Cryptography and SSL/TLS Toolkit



OSSL_HTTP_REQ_CTX, OSSL_HTTP_REQ_CTX_new, OSSL_HTTP_REQ_CTX_free, OSSL_HTTP_REQ_CTX_set_request_line, OSSL_HTTP_REQ_CTX_add1_header, OSSL_HTTP_REQ_CTX_set_expected, OSSL_HTTP_REQ_CTX_set1_req, OSSL_HTTP_REQ_CTX_nbio, OSSL_HTTP_REQ_CTX_nbio_d2i, OSSL_HTTP_REQ_CTX_exchange, OSSL_HTTP_REQ_CTX_get0_mem_bio, OSSL_HTTP_REQ_CTX_get_resp_len, OSSL_HTTP_REQ_CTX_set_max_response_length, OSSL_HTTP_is_alive - HTTP client low-level functions


#include <openssl/http.h>

typedef struct ossl_http_req_ctx_st OSSL_HTTP_REQ_CTX;

OSSL_HTTP_REQ_CTX *OSSL_HTTP_REQ_CTX_new(BIO *wbio, BIO *rbio, int buf_size);

int OSSL_HTTP_REQ_CTX_set_request_line(OSSL_HTTP_REQ_CTX *rctx, int method_POST,
                                       const char *server, const char *port,
                                       const char *path);
int OSSL_HTTP_REQ_CTX_add1_header(OSSL_HTTP_REQ_CTX *rctx,
                                  const char *name, const char *value);

int OSSL_HTTP_REQ_CTX_set_expected(OSSL_HTTP_REQ_CTX *rctx,
                                   const char *content_type, int asn1,
                                   int timeout, int keep_alive);
int OSSL_HTTP_REQ_CTX_set1_req(OSSL_HTTP_REQ_CTX *rctx, const char *content_type,
                               const ASN1_ITEM *it, const ASN1_VALUE *req);
                               ASN1_VALUE **pval, const ASN1_ITEM *it);

BIO *OSSL_HTTP_REQ_CTX_get0_mem_bio(const OSSL_HTTP_REQ_CTX *rctx);
size_t OSSL_HTTP_REQ_CTX_get_resp_len(const OSSL_HTTP_REQ_CTX *rctx);
void OSSL_HTTP_REQ_CTX_set_max_response_length(OSSL_HTTP_REQ_CTX *rctx,
                                               unsigned long len);

int OSSL_HTTP_is_alive(const OSSL_HTTP_REQ_CTX *rctx);


OSSL_HTTP_REQ_CTX is a context structure for an HTTP request and response, used to collect all the necessary data to perform that request.

This file documents low-level HTTP functions rarely used directly. High-level HTTP client functions like OSSL_HTTP_get(3) and OSSL_HTTP_transfer(3) should be preferred.

OSSL_HTTP_REQ_CTX_new() allocates a new HTTP request context structure, which gets populated with the BIO to write/send the request to (wbio), the BIO to read/receive the response from (rbio, which may be equal to wbio), and the maximum expected response header line length buf_size. A value <= 0 indicates that the OSSL_HTTP_DEFAULT_MAX_LINE_LEN of 4KiB should be used. buf_size is also used as the number of content bytes that are read at a time. The allocated context structure includes an internal memory BIO, which collects the HTTP request header lines.

OSSL_HTTP_REQ_CTX_free() frees up the HTTP request context rctx. The rbio is not free'd, wbio will be free'd if free_wbio is set. If the argument is NULL, nothing is done.

OSSL_HTTP_REQ_CTX_set_request_line() adds the 1st HTTP request line to rctx. The HTTP method is determined by method_POST, which should be 1 to indicate POST or 0 to indicate GET. server and port may be set to give the server and the optional port that an HTTP proxy shall forward the request to, otherwise they must be left NULL. path provides the HTTP request path; if left NULL, / is used. For backward compatibility, path may begin with http:// and thus convey an absoluteURI. In this case it indicates HTTP proxy use and provides also the server (and optionally the port) that the proxy shall forward the request to. In this case the server and port arguments must be NULL.

OSSL_HTTP_REQ_CTX_add1_header() adds header name with value value to the context rctx. It can be called more than once to add multiple header lines. For example, to add a Host header for you would call:

OSSL_HTTP_REQ_CTX_add1_header(ctx, "Host", "");

OSSL_HTTP_REQ_CTX_set_expected() optionally sets in rctx some expectations of the HTTP client on the response. Due to the structure of an HTTP request, if the keep_alive argument is nonzero the function must be used before calling OSSL_HTTP_REQ_CTX_set1_req(). If the content_type parameter is not NULL then the client will check that the given content type string is included in the HTTP header of the response and return an error if not. If the asn1 parameter is nonzero a structure in ASN.1 encoding will be expected as the response content and input streaming is disabled. This means that an ASN.1 sequence header is required, its length field is checked, and OSSL_HTTP_REQ_CTX_get0_mem_bio() should be used to get the buffered response. Otherwise (by default) any input format is allowed without length checks. In this case the BIO given as rbio argument to OSSL_HTTP_REQ_CTX_new() should be used directly to read the response contents, which may support streaming. If the timeout parameter is > 0 this indicates the maximum number of seconds the subsequent HTTP transfer (sending the request and receiving a response) is allowed to take. timeout == 0 enables waiting indefinitely, i.e., no timeout can occur. This is the default. timeout < 0 takes over any value set via the overall_timeout argument of OSSL_HTTP_open(3) with the default being 0, which means no timeout. If the keep_alive parameter is 0, which is the default, the connection is not kept open after receiving a response. This is the default behavior for HTTP 1.0. If the value is 1 or 2 then a persistent connection is requested. If the value is 2 then a persistent connection is required, i.e., an error occurs in case the server does not grant it.

OSSL_HTTP_REQ_CTX_set1_req() finalizes the HTTP request context. It is needed if the method_POST parameter in the OSSL_HTTP_REQ_CTX_set_request_line() call was 1 and an ASN.1-encoded request should be sent. It must also be used when requesting "keep-alive", even if a GET request is going to be sent, in which case req must be NULL. Unless req is NULL, the function adds the DER encoding of req using the ASN.1 template it to do the encoding (which does not support streaming). The HTTP header Content-Length is filled out with the length of the request. content_type must be NULL if req is NULL. If content_type isn't NULL, the HTTP header Content-Type is also added with the given string value. The header lines are added to the internal memory BIO for the request header.

OSSL_HTTP_REQ_CTX_nbio() attempts to send the request prepared in rctx and to gather the response via HTTP, using the wbio and rbio that were given when calling OSSL_HTTP_REQ_CTX_new(). The function may need to be called again if its result is -1, which indicates BIO_should_retry(3). In such a case it is advisable to sleep a little in between, using BIO_wait(3) on the read BIO to prevent a busy loop.

OSSL_HTTP_REQ_CTX_nbio_d2i() is like OSSL_HTTP_REQ_CTX_nbio() but on success in addition parses the response, which must be a DER-encoded ASN.1 structure, using the ASN.1 template it and places the result in *pval.

OSSL_HTTP_REQ_CTX_exchange() calls OSSL_HTTP_REQ_CTX_nbio() as often as needed in order to exchange a request and response or until a timeout is reached. On success it returns a pointer to the BIO that can be used to read the result. If an ASN.1-encoded response was expected, this is the BIO returned by OSSL_HTTP_REQ_CTX_get0_mem_bio() when called after the exchange. This memory BIO does not support streaming. Otherwise the returned BIO is the rbio given to OSSL_HTTP_REQ_CTX_new(), which may support streaming. When this BIO is returned, it has been read past the end of the response header, such that the actual response body can be read from it. The returned BIO pointer MUST NOT be freed by the caller.

OSSL_HTTP_REQ_CTX_get0_mem_bio() returns the internal memory BIO. Before the HTTP request is sent, this could be used to adapt its header lines. Use with caution! After receiving a response via HTTP, the BIO represents the current state of reading the response header. If the response was expected to be ASN.1 encoded, its contents can be read via this BIO, which does not support streaming. The returned BIO pointer must not be freed by the caller.

OSSL_HTTP_REQ_CTX_get_resp_len() returns the size of the response contents in rctx if provided by the server as <Content-Length> header field, else 0.

OSSL_HTTP_REQ_CTX_set_max_response_length() sets the maximum allowed response content length for rctx to len. If not set or len is 0 then the OSSL_HTTP_DEFAULT_MAX_RESP_LEN is used, which currently is 100 KiB. If the Content-Length header is present and exceeds this value or the content is an ASN.1 encoded structure with a length exceeding this value or both length indications are present but disagree then an error occurs.

OSSL_HTTP_is_alive() can be used to query if the HTTP connection given by rctx is still alive, i.e., has not been closed. It returns 0 if rctx is NULL.

If the client application requested or required a persistent connection and this was granted by the server, it can keep rctx as long as it wants to send further requests and OSSL_HTTP_is_alive() returns nonzero, else it should call OSSL_HTTP_REQ_CTX_free(rctx) or OSSL_HTTP_close(3). In case the client application keeps rctx but the connection then dies for any reason at the server side, it will notice this obtaining an I/O error when trying to send the next request via rctx.


The server's response may be unexpected if the hostname that was used to create the wbio, any Host header, and the host specified in the request URL do not match.

Many of these functions must be called in a certain order.

First, the HTTP request context must be allocated: OSSL_HTTP_REQ_CTX_new().

Then, the HTTP request must be prepared with request data:

  1. Calling OSSL_HTTP_REQ_CTX_set_request_line().

  2. Adding extra header lines with OSSL_HTTP_REQ_CTX_add1_header(). This is optional and may be done multiple times with different names.

  3. Finalize the request using OSSL_HTTP_REQ_CTX_set1_req(). This may be omitted if the GET method is used and "keep-alive" is not requested.

When the request context is fully prepared, the HTTP exchange may be performed with OSSL_HTTP_REQ_CTX_nbio() or OSSL_HTTP_REQ_CTX_exchange().


When built with tracing enabled, OSSL_HTTP_REQ_CTX_nbio() and all functions using it, such as OSSL_HTTP_REQ_CTX_exchange() and OSSL_HTTP_transfer(3), may be traced using OSSL_TRACE_CATEGORY_HTTP. See also OSSL_trace_enabled(3) and "ENVIRONMENT" in openssl(1).


OSSL_HTTP_REQ_CTX_new() returns a pointer to a OSSL_HTTP_REQ_CTX, or NULL on error.

OSSL_HTTP_REQ_CTX_free() and OSSL_HTTP_REQ_CTX_set_max_response_length() do not return values.

OSSL_HTTP_REQ_CTX_set_request_line(), OSSL_HTTP_REQ_CTX_add1_header(), OSSL_HTTP_REQ_CTX_set1_req(), and OSSL_HTTP_REQ_CTX_set_expected() return 1 for success and 0 for failure.

OSSL_HTTP_REQ_CTX_nbio() and OSSL_HTTP_REQ_CTX_nbio_d2i() return 1 for success, 0 on error or redirection, -1 if retry is needed.

OSSL_HTTP_REQ_CTX_exchange() and OSSL_HTTP_REQ_CTX_get0_mem_bio() return a pointer to a BIO on success as described above or NULL on failure. The returned BIO must not be freed by the caller.

OSSL_HTTP_REQ_CTX_get_resp_len() returns the size of the response contents or 0 if not available or an error occurred.

OSSL_HTTP_is_alive() returns 1 if its argument is non-NULL and the client requested a persistent connection and the server did not disagree on keeping the connection open, else 0.


BIO_should_retry(3), BIO_wait(3), ASN1_item_d2i_bio(3), ASN1_item_i2d_mem_bio(3), OSSL_HTTP_open(3), OSSL_HTTP_get(3), OSSL_HTTP_transfer(3), OSSL_HTTP_close(3), OSSL_trace_enabled(3)


The functions described here were added in OpenSSL 3.0.

Copyright 2015-2023 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at