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BIO_f_ssl(3)                        OpenSSL                       BIO_f_ssl(3)

       BIO_do_handshake, 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

       libcrypto, -lcrypto

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

        const BIO_METHOD *BIO_f_ssl(void);

        long BIO_set_ssl(BIO *b, SSL *ssl, long c);
        long BIO_get_ssl(BIO *b, SSL **sslp);
        long BIO_set_ssl_mode(BIO *b, long client);
        long BIO_set_ssl_renegotiate_bytes(BIO *b, long num);
        long BIO_set_ssl_renegotiate_timeout(BIO *b, long seconds);
        long BIO_get_num_renegotiates(BIO *b);

        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);

        long BIO_do_handshake(BIO *b);

       BIO_f_ssl() returns the SSL BIO method. This is a filter BIO which is a
       wrapper round the OpenSSL SSL routines adding a BIO "flavour" to SSL

       I/O performed on an SSL BIO communicates using the SSL protocol with
       the SSLs 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() it is automatically
       used as the SSL BIOs read and write BIOs.

       Calling BIO_reset() on an SSL BIO closes down any current SSL
       connection by calling SSL_shutdown(). 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

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

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

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

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

       BIO_set_ssl_renegotiate_bytes() 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.

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

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

       BIO_new_ssl() allocates an SSL BIO using SSL_CTX ctx and using client
       mode if client is non zero.

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

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

       BIO_ssl_copy_session_id() 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() on the internal SSL pointer.

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

       BIO_do_handshake() 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() 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_ex() operation, one case where this happens is when
       step up occurs.

       The SSL flag SSL_AUTO_RETRY can be set to disable this behaviour. That
       is when this flag is set an SSL BIO using a blocking transport will
       never request a retry.

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

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

       BIO_set_ssl(), BIO_get_ssl(), BIO_set_ssl_mode(),
       BIO_set_ssl_renegotiate_bytes(), BIO_set_ssl_renegotiate_timeout(),
       BIO_get_num_renegotiates(), and BIO_do_handshake() are implemented as

       BIO_f_ssl() returns the SSL BIO_METHOD structure.

       BIO_set_ssl(), BIO_get_ssl(), BIO_set_ssl_mode(),
       BIO_set_ssl_renegotiate_bytes(), BIO_set_ssl_renegotiate_timeout() and
       BIO_get_num_renegotiates() return 1 on success or a value which is less
       than or equal to 0 if an error occurred.

       BIO_new_ssl(), BIO_new_ssl_connect() and BIO_new_buffer_ssl_connect()
       return a valid BIO structure on success or NULL if an error occurred.

       BIO_ssl_copy_session_id() returns 1 on success or 0 on error.

       BIO_do_handshake() returns 1 if the connection was established
       successfully.  A zero or negative value is returned if the connection
       could not be established.

       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;

        /* XXX Seed the PRNG if needed. */

        ctx = SSL_CTX_new(TLS_client_method());

        /* XXX Set verify paths and mode here. */

        sbio = BIO_new_ssl_connect(ctx);
        BIO_get_ssl(sbio, &ssl);
        if (ssl == NULL) {
            fprintf(stderr, "Can't locate SSL pointer\n");

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

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

        /* An empty host part means the loopback address */
        BIO_set_conn_hostname(sbio, ":https");

        out = BIO_new_fp(stdout, BIO_NOCLOSE);
        if (BIO_do_connect(sbio) <= 0) {
            fprintf(stderr, "Error connecting to server\n");
        if (BIO_do_handshake(sbio) <= 0) {
            fprintf(stderr, "Error establishing SSL connection\n");

        /* XXX 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)
            BIO_write(out, tmpbuf, len);

       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.  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;

        /* XXX Seed the PRNG if needed. */

        ctx = SSL_CTX_new(TLS_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");

        /* XXX Other things like set verify locations, EDH temp callbacks. */

        /* New SSL BIO setup as server */
        sbio = BIO_new_ssl(ctx, 0);
        BIO_get_ssl(sbio, &ssl);
        if (ssl == NULL) {
            fprintf(stderr, "Can't locate SSL pointer\n");

        SSL_set_mode(ssl, SSL_MODE_AUTO_RETRY);
        bbio = BIO_new(BIO_f_buffer());
        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");

        /* We only want one connection so remove and free accept BIO */
        sbio = BIO_pop(acpt);

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

        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)
            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')

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

       In OpenSSL before 1.0.0 the BIO_pop() 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.

       Copyright 2000-2019 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the OpenSSL license (the "License").  You may not use
       this file except in compliance with the License.  You can obtain a copy
       in the file LICENSE in the source distribution or at

1.1.1i                            2020-01-23                      BIO_f_ssl(3)