BIO_F_SSL(3)

✨ BIO_F_SSL(3) — SSL BIO ✨

okay bestie, you're reading the BIO_F_SSL(3) page from libssl/libcrypto, and it's giving “SSL but make it BIO.” We got a whole squad of functions here, so let's get into it 💅

🌸 NAME 🌸

The whole squad is: 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, and BIO_do_handshake — all here for the SSL BIO. Periodt.

🦋 SYNOPSIS 🦋

library “libssl” libcrypto
#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() hands you the SSL BIO method. It’s a filter BIO — think of it as a little wrapper around the OpenSSL SSL routines that adds a BIO “flavor” to SSL I/O. Cute, right? 🎀

When you do I/O on an SSL BIO, it communicates using the SSL protocol with the SSL’s read and write BIOs. If the SSL connection is not established yet, it tries to establish one on the first I/O call. No cap.

If you append a BIO to an SSL BIO using BIO_push(3), that BIO 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). Then BIO_reset(3) is sent to the next BIO in the chain; this will typically disconnect the underlying transport. After that, the SSL BIO is 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).

Here’s the function-by-function tea ☕:

• 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; after that you can manipulate it 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.

🚨 bestie, yes, num must be at least 512 bytes. That part is fr fr important.

• 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 nonzero.
• 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(3) 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(3) 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(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.

When a chain containing an SSL BIO is copied with BIO_dup_chain(3), SSL_dup(3) is called internally to copy the SSL object from the existing BIO object to the new BIO object, and the internal data related to BIO_set_ssl_renegotiate_bytes() and BIO_set_ssl_renegotiate_timeout() is also copied.

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 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_conn_hostname(3) and BIO_set_conn_port(3) on the BIO returned by BIO_new_ssl_connect() without having to locate the connect BIO first. Lowkey convenient.

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.

And just so you know: 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. 💖

🌈 SEE ALSO 🌈

okay bestie, if you wanna go deeper (and you know you do 👀), go check these out:

• BIO_new(3) — the whole how-to-make-a-BIO situation ✨
• ssl(3) — the SSL of it all, obvs 🎀

---

💜 HISTORY 💜

ngl, this is the part where we stan the elders 🫶 — here's the whole timeline, and yes, every single version number matters, so don't skip:

• BIO_f_ssl(), BIO_set_ssl(), and BIO_get_ssl() — first showed up in SSLeay 0.6.0. OG behaviour 💅
• BIO_set_ssl_mode(), BIO_new_ssl(), and BIO_ssl_copy_session_id() — first appeared in SSLeay 0.8.0. glow-up era ✨
• BIO_ssl_shutdown() and BIO_do_handshake() — first appeared in SSLeay 0.8.1. iconic arrivals 🦋
• BIO_set_ssl_renegotiate_bytes(), BIO_get_num_renegotiates(), BIO_set_ssl_renegotiate_timeout(), BIO_new_ssl_connect(), and BIO_new_buffer_ssl_connect() — first appeared in SSLeay 0.9.0. the sequel that ate 🔥

and then, the moment you've been waiting for: all these functions have been available since OpenBSD 2.4. literally vintage, main character energy, no cap 👑

---

so yeah, bestie — that's the whole page in one vibe check. every flag, every function, every version, every era. you're basically a BIO_f_ssl expert now 🎓💖

🌸 mwah 💋 — this one shipped from Debian, dated June 8, 2025, and it's the BIO_F_SSL(3) page. go forth and slay your TLS, queen 🫶🎀