curl/docs/libcurl/opts/CURLOPT_SSL_CTX_FUNCTION.3

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.TH CURLOPT_SSL_CTX_FUNCTION 3 "19 Jun 2014" "libcurl 7.37.0" "curl_easy_setopt options"
.SH NAME
CURLOPT_SSL_CTX_FUNCTION \- SSL context callback for OpenSSL, wolfSSL or mbedTLS
.SH SYNOPSIS
.nf
#include <curl/curl.h>

CURLcode ssl_ctx_callback(CURL *curl, void *ssl_ctx, void *userptr);

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSL_CTX_FUNCTION,
                          ssl_ctx_callback);
.SH DESCRIPTION
This option only works for libcurl powered by OpenSSL, wolfSSL or mbedTLS. If
libcurl was built against another SSL library this functionality is absent.

Pass a pointer to your callback function, which should match the prototype
shown above.

This callback function gets called by libcurl just before the initialization
of an SSL connection after having processed all other SSL related options to
give a last chance to an application to modify the behavior of the SSL
initialization. The \fIssl_ctx\fP parameter is actually a pointer to the SSL
library's \fISSL_CTX\fP for OpenSSL or wolfSSL, and a pointer to
\fImbedtls_ssl_config\fP for mbedTLS. If an error is returned from the
callback no attempt to establish a connection is made and the perform
operation will return the callback's error code. Set the \fIuserptr\fP
argument with the \fICURLOPT_SSL_CTX_DATA(3)\fP option.

This function will get called on all new connections made to a server, during
the SSL negotiation. The \fIssl_ctx\fP will point to a newly initialized object
each time, but note the pointer may be the same as from a prior call.

To use this properly, a non-trivial amount of knowledge of your SSL library is
necessary. For example, you can use this function to call library-specific
callbacks to add additional validation code for certificates, and even to
change the actual URI of an HTTPS request.

WARNING: The \fICURLOPT_SSL_CTX_FUNCTION(3)\fP callback allows the application
to reach in and modify SSL details in the connection without libcurl itself
knowing anything about it, which then subsequently can lead to libcurl
unknowingly reusing SSL connections with different properties. To remedy this
you may set \fICURLOPT_FORBID_REUSE(3)\fP from the callback function.

WARNING: If you are using DNS-over-HTTPS (DoH) via \fICURLOPT_DOH_URL(3)\fP
then the CTX callback will also be called for those transfers and the curl
handle is set to an internal handle. \fBThis behavior is subject to change.\fP
We recommend before performing your transfer set \fICURLOPT_PRIVATE(3)\fP on
your curl handle so you can identify it in the CTX callback. If you have a
reason to modify DoH SSL context please let us know on the curl-library mailing
list because we are considering removing this capability.
.SH DEFAULT
NULL
.SH PROTOCOLS
All TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.
.SH EXAMPLE
See cacertinmem.c in docs/examples directory for usage example.

https://curl.se/libcurl/c/cacertinmem.html
.SH AVAILABILITY
Added in 7.11.0 for OpenSSL, in 7.42.0 for wolfSSL and in 7.54.0 for
mbedTLS. Other SSL backends are not supported.
.SH RETURN VALUE
CURLE_OK if supported; or an error such as:

CURLE_NOT_BUILT_IN - Not supported by the SSL backend

CURLE_UNKNOWN_OPTION
.SH "SEE ALSO"
.BR CURLOPT_SSL_CTX_DATA "(3), " CURLOPT_SSL_VERIFYPEER "(3), "