.\" $OpenBSD: BIO_f_ssl.3,v 1.13 2022/12/18 20:24:52 schwarze Exp $
.\" full merge up to: OpenSSL f672aee4 Feb 9 11:52:40 2016 -0500
.\" selective merge up to: OpenSSL 61f805c1 Jan 16 01:01:46 2018 +0800
.\"
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.Dd $Mdocdate: December 18 2022 $
.Dt BIO_F_SSL 3
.Os
.Sh NAME
.Nm BIO_f_ssl ,
.Nm BIO_set_ssl ,
.Nm BIO_get_ssl ,
.Nm BIO_set_ssl_mode ,
.Nm BIO_set_ssl_renegotiate_bytes ,
.Nm BIO_get_num_renegotiates ,
.Nm BIO_set_ssl_renegotiate_timeout ,
.Nm BIO_new_ssl ,
.Nm BIO_new_ssl_connect ,
.Nm BIO_new_buffer_ssl_connect ,
.Nm BIO_ssl_copy_session_id ,
.Nm BIO_ssl_shutdown ,
.Nm BIO_do_handshake
.Nd SSL BIO
.Sh SYNOPSIS
.In openssl/bio.h
.In openssl/ssl.h
.Ft const BIO_METHOD *
.Fn BIO_f_ssl void
.Ft long
.Fo BIO_set_ssl
.Fa "BIO *b"
.Fa "SSL *ssl"
.Fa "long c"
.Fc
.Ft long
.Fo BIO_get_ssl
.Fa "BIO *b"
.Fa "SSL *sslp"
.Fc
.Ft long
.Fo BIO_set_ssl_mode
.Fa "BIO *b"
.Fa "long client"
.Fc
.Ft long
.Fo BIO_set_ssl_renegotiate_bytes
.Fa "BIO *b"
.Fa "long num"
.Fc
.Ft long
.Fo BIO_set_ssl_renegotiate_timeout
.Fa "BIO *b"
.Fa "long seconds"
.Fc
.Ft long
.Fo BIO_get_num_renegotiates
.Fa "BIO *b"
.Fc
.Ft BIO *
.Fn BIO_new_ssl "SSL_CTX *ctx" "int client"
.Ft BIO *
.Fn BIO_new_ssl_connect "SSL_CTX *ctx"
.Ft BIO *
.Fn BIO_new_buffer_ssl_connect "SSL_CTX *ctx"
.Ft int
.Fn BIO_ssl_copy_session_id "BIO *to" "BIO *from"
.Ft void
.Fn BIO_ssl_shutdown "BIO *bio"
.Ft long
.Fn BIO_do_handshake "BIO *b"
.Sh DESCRIPTION
.Fn BIO_f_ssl
returns the
.Vt SSL
.Vt BIO
method.
This is a filter
.Vt BIO
which is a wrapper around the OpenSSL
.Vt SSL
routines adding a
.Vt BIO
.Dq flavor
to SSL I/O.
.Pp
I/O performed on an
.Vt SSL
.Vt BIO
communicates using the SSL protocol with
the
.Vt SSL Ns 's
read and write
.Vt BIO Ns s .
If an SSL connection is not established then an attempt is made to establish
one on the first I/O call.
.Pp
If a
.Vt BIO
is appended to an
.Vt SSL
.Vt BIO
using
.Xr BIO_push 3 ,
it is automatically used as the
.Vt SSL
.Vt BIO Ns 's read and write
.Vt BIO Ns s .
.Pp
Calling
.Xr BIO_reset 3
on an
.Vt SSL
.Vt BIO
closes down any current SSL connection by calling
.Xr SSL_shutdown 3 .
.Xr BIO_reset 3
is then sent to the next
.Vt BIO
in the chain; this will typically disconnect the underlying transport.
The
.Vt SSL
.Vt BIO
is then reset to the initial accept or connect state.
.Pp
If the close flag is set when an
.Vt SSL
.Vt BIO
is freed then the internal
.Vt SSL
structure is also freed using
.Xr SSL_free 3 .
.Pp
.Fn BIO_set_ssl
sets the internal
.Vt SSL
pointer of
.Vt BIO
.Fa b
to
.Fa ssl
using
the close flag
.Fa c .
.Pp
.Fn BIO_get_ssl
retrieves the
.Vt SSL
pointer of
.Vt BIO
.Fa b ;
it can then be manipulated using the standard SSL library functions.
.Pp
.Fn BIO_set_ssl_mode
sets the
.Vt SSL
.Vt BIO
mode to
.Fa client .
If
.Fa client
is 1, client mode is set.
If
.Fa client
is 0, server mode is set.
.Pp
.Fn BIO_set_ssl_renegotiate_bytes
sets the renegotiate byte count to
.Fa num .
When set, after every
.Fa num
bytes of I/O (read and write) the SSL session is automatically renegotiated.
.Fa num
must be at least 512 bytes.
.Pp
.Fn BIO_set_ssl_renegotiate_timeout
sets the renegotiate timeout to
.Fa seconds .
When the renegotiate timeout elapses, the session is automatically renegotiated.
.Pp
.Fn BIO_get_num_renegotiates
returns the total number of session renegotiations due to I/O or timeout.
.Pp
.Fn BIO_new_ssl
allocates an
.Vt SSL
.Vt BIO
using
.Vt SSL_CTX
.Va ctx
and using client mode if
.Fa client
is nonzero.
.Pp
.Fn BIO_new_ssl_connect
creates a new
.Vt BIO
chain consisting of an
.Vt SSL
.Vt BIO
(using
.Fa ctx )
followed by a connect BIO.
.Pp
.Fn BIO_new_buffer_ssl_connect
creates a new
.Vt BIO
chain consisting of a buffering
.Vt BIO ,
an
.Vt SSL
.Vt BIO
(using
.Fa ctx )
and a connect
.Vt BIO .
.Pp
.Fn BIO_ssl_copy_session_id
copies an SSL session id between
.Vt BIO
chains
.Fa from
and
.Fa to .
It does this by locating the
.Vt SSL
.Vt BIO Ns s
in each chain and calling
.Xr SSL_copy_session_id 3
on the internal
.Vt SSL
pointer.
.Pp
.Fn BIO_ssl_shutdown
closes down an SSL connection on
.Vt BIO
chain
.Fa bio .
It does this by locating the
.Vt SSL
.Vt BIO
in the
chain and calling
.Xr SSL_shutdown 3
on its internal
.Vt SSL
pointer.
.Pp
.Fn BIO_do_handshake
attempts to complete an SSL handshake on the supplied
.Vt 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
.Xr BIO_should_retry 3
should be used for non blocking connect
.Vt BIO Ns s
to determine if the call should be retried.
If an SSL connection has already been established, this call has no effect.
.Pp
When a chain containing an SSL BIO is copied with
.Xr BIO_dup_chain 3 ,
.Xr SSL_dup 3
is called internally to copy the
.Vt SSL
object from the existing BIO object to the new BIO object,
and the internal data related to
.Fn BIO_set_ssl_renegotiate_bytes
and
.Fn BIO_set_ssl_renegotiate_timeout
is also copied.
.Pp
.Vt SSL
.Vt BIO Ns s
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
.Xr BIO_read 3
operation.
One case where this happens is when step up occurs.
.Pp
In OpenSSL 0.9.6 and later the SSL flag
.Dv SSL_AUTO_RETRY
can be set to disable this behaviour.
In other words, when this flag is set an
.Vt SSL
.Vt BIO
using a blocking transport will never request a retry.
.Pp
Since unknown
.Xr BIO_ctrl 3
operations are sent through filter
.Vt BIO Ns s ,
the server name and port can be set using
.Xr BIO_set_conn_hostname 3
and
.Xr BIO_set_conn_port 3
on the
.Vt BIO
returned by
.Fn BIO_new_ssl_connect
without having to locate the connect
.Vt BIO
first.
.Pp
Applications do not have to call
.Fn BIO_do_handshake
but may wish to do so to separate the handshake process from other I/O
processing.
.Pp
.Fn BIO_set_ssl ,
.Fn BIO_get_ssl ,
.Fn BIO_set_ssl_mode ,
.Fn BIO_set_ssl_renegotiate_bytes ,
.Fn BIO_set_ssl_renegotiate_timeout ,
.Fn BIO_get_num_renegotiates ,
and
.Fn BIO_do_handshake
are implemented as macros.
.Sh RETURN VALUES
.Fn BIO_f_ssl
returns a pointer to a static
.Vt BIO_METHOD
structure.
.Pp
.Fn BIO_set_ssl ,
.Fn BIO_get_ssl ,
.Fn BIO_set_ssl_mode ,
.Fn BIO_set_ssl_renegotiate_bytes ,
.Fn BIO_set_ssl_renegotiate_timeout ,
and
.Fn BIO_get_num_renegotiates
return 1 on success or a value less than or equal to 0
if an error occurred.
.Pp
.Fn BIO_new_ssl ,
.Fn BIO_new_ssl_connect ,
and
.Fn BIO_new_buffer_ssl_connect
returns a pointer to a newly allocated
.Vt BIO
chain or
.Dv NULL
if an error occurred.
.Pp
.Fn BIO_ssl_copy_session_id
returns 1 on success or 0 on error.
.Pp
.Fn BIO_do_handshake
returns 1 if the connection was established successfully
or a value less than or equal to 0 otherwise.
.Sh EXAMPLES
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
.Xr BIO_s_connect 3 .
.Bd -literal
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\en");
	/* 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\en");
	ERR_print_errors_fp(stderr);
	/* whatever ... */
}

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

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

BIO_puts(sbio, "GET / HTTP/1.0\en\en");
for (;;) {
	len = BIO_read(sbio, tmpbuf, 1024);
	if(len <= 0) break;
	BIO_write(out, tmpbuf, len);
}
BIO_free_all(sbio);
BIO_free(out);
.Ed
.Pp
Here is a simple server example.
It makes use of a buffering
.Vt BIO
to allow lines to be read from the
.Vt SSL
.Vt BIO
using
.Xr BIO_gets 3 .
It creates a pseudo web page containing the actual request from a client and
also echoes the request to standard output.
.Bd -literal
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\en");
	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\en");
	/* 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);

/* Wait for incoming connection */
if (BIO_do_accept(acpt) <= 0) {
	fprintf(stderr, "Error setting up accept BIO\en");
	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\en");
	ERR_print_errors_fp(stderr);
	return 0;
}

BIO_puts(sbio, "HTTP/1.0 200 OK\er\enContent-type: text/plain\er\en\er\en");
BIO_puts(sbio, "\er\enConnection Established\er\enRequest headers:\er\en");
BIO_puts(sbio, "--------------------------------------------------\er\en");

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] == '\er') || (tmpbuf[0] == '\en'))
		break;
}

BIO_puts(sbio, "--------------------------------------------------\er\en");
BIO_puts(sbio, "\er\en");

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

BIO_free_all(sbio);
.Ed
.Sh SEE ALSO
.Xr BIO_new 3 ,
.Xr ssl 3
.Sh HISTORY
.Fn BIO_f_ssl ,
.Fn BIO_set_ssl ,
and
.Fn BIO_get_ssl
first appeared in SSLeay 0.6.0.
.Fn BIO_set_ssl_mode ,
.Fn BIO_new_ssl ,
and
.Fn BIO_ssl_copy_session_id
first appeared in SSLeay 0.8.0.
.Fn BIO_ssl_shutdown
and
.Fn BIO_do_handshake
first appeared in SSLeay 0.8.1.
.Fn BIO_set_ssl_renegotiate_bytes ,
.Fn BIO_get_num_renegotiates ,
.Fn BIO_set_ssl_renegotiate_timeout ,
.Fn BIO_new_ssl_connect ,
and
.Fn BIO_new_buffer_ssl_connect
first appeared in SSLeay 0.9.0.
All these functions have been available since
.Ox 2.4 .
.Pp
In OpenSSL versions before 1.0.0 the
.Xr BIO_pop 3
call was handled incorrectly:
the I/O BIO reference count was incorrectly incremented (instead of
decremented) and dissociated with the
.Vt SSL
.Vt BIO
even if the
.Vt SSL
.Vt 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
.Vt BIO .