SSL_shutdown 3 2005-03-25 0.9.9-dev OpenSSL
NAME
SSL_shutdown - shut down a TLS/SSL connection
LIBRARY
libcrypto, -lcrypto
SYNOPSIS
#include
int SSL_shutdown(SSL *ssl);
DESCRIPTION
_S_S_L___s_h_u_t_d_o_w_n_(_) shuts down an active TLS/SSL connection. It sends the
"close notify" shutdown alert to the peer.
NOTES
_S_S_L___s_h_u_t_d_o_w_n_(_) tries to send the "close notify" shutdown alert to the peer.
Whether the operation succeeds or not, the SSL_SENT_SHUTDOWN flag is set and
a currently open session is considered closed and good and will be kept in the
session cache for further reuse.
The shutdown procedure consists of 2 steps: the sending of the "close notify"
shutdown alert and the reception of the peer's "close notify" shutdown
alert. According to the TLS standard, it is acceptable for an application
to only send its shutdown alert and then close the underlying connection
without waiting for the peer's response (this way resources can be saved,
as the process can already terminate or serve another connection).
When the underlying connection shall be used for more communications, the
complete shutdown procedure (bidirectional "close notify" alerts) must be
performed, so that the peers stay synchronized.
_S_S_L___s_h_u_t_d_o_w_n_(_) supports both uni- and bidirectional shutdown by its 2 step
behaviour.
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When the application is the first party to send the "close notify" alert, _S_S_L___s_h_u_t_d_o_w_n_(_) will only send the alert and then set the SSL_SENT_SHUTDOWN flag (so that the session is considered good and will be kept in cache). _S_S_L___s_h_u_t_d_o_w_n_(_) will then return with 0. If a unidirectional shutdown is enough (the underlying connection shall be closed anyway), this first call to _S_S_L___s_h_u_t_d_o_w_n_(_) is sufficient. In order to complete the bidirectional shutdown handshake, _S_S_L___s_h_u_t_d_o_w_n_(_) must be called again. The second call will make _S_S_L___s_h_u_t_d_o_w_n_(_) wait for the peer's "close notify" shutdown alert. On success, the second call to _S_S_L___s_h_u_t_d_o_w_n_(_) will return with 1.
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If the peer already sent the "close notify" alert aanndd it was already processed implicitly inside another function (_S_S_L___r_e_a_d(3)), the SSL_RECEIVED_SHUTDOWN flag is set. _S_S_L___s_h_u_t_d_o_w_n_(_) will send the "close notify" alert, set the SSL_SENT_SHUTDOWN flag and will immediately return with 1. Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the _S_S_L___g_e_t___s_h_u_t_d_o_w_n_(_) (see also _S_S_L___s_e_t___s_h_u_t_d_o_w_n(3) call.
It is therefore recommended, to check the return value of _S_S_L___s_h_u_t_d_o_w_n_(_)
and call _S_S_L___s_h_u_t_d_o_w_n_(_) again, if the bidirectional shutdown is not yet
complete (return value of the first call is 0). As the shutdown is not
specially handled in the SSLv2 protocol, _S_S_L___s_h_u_t_d_o_w_n_(_) will succeed on
the first call.
The behaviour of _S_S_L___s_h_u_t_d_o_w_n_(_) additionally depends on the underlying BIO.
If the underlying BIO is bblloocckkiinngg, _S_S_L___s_h_u_t_d_o_w_n_(_) will only return once the
handshake step has been finished or an error occurred.
If the underlying BIO is nnoonn--bblloocckkiinngg, _S_S_L___s_h_u_t_d_o_w_n_(_) will also return
when the underlying BIO could not satisfy the needs of _S_S_L___s_h_u_t_d_o_w_n_(_)
to continue the handshake. In this case a call to _S_S_L___g_e_t___e_r_r_o_r_(_) with the
return value of _S_S_L___s_h_u_t_d_o_w_n_(_) will yield SSSSLL__EERRRROORR__WWAANNTT__RREEAADD or
SSSSLL__EERRRROORR__WWAANNTT__WWRRIITTEE. The calling process then must repeat the call after
taking appropriate action to satisfy the needs of _S_S_L___s_h_u_t_d_o_w_n_(_).
The action depends on the underlying BIO. When using a non-blocking socket,
nothing is to be done, but _s_e_l_e_c_t_(_) can be used to check for the required
condition. When using a buffering BIO, like a BIO pair, data must be written
into or retrieved out of the BIO before being able to continue.
_S_S_L___s_h_u_t_d_o_w_n_(_) can be modified to only set the connection to "shutdown"
state but not actually send the "close notify" alert messages,
see _S_S_L___C_T_X___s_e_t___q_u_i_e_t___s_h_u_t_d_o_w_n(3).
When "quiet shutdown" is enabled, _S_S_L___s_h_u_t_d_o_w_n_(_) will always succeed
and return 1.
RETURN VALUES
The following return values can occur:
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1
The shutdown was successfully completed. The "close notify" alert was sent
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and the peer's "close notify" alert was received.
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0
The shutdown is not yet finished. Call _S_S_L___s_h_u_t_d_o_w_n_(_) for a second time,
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if a bidirectional shutdown shall be performed.
The output of _S_S_L___g_e_t___e_r_r_o_r(3) may be misleading, as an
erroneous SSL_ERROR_SYSCALL may be flagged even though no error occurred.
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-1
The shutdown was not successful because a fatal error occurred either
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at the protocol level or a connection failure occurred. It can also occur if
action is need to continue the operation for non-blocking BIOs.
Call _S_S_L___g_e_t___e_r_r_o_r(3) with the return value rreett
to find out the reason.
SEE ALSO
_S_S_L___g_e_t___e_r_r_o_r(3), _S_S_L___c_o_n_n_e_c_t(3),
_S_S_L___a_c_c_e_p_t(3), _S_S_L___s_e_t___s_h_u_t_d_o_w_n(3),
_S_S_L___C_T_X___s_e_t___q_u_i_e_t___s_h_u_t_d_o_w_n(3),
_S_S_L___c_l_e_a_r(3), _S_S_L___f_r_e_e(3),
_s_s_l(3), _o_p_e_n_s_s_l___b_i_o(3)