BIO_F_SSL(3ossl)

📦
version 3.5.9 · 8 versions
, in fedora 43, fedora 44

💖 BIO_F_SSL(3ossl) 💖

okay bestie, gather round, because this is the SSL BIO page from OpenSSL and it is lowkey a whole vibe. it's giving "I wrap the entire SSL stack in a BIO so you can treat TLS like a file descriptor" and honestly? iconic. 🎀

✨ NAME ✨

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 — the whole SSL BIO friend group 💅

🌸 SYNOPSIS 🌸

#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);

🦋 DESCRIPTION 🦋

BIO_f_ssl() returns the SSL BIO method. it's a filter BIO, meaning it's a little wrapper round the OpenSSL SSL routines that adds a BIO "flavour" to SSL I/O. bestie it's literally a glow-up filter but for sockets. ✨

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 yet, it tries to establish one on the first I/O call. no pressure, it just does it. 🫶

if a BIO is appended to an SSL BIO using BIO_push() it automatically becomes the SSL BIO's read and write BIOs. automatic. effortless. main character energy. 👑

calling BIO_reset() on an SSL BIO closes down any current SSL connection by calling SSL_shutdown(). then BIO_reset() gets sent to the next BIO in the chain — which will typically disconnect the underlying transport. after that, the SSL BIO is reset to the initial accept or connect state. full factory reset, she's fresh again 💅

if the close flag is set when an SSL BIO is freed, then the internal SSL structure is also freed using SSL_free(). so like, watch that flag, it has opinions.

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

BIO_get_ssl() retrieves the SSL pointer of SSL BIO b — and then you can go manipulate it with the standard SSL library functions. get you a girl who can do both 💖

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. binary. no in-between. she's decisive. 🎀

BIO_set_ssl_renegotiate_bytes() sets the renegotiate byte count of SSL BIO b to num. once it's set, after every num bytes of I/O (read and write) the SSL session is automatically renegotiated. > num must be at least 512 bytes. that's the floor, don't test her. 🚧

BIO_set_ssl_renegotiate_timeout() sets the renegotiate timeout of SSL BIO b to seconds. when the renegotiate timeout elapses, the session is automatically renegotiated. clock's ticking bestie ⏰

BIO_get_num_renegotiates() returns the total number of session renegotiations due to I/O or timeout of SSL BIO b. it's the receipts. 🧾

BIO_new_ssl() allocates an SSL BIO using SSL_CTX ctx, and uses 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. chain? slay. 🔗

BIO_new_buffer_ssl_connect() creates a new BIO chain consisting of a buffering BIO, an SSL BIO (using ctx), and a connect BIO. three-stack tower, absolutely stacked. 🍓

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. matchy matchy 🫶

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. goodbye, say it back 😭

BIO_do_handshake() attempts to complete an SSL handshake on the supplied BIO and establish the SSL connection. for non-SSL BIOs the connection is done typically at TCP level. if domain name resolution yields multiple IP addresses, all of them are tried after connect() failures. the function 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 nonblocking connect BIOs to determine if the call should be retried. if a connection has already been established, this call has no effect. she said what she said. 💅

🔥 NOTES 🔥

SSL BIOs are exceptional (as they should be, honestly) 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. no takesies-backsies.

since unknown BIO_ctrl() operations are sent through filter BIOs, the server's 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. lazy girl win, we love efficiency 🧚

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. boundaries! healthy! 🌸

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 macros. surprise! they're not even real functions bestie 💀

BIO_ssl_copy_session_id() is not currently supported on QUIC SSL objects and fails if called on such an object. so don't try it, she will not be doing that today 🙈

⭐ RETURN VALUES ⭐

  • 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, or if called on a QUIC SSL object.
  • 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.

🌈 EXAMPLES 🌈

💜 a lil SSL/TLS client 💜

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). same energy, different encryption. ✨

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");
    ERR_print_errors_fp(stderr);
    exit(1);
}

/* 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");
    ERR_print_errors_fp(stderr);
    exit(1);
}

/* 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)
        break;
    BIO_write(out, tmpbuf, len);
}
BIO_free_all(sbio);
BIO_free(out);

🎀 a simple server 🎀

here's 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. multitasking queen 💅

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");
    ERR_print_errors_fp(stderr);
    exit(1);
}

/* 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");
    ERR_print_errors_fp(stderr);
    exit(1);
}

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);

/* First call to BIO_do_accept() sets up accept BIO */
if (BIO_do_accept(acpt) <= 0) {
    fprintf(stderr, "Error setting up accept BIO\n");
    ERR_print_errors_fp(stderr);
    exit(1);
}

/* Second call to BIO_do_accept() waits for incoming connection */
if (BIO_do_accept(acpt) <= 0) {
   fprintf(stderr, "Error accepting connection\n");
   ERR_print_errors_fp(stderr);
   exit(1);
}

/* 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);
    exit(1);
}

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");
BIO_flush(sbio);
BIO_free_all(sbio);

🧚 HISTORY 🧚

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). messy behaviour, we don't claim her 💀

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. > that's the dangerous kind of double-dipping, bestie, please update your code 🚨

okay bestie, last section, and it's the legal one — the ✨receipts✨, if you will. this is the part where the OpenSSL girlies say "we made this, please be normal about it." 🫶

Copyright 2000-2023 The OpenSSL Project Authors. All Rights Reserved.

periodt. that's the whole copyright line, characters and all — 2000 through 2023, all rights reserved, The OpenSSL Project Authors. no cap. 💅

🎀 the license tea

Licensed under the Apache License 2.0 (the "License"). so like, you can use this file — but only if you actually follow the License. it's giving "yes you may, but read the rules first, bestie."

You may not use this file except in compliance with the License.

translation: don't just vibe your way past the license terms. that's not main character energy, that's a lawsuit arc. 💀

🦋 where to find the actual license text

You can obtain a copy:

two options, both iconic. pick whichever one your vibe check prefers. 🌸

💜 the sign-off

and that's the whole page, bestie!! you made it to the bottom, which honestly is a slay in itself. this was BIO_F_SSL(3ossl), straight out of the OpenSSL source, version 3.5.9, dated 2026-09-29 — so if you're reading this in a future where the date looks unhinged, that's just the manpage being mysterious. ⭐

go forth, filter your BIOs, and stay compliant with the License fr fr. 🎀💖✨

📦 where this page comes from