curl / libcurl / API / curl_easy_getinfo / CURLINFO_TLS_SSL_PTR

CURLINFO_TLS_SSL_PTR explained

Name

CURLINFO_TLS_SESSION, CURLINFO_TLS_SSL_PTR - get TLS session info

Synopsis

#include <curl/curl.h>
 
CURLcode curl_easy_getinfo(CURL *handle, CURLINFO_TLS_SSL_PTR,
                           struct curl_tlssessioninfo **session);
 
/* if you need compatibility with libcurl < 7.48.0 use
   CURLINFO_TLS_SESSION instead: */
 
CURLcode curl_easy_getinfo(CURL *handle, CURLINFO_TLS_SESSION,
                           struct curl_tlssessioninfo **session);

Description

Pass a pointer to a struct curl_tlssessioninfo *. The pointer is initialized to refer to a struct curl_tlssessioninfo * that contains an enum indicating the SSL library used for the handshake and a pointer to the respective internal TLS session structure of this underlying SSL library.

This option may be useful for example to extract certificate information in a format convenient for further processing, such as manual validation. Refer to the LIMITATIONS section.

struct curl_tlssessioninfo {
  curl_sslbackend backend;
  void *internals;
};

The backend struct member is one of the defines in the CURLSSLBACKEND_* series: CURLSSLBACKEND_NONE (when built without TLS support), CURLSSLBACKEND_WOLFSSL, CURLSSLBACKEND_SECURETRANSPORT, CURLSSLBACKEND_GNUTLS, CURLSSLBACKEND_MBEDTLS, CURLSSLBACKEND_NSS, CURLSSLBACKEND_OPENSSL or CURLSSLBACKEND_SCHANNEL. (Note that the OpenSSL forks are all reported as just OpenSSL here.)

The internals struct member points to a TLS library specific pointer for the active ("in use") SSL connection, with the following underlying types:

GnuTLS

gnutls_session_t

OpenSSL

CURLINFO_TLS_SESSION: SSL_CTX *

CURLINFO_TLS_SSL_PTR: SSL * Since 7.48.0 the internals member can point to these other SSL backends as well:

mbedTLS

mbedTLS_ssl_context *

Secure Channel

CtxtHandle *

Secure Transport

SSLContext *

wolfSSL

SSL *

If the internals pointer is NULL then either the SSL backend is not supported, an SSL session has not yet been established or the connection is no longer associated with the easy handle (e.g. curl_easy_perform has returned).

Limitations

This option has some limitations that could make it unsafe when it comes to the manual verification of certificates.

This option only retrieves the first in-use SSL session pointer for your easy handle, however your easy handle may have more than one in-use SSL session if using FTP over SSL. That is because the FTP protocol has a control channel and a data channel and one or both may be over SSL. Currently there is no way to retrieve a second in-use SSL session associated with an easy handle.

This option has not been thoroughly tested with clear text protocols that can be upgraded/downgraded to/from SSL: FTP, SMTP, POP3, IMAP when used with CURLOPT_USE_SSL. Though you can to retrieve the SSL pointer, it is possible that before you can do that, data (including auth) may have already been sent over a connection after it was upgraded.

Renegotiation. If unsafe renegotiation or renegotiation in a way that the certificate is allowed to change is allowed by your SSL library this may occur and the certificate may change, and data may continue to be sent or received after renegotiation but before you are able to get the (possibly) changed SSL pointer, with the (possibly) changed certificate information.

Instead of using this option to poll for certificate changes use CURLOPT_SSL_CTX_FUNCTION to set a verification callback, if supported. That is safer and does not suffer from any of the problems above.

How are you using this option? Are you affected by any of these limitations? Please let us know by making a comment at https://github.com/curl/curl/issues/685

Protocols

This functionality affects all TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.

This option works only with the following TLS backends: BearSSL, GnuTLS, OpenSSL, Schannel, Secure Transport, mbedTLS and wolfSSL

Example

#include <curl/curl.h>
#include <openssl/ssl.h>
 
CURL *curl;
static size_t wf(void *ptr, size_t size, size_t nmemb, void *stream)
{
  const struct curl_tlssessioninfo *info = NULL;
  CURLcode res = curl_easy_getinfo(curl, CURLINFO_TLS_SSL_PTR, &info);
  if(info && !res) {
    if(CURLSSLBACKEND_OPENSSL == info->backend) {
      printf("OpenSSL ver. %s\n", SSL_get_version((SSL*)info->internals));
    }
  }
  return size * nmemb;
}
 
int main(int argc, char **argv)
{
  CURLcode res;
  curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, wf);
    res = curl_easy_perform(curl);
    curl_easy_cleanup(curl);
  }
  return res;
}

History

This option supersedes CURLINFO_TLS_SESSION which was added in 7.34.0. This option is exactly the same as that option except in the case of OpenSSL.

Availability

Added in curl 7.48.0

Return value

Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.