1CURLOPT_SSL_CTX_FUNCTION(3)curl_easy_setopt optionsCURLOPT_SSL_CTX_FUNCTION(3)
2
6 CURLOPT_SSL_CTX_FUNCTION - SSL context callback for OpenSSL, wolfSSL or
7 mbedTLS
8
10 #include <curl/curl.h>
11 CURLcode ssl_ctx_callback(CURL *curl, void *ssl_ctx, void *userptr);
13 CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSL_CTX_FUNCTION,
15 ssl_ctx_callback);
16
18 This option only works for libcurl powered by OpenSSL, wolfSSL or
19 mbedTLS. If libcurl was built against another SSL library this func‐
20 tionality is absent.
21 Pass a pointer to your callback function, which should match the proto‐
23 type shown above.
24 This callback function gets called by libcurl just before the initial‐
26 ization of an SSL connection after having processed all other SSL re‐
27 lated options to give a last chance to an application to modify the be‐
28 havior of the SSL initialization. The ssl_ctx parameter is actually a
29 pointer to the SSL library's SSL_CTX for OpenSSL or wolfSSL, and a
30 pointer to mbedtls_ssl_config for mbedTLS. If an error is returned from
31 the callback no attempt to establish a connection is made and the per‐
32 form operation will return the callback's error code. Set the userptr
33 argument with the CURLOPT_SSL_CTX_DATA(3) option.
34 This function will get called on all new connections made to a server,
36 during the SSL negotiation. The ssl_ctx will point to a newly initial‐
37 ized object each time, but note the pointer may be the same as from a
38 prior call.
39 To use this properly, a non-trivial amount of knowledge of your SSL li‐
41 brary is necessary. For example, you can use this function to call li‐
42 brary-specific callbacks to add additional validation code for certifi‐
43 cates, and even to change the actual URI of an HTTPS request.
44 WARNING: The CURLOPT_SSL_CTX_FUNCTION(3) callback allows the applica‐
46 tion to reach in and modify SSL details in the connection without
47 libcurl itself knowing anything about it, which then subsequently can
48 lead to libcurl unknowingly reusing SSL connections with different
49 properties. To remedy this you may set CURLOPT_FORBID_REUSE(3) from the
50 callback function.
51 WARNING: If you are using DNS-over-HTTPS (DoH) via CURLOPT_DOH_URL(3)
53 then the CTX callback will also be called for those transfers and the
54 curl handle is set to an internal handle. This behavior is subject to
55 change. We recommend before performing your transfer set CURLOPT_PRI‐
56 VATE(3) on your curl handle so you can identify it in the CTX callback.
57 If you have a reason to modify DoH SSL context please let us know on
58 the curl-library mailing list because we are considering removing this
59 capability.
60
62 NULL
63
65 All TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.
66
68 /* OpenSSL specific */
69 #include <openssl/ssl.h>
71 #include <curl/curl.h>
72 #include <stdio.h>
73 static CURLcode sslctx_function(CURL *curl, void *sslctx, void *parm)
75 {
76 X509_STORE *store;
77 X509 *cert = NULL;
78 BIO *bio;
79 char *mypem = parm;
80 /* get a BIO */
81 bio = BIO_new_mem_buf(mypem, -1);
82 /* use it to read the PEM formatted certificate from memory into an
83 * X509 structure that SSL can use
84 */
85 PEM_read_bio_X509(bio, &cert, 0, NULL);
86 if(cert == NULL)
87 printf("PEM_read_bio_X509 failed...\n");
88 /* get a pointer to the X509 certificate store (which may be empty) */
90 store = SSL_CTX_get_cert_store((SSL_CTX *)sslctx);
91 /* add our certificate to this store */
93 if(X509_STORE_add_cert(store, cert) == 0)
94 printf("error adding certificate\n");
95 /* decrease reference counts */
97 X509_free(cert);
98 BIO_free(bio);
99 /* all set to go */
101 return CURLE_OK;
102 }
103 int main(void)
105 {
106 CURL * ch;
107 CURLcode rv;
108 char *mypem = /* example CA cert PEM - shortened */
109 "-----BEGIN CERTIFICATE-----\n"
110 "MIIHPTCCBSWgAwIBAgIBADANBgkqhkiG9w0BAQQFADB5MRAwDgYDVQQKEwdSb290\n"
111 "IENBMR4wHAYDVQQLExVodHRwOi8vd3d3LmNhY2VydC5vcmcxIjAgBgNVBAMTGUNB\n"
112 "IENlcnQgU2lnbmluZyBBdXRob3JpdHkxITAfBgkqhkiG9w0BCQEWEnN1cHBvcnRA\n"
113 "Y2FjZXJ0Lm9yZzAeFw0wMzAzMzAxMjI5NDlaFw0zMzAzMjkxMjI5NDlaMHkxEDAO\n"
114 "GCSNe9FINSkYQKyTYOGWhlC0elnYjyELn8+CkcY7v2vcB5G5l1YjqrZslMZIBjzk\n"
115 "zk6q5PYvCdxTby78dOs6Y5nCpqyJvKeyRKANihDjbPIky/qbn3BHLt4Ui9SyIAmW\n"
116 "omTxJBzcoTWcFbLUvFUufQb1nA5V9FrWk9p2rSVzTMVD\n"
117 "-----END CERTIFICATE-----\n";
118 curl_global_init(CURL_GLOBAL_ALL);
120 ch = curl_easy_init();
121 curl_easy_setopt(ch, CURLOPT_SSLCERTTYPE, "PEM");
123 curl_easy_setopt(ch, CURLOPT_SSL_VERIFYPEER, 1L);
124 curl_easy_setopt(ch, CURLOPT_URL, "https://www.example.com/");
125 /* Retrieve page using cacerts' certificate -> will succeed
127 * load the certificate by installing a function doing the necessary
128 * "modifications" to the SSL CONTEXT just before link init
129 */
130 curl_easy_setopt(ch, CURLOPT_SSL_CTX_FUNCTION, *sslctx_function);
131 curl_easy_setopt(ch, CURLOPT_SSL_CTX_DATA, mypem);
132 rv = curl_easy_perform(ch);
133 if(!rv)
134 printf("*** transfer succeeded ***\n");
135 else
136 printf("*** transfer failed ***\n");
137 curl_easy_cleanup(ch);
139 curl_global_cleanup();
140 return rv;
141 }
142
144 Added in 7.11.0 for OpenSSL, in 7.42.0 for wolfSSL and in 7.54.0 for
145 mbedTLS. Other SSL backends are not supported.
146
148 CURLE_OK if supported; or an error such as:
149 CURLE_NOT_BUILT_IN - Not supported by the SSL backend
151 CURLE_UNKNOWN_OPTION
153
155 CURLOPT_SSL_CTX_DATA(3), CURLOPT_SSL_VERIFYPEER(3),
156libcurl 7.82.0 July 26, 2021 CURLOPT_SSL_CTX_FUNCTION(3)