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149 lines
5.4 KiB
Groff
149 lines
5.4 KiB
Groff
.\" generated by cd2nroff 0.1 from CURLOPT_SSL_CTX_FUNCTION.md
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.TH CURLOPT_SSL_CTX_FUNCTION 3 "2024-06-01" libcurl
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.SH NAME
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CURLOPT_SSL_CTX_FUNCTION \- SSL context callback
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.SH SYNOPSIS
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.nf
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#include <curl/curl.h>
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CURLcode ssl_ctx_callback(CURL *curl, void *ssl_ctx, void *clientp);
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CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSL_CTX_FUNCTION,
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ssl_ctx_callback);
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.fi
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.SH DESCRIPTION
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Pass a pointer to your callback function, which should match the prototype
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shown above.
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This callback function gets called by libcurl just before the initialization
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of an SSL connection after having processed all other SSL related options to
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give a last chance to an application to modify the behavior of the SSL
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initialization. The \fIssl_ctx\fP parameter is a pointer to the SSL library\(aqs
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\fISSL_CTX\fP for OpenSSL or wolfSSL, a pointer to \fImbedtls_ssl_config\fP for
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mbedTLS or a pointer to \fIbr_ssl_client_context\fP for BearSSL. If an error is
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returned from the callback no attempt to establish a connection is made and
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the perform operation returns the callback\(aqs error code. Set the \fIclientp\fP
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argument passed in to this callback with the \fICURLOPT_SSL_CTX_DATA(3)\fP option.
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This function gets called for all new connections made to a server, during the
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SSL negotiation. While \fIssl_ctx\fP points to a newly initialized object each
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time, the pointer may still be the same as in a prior call.
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To use this callback, a non\-trivial amount of knowledge of your SSL library is
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necessary. For example, you can use this function to call library\-specific
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callbacks to add additional validation code for certificates, and even to
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change the actual URI of an HTTPS request.
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For OpenSSL, asynchronous certificate verification via \fISSL_set_retry_verify\fP
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is supported. (Added in 8.3.0)
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The \fICURLOPT_SSL_CTX_FUNCTION(3)\fP callback allows the application to reach in
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and modify SSL details in the connection without libcurl itself knowing
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anything about it, which then subsequently can lead to libcurl unknowingly
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reusing SSL connections with different properties. To remedy this you may set
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\fICURLOPT_FORBID_REUSE(3)\fP from the callback function.
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If you are using DNS\-over\-HTTPS (DoH) via \fICURLOPT_DOH_URL(3)\fP then this
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callback is also called for those transfers and the curl handle is set to an
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internal handle. \fBThis behavior is subject to change.\fP We recommend setting
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\fICURLOPT_PRIVATE(3)\fP on your curl handle so you can identify it correctly in the
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context callback. If you have a reason to modify DoH SSL context please let us
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know on the curl\-library mailing list because we are considering removing this
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capability.
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libcurl does not guarantee the lifetime of the passed in object once this
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callback function has returned. Your application must not assume that it can
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keep using the SSL context or data derived from it once this function is
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completed.
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.SH DEFAULT
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NULL
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.SH PROTOCOLS
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All TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.
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This option works only with the following TLS backends:
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BearSSL, OpenSSL, mbedTLS and wolfSSL
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.SH EXAMPLE
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.nf
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/* OpenSSL specific */
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#include <openssl/ssl.h>
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#include <curl/curl.h>
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#include <stdio.h>
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static CURLcode sslctx_function(CURL *curl, void *sslctx, void *parm)
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{
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X509_STORE *store;
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X509 *cert = NULL;
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BIO *bio;
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char *mypem = parm;
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/* get a BIO */
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bio = BIO_new_mem_buf(mypem, -1);
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/* use it to read the PEM formatted certificate from memory into an
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* X509 structure that SSL can use
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*/
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PEM_read_bio_X509(bio, &cert, 0, NULL);
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if(!cert)
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printf("PEM_read_bio_X509 failed...\\n");
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/* get a pointer to the X509 certificate store (which may be empty) */
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store = SSL_CTX_get_cert_store((SSL_CTX *)sslctx);
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/* add our certificate to this store */
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if(X509_STORE_add_cert(store, cert) == 0)
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printf("error adding certificate\\n");
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/* decrease reference counts */
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X509_free(cert);
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BIO_free(bio);
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/* all set to go */
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return CURLE_OK;
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}
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int main(void)
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{
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CURL *ch;
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CURLcode rv;
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char *mypem = /* example CA cert PEM - shortened */
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"-----BEGIN CERTIFICATE-----\\n"
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"MIIHPTCCBSWgAwIBAgIBADANBgkqhkiG9w0BAQQFADB5MRAwDgYDVQQKEwdSb290\\n"
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"IENBMR4wHAYDVQQLExVodHRwOi8vd3d3LmNhY2VydC5vcmcxIjAgBgNVBAMTGUNB\\n"
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"IENlcnQgU2lnbmluZyBBdXRob3JpdHkxITAfBgkqhkiG9w0BCQEWEnN1cHBvcnRA\\n"
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"Y2FjZXJ0Lm9yZzAeFw0wMzAzMzAxMjI5NDlaFw0zMzAzMjkxMjI5NDlaMHkxEDAO\\n"
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"GCSNe9FINSkYQKyTYOGWhlC0elnYjyELn8+CkcY7v2vcB5G5l1YjqrZslMZIBjzk\\n"
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"zk6q5PYvCdxTby78dOs6Y5nCpqyJvKeyRKANihDjbPIky/qbn3BHLt4Ui9SyIAmW\\n"
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"omTxJBzcoTWcFbLUvFUufQb1nA5V9FrWk9p2rSVzTMVD\\n"
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"-----END CERTIFICATE-----\\n";
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curl_global_init(CURL_GLOBAL_ALL);
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ch = curl_easy_init();
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curl_easy_setopt(ch, CURLOPT_SSLCERTTYPE, "PEM");
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curl_easy_setopt(ch, CURLOPT_SSL_VERIFYPEER, 1L);
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curl_easy_setopt(ch, CURLOPT_URL, "https://www.example.com/");
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curl_easy_setopt(ch, CURLOPT_SSL_CTX_FUNCTION, *sslctx_function);
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curl_easy_setopt(ch, CURLOPT_SSL_CTX_DATA, mypem);
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rv = curl_easy_perform(ch);
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if(!rv)
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printf("*** transfer succeeded ***\\n");
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else
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printf("*** transfer failed ***\\n");
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curl_easy_cleanup(ch);
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curl_global_cleanup();
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return rv;
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}
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.fi
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.SH AVAILABILITY
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OpenSSL (added in 7.11.0), wolfSSL (added in 7.42.0), mbedTLS (added in
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7.54.0) or BearSSL (added in 7.83.0)
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.SH RETURN VALUE
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CURLE_OK if supported; or an error such as:
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CURLE_NOT_BUILT_IN \- Not supported by the SSL backend
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.SH SEE ALSO
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.BR CURLOPT_CAINFO (3),
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.BR CURLOPT_SSL_CTX_DATA (3),
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.BR CURLOPT_SSL_VERIFYPEER (3)
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