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deps/curl/docs/libcurl/opts/CURLOPT_SSL_CTX_FUNCTION.md

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+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLOPT_SSL_CTX_FUNCTION
+Section: 3
+Source: libcurl
+See-also:
+  - CURLOPT_SSL_CTX_DATA (3)
+  - CURLOPT_SSL_VERIFYPEER (3)
+---
+
+# NAME
+
+CURLOPT_SSL_CTX_FUNCTION - SSL context callback for OpenSSL, wolfSSL or mbedTLS
+
+# SYNOPSIS
+
+~~~c
+#include <curl/curl.h>
+
+CURLcode ssl_ctx_callback(CURL *curl, void *ssl_ctx, void *clientp);
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSL_CTX_FUNCTION,
+                          ssl_ctx_callback);
+~~~
+
+# DESCRIPTION
+
+This option only works for libcurl powered by OpenSSL, wolfSSL, mbedTLS or
+BearSSL. 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 *ssl_ctx* parameter is actually a pointer to the SSL
+library's *SSL_CTX* for OpenSSL or wolfSSL, a pointer to
+*mbedtls_ssl_config* for mbedTLS or a pointer to
+*br_ssl_client_context* for BearSSL. If an error is returned from the
+callback no attempt to establish a connection is made and the perform
+operation returns the callback's error code. Set the *clientp* argument
+with the CURLOPT_SSL_CTX_DATA(3) option.
+
+This function gets called on all new connections made to a server, during the
+SSL negotiation. The *ssl_ctx* points 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.
+
+For OpenSSL, asynchronous certificate verification via
+*SSL_set_retry_verify* is supported. (Added in 8.3.0)
+
+WARNING: The CURLOPT_SSL_CTX_FUNCTION(3) 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 CURLOPT_FORBID_REUSE(3) from the callback function.
+
+WARNING: If you are using DNS-over-HTTPS (DoH) via CURLOPT_DOH_URL(3)
+then this callback is also called for those transfers and the curl handle is
+set to an internal handle. **This behavior is subject to change.** We
+recommend before performing your transfer set CURLOPT_PRIVATE(3) on your
+curl handle so you can identify it in the context 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.
+
+# DEFAULT
+
+NULL
+
+# PROTOCOLS
+
+All TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.
+
+# EXAMPLE
+
+~~~c
+/* OpenSSL specific */
+
+#include <openssl/ssl.h>
+#include <curl/curl.h>
+#include <stdio.h>
+
+static CURLcode sslctx_function(CURL *curl, void *sslctx, void *parm)
+{
+  X509_STORE *store;
+  X509 *cert = NULL;
+  BIO *bio;
+  char *mypem = parm;
+  /* get a BIO */
+  bio = BIO_new_mem_buf(mypem, -1);
+  /* use it to read the PEM formatted certificate from memory into an
+   * X509 structure that SSL can use
+   */
+  PEM_read_bio_X509(bio, &cert, 0, NULL);
+  if(!cert)
+    printf("PEM_read_bio_X509 failed...\n");
+
+  /* get a pointer to the X509 certificate store (which may be empty) */
+  store = SSL_CTX_get_cert_store((SSL_CTX *)sslctx);
+
+  /* add our certificate to this store */
+  if(X509_STORE_add_cert(store, cert) == 0)
+    printf("error adding certificate\n");
+
+  /* decrease reference counts */
+  X509_free(cert);
+  BIO_free(bio);
+
+  /* all set to go */
+  return CURLE_OK;
+}
+
+int main(void)
+{
+  CURL *ch;
+  CURLcode rv;
+  char *mypem = /* example CA cert PEM - shortened */
+    "-----BEGIN CERTIFICATE-----\n"
+    "MIIHPTCCBSWgAwIBAgIBADANBgkqhkiG9w0BAQQFADB5MRAwDgYDVQQKEwdSb290\n"
+    "IENBMR4wHAYDVQQLExVodHRwOi8vd3d3LmNhY2VydC5vcmcxIjAgBgNVBAMTGUNB\n"
+    "IENlcnQgU2lnbmluZyBBdXRob3JpdHkxITAfBgkqhkiG9w0BCQEWEnN1cHBvcnRA\n"
+    "Y2FjZXJ0Lm9yZzAeFw0wMzAzMzAxMjI5NDlaFw0zMzAzMjkxMjI5NDlaMHkxEDAO\n"
+    "GCSNe9FINSkYQKyTYOGWhlC0elnYjyELn8+CkcY7v2vcB5G5l1YjqrZslMZIBjzk\n"
+    "zk6q5PYvCdxTby78dOs6Y5nCpqyJvKeyRKANihDjbPIky/qbn3BHLt4Ui9SyIAmW\n"
+    "omTxJBzcoTWcFbLUvFUufQb1nA5V9FrWk9p2rSVzTMVD\n"
+    "-----END CERTIFICATE-----\n";
+
+  curl_global_init(CURL_GLOBAL_ALL);
+  ch = curl_easy_init();
+
+  curl_easy_setopt(ch, CURLOPT_SSLCERTTYPE, "PEM");
+  curl_easy_setopt(ch, CURLOPT_SSL_VERIFYPEER, 1L);
+  curl_easy_setopt(ch, CURLOPT_URL, "https://www.example.com/");
+
+  curl_easy_setopt(ch, CURLOPT_SSL_CTX_FUNCTION, *sslctx_function);
+  curl_easy_setopt(ch, CURLOPT_SSL_CTX_DATA, mypem);
+  rv = curl_easy_perform(ch);
+  if(!rv)
+    printf("*** transfer succeeded ***\n");
+  else
+    printf("*** transfer failed ***\n");
+
+  curl_easy_cleanup(ch);
+  curl_global_cleanup();
+  return rv;
+}
+~~~
+
+# AVAILABILITY
+
+Added in 7.11.0 for OpenSSL, in 7.42.0 for wolfSSL, in 7.54.0 for mbedTLS,
+in 7.83.0 in BearSSL. Other SSL backends are not supported.
+
+# RETURN VALUE
+
+CURLE_OK if supported; or an error such as:
+
+CURLE_NOT_BUILT_IN - Not supported by the SSL backend
+
+CURLE_UNKNOWN_OPTION