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BIO_F_SSL(3) Library Functions Manual BIO_F_SSL(3)

BIO_f_ssl, BIO_set_ssl, BIO_get_ssl, BIO_set_ssl_mode, BIO_set_ssl_renegotiate_bytes, BIO_get_num_renegotiates, BIO_set_ssl_renegotiate_timeout, BIO_new_ssl, BIO_new_ssl_connect, BIO_new_buffer_ssl_connect, BIO_ssl_copy_session_id, BIO_ssl_shutdown — SSL BIO

#include <openssl/bio.h>
#include <openssl/ssl.h>

BIO_METHOD *
BIO_f_ssl(void);

#define BIO_set_ssl(b,ssl,c) BIO_ctrl(b,BIO_C_SET_SSL,c,(char *)ssl)
#define BIO_get_ssl(b,sslp) BIO_ctrl(b,BIO_C_GET_SSL,0,(char *)sslp)
#define BIO_set_ssl_mode(b,client) BIO_ctrl(b,BIO_C_SSL_MODE,client,NULL)
#define BIO_set_ssl_renegotiate_bytes(b,num) BIO_ctrl(b,BIO_C_SET_SSL_RENEGOTIATE_BYTES,num,NULL)
#define BIO_set_ssl_renegotiate_timeout(b,seconds) BIO_ctrl(b,BIO_C_SET_SSL_RENEGOTIATE_TIMEOUT,seconds,NULL)
#define BIO_get_num_renegotiates(b) BIO_ctrl(b,BIO_C_SET_SSL_NUM_RENEGOTIATES,0,NULL)

BIO *
BIO_new_ssl(SSL_CTX *ctx, int client);

BIO *
BIO_new_ssl_connect(SSL_CTX *ctx);

BIO *
BIO_new_buffer_ssl_connect(SSL_CTX *ctx);

int
BIO_ssl_copy_session_id(BIO *to, BIO *from);

void
BIO_ssl_shutdown(BIO *bio);

#define BIO_do_handshake(b) BIO_ctrl(b,BIO_C_DO_STATE_MACHINE,0,NULL)

() returns the SSL BIO method. This is a filter BIO which is a wrapper around the OpenSSL SSL routines adding a BIO “flavor” to SSL I/O.

I/O performed on an SSL BIO communicates using the SSL protocol with the SSL's read and write BIOs. If an SSL connection is not established then an attempt is made to establish one on the first I/O call.

If a BIO is appended to an SSL BIO using BIO_push(3) it is automatically used as the SSL BIO's read and write BIOs.

Calling BIO_reset(3) on an SSL BIO closes down any current SSL connection by calling SSL_shutdown(3). BIO_reset is then sent to the next BIO in the chain; this will typically disconnect the underlying transport. The SSL BIO is then reset to the initial accept or connect state.

If the close flag is set when an SSL BIO is freed then the internal SSL structure is also freed using SSL_free(3).

() sets the internal SSL pointer of BIO b to ssl using the close flag c.

() retrieves the SSL pointer of BIO b; it can then be manipulated using the standard SSL library functions.

() sets the SSL BIO mode to client. If client is 1, client mode is set. If client is 0, server mode is set.

() sets the renegotiate byte count to num. When set after every num bytes of I/O (read and write) the SSL session is automatically renegotiated. num must be at least 512 bytes.

() sets the renegotiate timeout to seconds. When the renegotiate timeout elapses the session is automatically renegotiated.

() returns the total number of session renegotiations due to I/O or timeout.

() allocates an SSL BIO using SSL_CTX ctx and using client mode if client is nonzero.

() creates a new BIO chain consisting of an SSL BIO (using ctx) followed by a connect BIO.

() creates a new BIO chain consisting of a buffering BIO, an SSL BIO (using ctx) and a connect BIO.

() copies an SSL session id between BIO chains from and to. It does this by locating the SSL BIOs in each chain and calling SSL_copy_session_id(3) on the internal SSL pointer.

() closes down an SSL connection on BIO chain bio. It does this by locating the SSL BIO in the chain and calling SSL_shutdown(3) on its internal SSL pointer.

() attempts to complete an SSL handshake on the supplied BIO and establish the SSL connection. It returns 1 if the connection was established successfully. A zero or negative value is returned if the connection could not be established; the call BIO_should_retry(3) should be used for non blocking connect BIOs to determine if the call should be retried. If an SSL connection has already been established this call has no effect.

SSL BIOs are exceptional in that if the underlying transport is non-blocking they can still request a retry in exceptional circumstances. Specifically this will happen if a session renegotiation takes place during a BIO_read(3) operation. One case where this happens is when SGC or step up occurs.

In OpenSSL 0.9.6 and later the SSL flag SSL_AUTO_RETRY can be set to disable this behaviour. In other words, when this flag is set an SSL BIO using a blocking transport will never request a retry.

Since unknown BIO_ctrl(3) operations are sent through filter BIOs the server name and port can be set using BIO_set_host(3) on the BIO returned by () without having to locate the connect BIO first.

Applications do not have to call () but may wish to do so to separate the handshake process from other I/O processing.

This section is incomplete.

This SSL/TLS client example attempts to retrieve a page from an SSL/TLS web server. The I/O routines are identical to those of the unencrypted example in BIO_s_connect(3).

BIO *sbio, *out;
int len;
char tmpbuf[1024];
SSL_CTX *ctx;
SSL *ssl;

ERR_load_crypto_strings();
ERR_load_SSL_strings();
OpenSSL_add_all_algorithms();

/*
 * We would seed the PRNG here if the platform didn't do it automatically
 */

ctx = SSL_CTX_new(SSLv23_client_method());

/*
 * We'd normally set some stuff like the verify paths and mode here because
 * as things stand this will connect to any server whose certificate is
 * signed by any CA.
 */

sbio = BIO_new_ssl_connect(ctx);

BIO_get_ssl(sbio, &ssl);

if (!ssl) {
	fprintf(stderr, "Can't locate SSL pointer\n");
	/* whatever ... */
}

/* Don't want any retries */
SSL_set_mode(ssl, SSL_MODE_AUTO_RETRY);

/* We might want to do other things with ssl here */

BIO_set_conn_hostname(sbio, "localhost:https");

out = BIO_new_fp(stdout, BIO_NOCLOSE);
if (BIO_do_connect(sbio) <= 0) {
	fprintf(stderr, "Error connecting to server\n");
	ERR_print_errors_fp(stderr);
	/* whatever ... */
}

if (BIO_do_handshake(sbio) <= 0) {
	fprintf(stderr, "Error establishing SSL connection\n");
	ERR_print_errors_fp(stderr);
	/* whatever ... */
}

/* Could examine ssl here to get connection info */

BIO_puts(sbio, "GET / HTTP/1.0\n\n");
for (;;) {
	len = BIO_read(sbio, tmpbuf, 1024);
	if(len <= 0) break;
	BIO_write(out, tmpbuf, len);
}
BIO_free_all(sbio);
BIO_free(out);

Here is a simple server example. It makes use of a buffering BIO to allow lines to be read from the SSL BIO using BIO_gets(3). It creates a pseudo web page containing the actual request from a client and also echoes the request to standard output.

BIO *sbio, *bbio, *acpt, *out;
int len;
char tmpbuf[1024];
SSL_CTX *ctx;
SSL *ssl;

ERR_load_crypto_strings();
ERR_load_SSL_strings();
OpenSSL_add_all_algorithms();

/* Might seed PRNG here */

ctx = SSL_CTX_new(SSLv23_server_method());

if (!SSL_CTX_use_certificate_file(ctx,"server.pem",SSL_FILETYPE_PEM)
    || !SSL_CTX_use_PrivateKey_file(ctx,"server.pem",SSL_FILETYPE_PEM)
    || !SSL_CTX_check_private_key(ctx)) {
	fprintf(stderr, "Error setting up SSL_CTX\n");
	ERR_print_errors_fp(stderr);
	return 0;
}

/*
 * Might do other things here like setting verify locations and DH and/or
 * RSA temporary key callbacks
 */

/* New SSL BIO setup as server */
sbio = BIO_new_ssl(ctx,0);

BIO_get_ssl(sbio, &ssl);

if (!ssl) {
	fprintf(stderr, "Can't locate SSL pointer\n");
	/* whatever ... */
}

/* Don't want any retries */
SSL_set_mode(ssl, SSL_MODE_AUTO_RETRY);

/* Create the buffering BIO */

bbio = BIO_new(BIO_f_buffer());

/* Add to chain */
sbio = BIO_push(bbio, sbio);

acpt = BIO_new_accept("4433");

/*
 * By doing this when a new connection is established we automatically
 * have sbio inserted into it. The BIO chain is now 'swallowed' by the
 * accept BIO and will be freed when the accept BIO is freed.
 */

BIO_set_accept_bios(acpt,sbio);

out = BIO_new_fp(stdout, BIO_NOCLOSE);

/* Setup accept BIO */
if (BIO_do_accept(acpt) <= 0) {
	fprintf(stderr, "Error setting up accept BIO\n");
	ERR_print_errors_fp(stderr);
	return 0;
}

/* Now wait for incoming connection */
if (BIO_do_accept(acpt) <= 0) {
	fprintf(stderr, "Error in connection\n");
	ERR_print_errors_fp(stderr);
	return 0;
}

/* We only want one connection so remove and free accept BIO */

sbio = BIO_pop(acpt);

BIO_free_all(acpt);

if (BIO_do_handshake(sbio) <= 0) {
	fprintf(stderr, "Error in SSL handshake\n");
	ERR_print_errors_fp(stderr);
	return 0;
}

BIO_puts(sbio, "HTTP/1.0 200 OK\r\nContent-type: text/plain\r\n\r\n");
BIO_puts(sbio, "\r\nConnection Established\r\nRequest headers:\r\n");
BIO_puts(sbio, "--------------------------------------------------\r\n");

for (;;) {
	len = BIO_gets(sbio, tmpbuf, 1024);
	if (len <= 0)
		break;
	BIO_write(sbio, tmpbuf, len);
	BIO_write(out, tmpbuf, len);
	/* Look for blank line signifying end of headers */
	if ((tmpbuf[0] == '\r') || (tmpbuf[0] == '\n'))
		break;
}

BIO_puts(sbio, "--------------------------------------------------\r\n");
BIO_puts(sbio, "\r\n");

/* Since there is a buffering BIO present we had better flush it */
BIO_flush(sbio);

BIO_free_all(sbio);

In OpenSSL versions before 1.0.0 the BIO_pop(3) call was handled incorrectly: the I/O BIO reference count was incorrectly incremented (instead of decremented) and dissociated with the SSL BIO even if the SSL BIO was not explicitly being popped (e.g., a pop higher up the chain). Applications which included workarounds for this bug (e.g., freeing BIOs more than once) should be modified to handle this fix or they may free up an already freed BIO.

December 2, 2014 OpenBSD-5.7