gnutls\_transport\_set\_fastopen(3) gnutls gnutls\_transport\_set\_fastopen(3)

# [NAME](#NAME)

gnutls\_transport\_set\_fastopen - API function

# [SYNOPSIS](#SYNOPSIS)

**#include <gnutls/socket.h>**

**void gnutls\_transport\_set\_fastopen(gnutls\_session\_t session, int fd, struct sockaddr * connect\_addr, socklen\_t connect\_addrlen, unsigned int flags);**

# [ARGUMENTS](#ARGUMENTS)

[gnutls\_session\_t session](#gnutls_session_t)

is a **gnutls\_session\_t** type.

[int fd](#int)

is the session's socket descriptor

[struct sockaddr * connect\_addr](#struct)

is the address we want to connect to

[socklen\_t connect\_addrlen](#socklen_t)

is the length of *connect\_addr*

[unsigned int flags](#unsigned)

must be zero

# [DESCRIPTION](#DESCRIPTION)

Enables TCP Fast Open (TFO) for the specified TLS client session. That means that TCP connection establishment and the transmission of the first TLS client hello packet are combined. The peer's address must be specified in *connect\_addr* and *connect\_addrlen* , and the socket specified by *fd* should not be connected.

TFO only works for TCP sockets of type AF\_INET and AF\_INET6. If the OS doesn't support TCP fast open this function will result to gnutls using **connect()** transparently during the first write.

# [NOTE](#NOTE)

This function overrides all the transport callback functions. If this is undesirable, TCP Fast Open must be implemented on the user callback functions without calling this function. When using this function, transport callbacks must not be set, and **gnutls\_transport\_set\_ptr()** or **gnutls\_transport\_set\_int()** must not be called.

On GNU/Linux TFO has to be enabled at the system layer, that is in /proc/sys/net/ipv4/tcp\_fastopen, bit 0 has to be set.

This function has no effect on server sessions.

# [SINCE](#SINCE)

3.5.3

# [REPORTING BUGS](#REPORTING_BUGS)

Report bugs to <bugs@gnutls.org>.  
Home page: https://www.gnutls.org

# [COPYRIGHT](#COPYRIGHT)

Copyright © 2001-2023 Free Software Foundation, Inc., and others.  
Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved.

# [SEE ALSO](#SEE_ALSO)

The full documentation for **gnutls** is maintained as a Texinfo manual. If the /usr/share/doc/gnutls/ directory does not contain the HTML form visit

[https://www.gnutls.org/manual/](https://www.gnutls.org/manual/)

3.8.13 gnutls

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Document: gnutls_transport_set_fastopen(3) from gnutls 3.8.13-r3 (Chainguard OS)
Source: https://gitlab.com/gnutls/gnutls