TLS_CONNECT(3) | Library Functions Manual | TLS_CONNECT(3)

# NAME

`tls_connect`, `tls_connect_fds`, `tls_connect_servername`, `tls_connect_socket`, `tls_connect_cbs` — instruct a TLS client to establish a connection

# SYNOPSIS

library “libtls” libssl libcrypto
`#include <tls.h>`

int
`tls_connect`(struct tls *ctx, const char *host, const char *port);

int
`tls_connect_fds`(struct tls *ctx, int fd_read, int fd_write, const char *servername);

int
`tls_connect_servername`(struct tls *ctx, const char *host, const char *port, const char *servername);

int
`tls_connect_socket`(struct tls *ctx, int s, const char *servername);

int
`tls_connect_cbs`(struct tls *ctx, ssize_t (*tls_read_cb)(struct tls *ctx, void *buf, size_t buflen, void *cb_arg), ssize_t (*tls_write_cb)(struct tls *ctx, const void *buf, size_t buflen, void *cb_arg), void *cb_arg, const char *servername);

# DESCRIPTION

After creating a TLS client context with tls_client(3) and configuring it with tls_configure(3), a client connection is initiated by calling `tls_connect`(). This function will create a new socket, connect to the specified host and port, and then establish a secure connection. The port may be numeric or a service name. If it is `NULL`, then a host of the format "hostname:port" is permitted. The name to use for verification is inferred from the host value.

The `tls_connect_servername`() function has the same behaviour, however the name to use for verification is explicitly provided, for the case where the TLS server name differs from the DNS name.

An already existing socket can be upgraded to a secure connection by calling `tls_connect_socket`().

Alternatively, a secure connection can be established over a pair of existing file descriptors by calling `tls_connect_fds`().

Calling `tls_connect_cbs`() allows read and write callback functions to handle data transfers. The specified cb_arg parameter is passed back to the functions, and can contain a pointer to any caller-specified data.

# RETURN VALUES

These functions return 0 on success or -1 on error.

# SEE ALSO

tls_accept_socket(3), tls_client(3), tls_close(3), tls_config_ocsp_require_stapling(3), tls_configure(3), tls_handshake(3), tls_init(3)

# HISTORY

`tls_connect`() and `tls_connect_socket`() appeared in OpenBSD 5.6 and got their final names in OpenBSD 5.7.

`tls_connect_fds`() and `tls_connect_servername`() appeared in OpenBSD 5.7 and `tls_connect_cbs`() in OpenBSD 6.1.

# AUTHORS

Joel Sing <jsing@openbsd.org>
Reyk Floeter <reyk@openbsd.org>

`tls_connect_cbs`() was written by Tobias Pape <tobias@netshed.de>.

July 7, 2025 | Debian