231]).

A proxy SHOULD NOT modify header fields that provide information about the endpoints of the communication chain, the resource state, or the selected representation (other than the payload) unless the field's definition specifically allows such modification or the modification is deemed necessary for privacy or security.