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.\" ========================================================================
.\"
.IX Title "SSL_SHUTDOWN 3"
.TH SSL_SHUTDOWN 3 "2018-09-11" "1.1.1" "OpenSSL"
.\" For nroff, turn off justification. Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
SSL_shutdown \- shut down a TLS/SSL connection
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/ssl.h>
\&
\& int SSL_shutdown(SSL *ssl);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fISSL_shutdown()\fR shuts down an active \s-1TLS/SSL\s0 connection. It sends the
\&\*(L"close notify\*(R" shutdown alert to the peer.
.SH "NOTES"
.IX Header "NOTES"
\&\fISSL_shutdown()\fR tries to send the \*(L"close notify\*(R" shutdown alert to the peer.
Whether the operation succeeds or not, the \s-1SSL_SENT_SHUTDOWN\s0 flag is set and
a currently open session is considered closed and good and will be kept in the
session cache for further reuse.
.PP
The shutdown procedure consists of 2 steps: the sending of the \*(L"close notify\*(R"
shutdown alert and the reception of the peer's \*(L"close notify\*(R" shutdown
alert. According to the \s-1TLS\s0 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 \*(L"close notify\*(R" alerts) must be
performed, so that the peers stay synchronized.
.PP
\&\fISSL_shutdown()\fR supports both uni\- and bidirectional shutdown by its 2 step
behaviour.
.PP
\&\fISSL_shutdown()\fR only closes the write direction.
It is not possible to call \fISSL_write()\fR after calling \fISSL_shutdown()\fR.
The read direction is closed by the peer.
.SS "First to close the connection"
.IX Subsection "First to close the connection"
When the application is the first party to send the \*(L"close notify\*(R"
alert, \fISSL_shutdown()\fR will only send the alert and then set the
\&\s-1SSL_SENT_SHUTDOWN\s0 flag (so that the session is considered good and will
be kept in the cache).
\&\fISSL_shutdown()\fR will then return with 0.
If a unidirectional shutdown is enough (the underlying connection shall be
closed anyway), this first call to \fISSL_shutdown()\fR is sufficient.
.PP
In order to complete the bidirectional shutdown handshake, the peer needs
to send back a \*(L"close notify\*(R" alert.
The \s-1SSL_RECEIVED_SHUTDOWN\s0 flag will be set after receiving and processing
it.
\&\fISSL_shutdown()\fR will return 1 when it has been received.
.PP
The peer is still allowed to send data after receiving the \*(L"close notify\*(R"
event.
If the peer did send data it needs to be processed by calling \fISSL_read()\fR
before calling \fISSL_shutdown()\fR a second time.
\&\fISSL_read()\fR will indicate the end of the peer data by returning <= 0
and \fISSL_get_error()\fR returning \s-1SSL_ERROR_ZERO_RETURN.\s0
It is recommended to call \fISSL_read()\fR between \fISSL_shutdown()\fR calls.
.SS "Peer closes the connection"
.IX Subsection "Peer closes the connection"
If the peer already sent the \*(L"close notify\*(R" alert \fBand\fR it was
already processed implicitly inside another function
(\fISSL_read\fR\|(3)), the \s-1SSL_RECEIVED_SHUTDOWN\s0 flag is set.
\&\fISSL_read()\fR will return <= 0 in that case, and \fISSL_get_error()\fR will return
\&\s-1SSL_ERROR_ZERO_RETURN.\s0
\&\fISSL_shutdown()\fR will send the \*(L"close notify\*(R" alert, set the \s-1SSL_SENT_SHUTDOWN\s0
flag and will immediately return with 1.
Whether \s-1SSL_RECEIVED_SHUTDOWN\s0 is already set can be checked using the
\&\fISSL_get_shutdown()\fR (see also \fISSL_set_shutdown\fR\|(3) call.
.SH "NOTES"
.IX Header "NOTES"
It is recommended to do a bidirectional shutdown by checking the return value
of \fISSL_shutdown()\fR and call it again until it returns 1 or a fatal error.
.PP
The behaviour of \fISSL_shutdown()\fR additionally depends on the underlying \s-1BIO.\s0
If the underlying \s-1BIO\s0 is \fBblocking\fR, \fISSL_shutdown()\fR will only return once the
handshake step has been finished or an error occurred.
.PP
If the underlying \s-1BIO\s0 is \fBnon-blocking\fR, \fISSL_shutdown()\fR will also return
when the underlying \s-1BIO\s0 could not satisfy the needs of \fISSL_shutdown()\fR
to continue the handshake. In this case a call to \fISSL_get_error()\fR with the
return value of \fISSL_shutdown()\fR will yield \fB\s-1SSL_ERROR_WANT_READ\s0\fR or
\&\fB\s-1SSL_ERROR_WANT_WRITE\s0\fR. The calling process then must repeat the call after
taking appropriate action to satisfy the needs of \fISSL_shutdown()\fR.
The action depends on the underlying \s-1BIO.\s0 When using a non-blocking socket,
nothing is to be done, but \fIselect()\fR can be used to check for the required
condition. When using a buffering \s-1BIO,\s0 like a \s-1BIO\s0 pair, data must be written
into or retrieved out of the \s-1BIO\s0 before being able to continue.
.PP
\&\fISSL_shutdown()\fR can be modified to only set the connection to \*(L"shutdown\*(R"
state but not actually send the \*(L"close notify\*(R" alert messages,
see \fISSL_CTX_set_quiet_shutdown\fR\|(3).
When \*(L"quiet shutdown\*(R" is enabled, \fISSL_shutdown()\fR will always succeed
and return 1.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
The following return values can occur:
.IP "0" 4
The shutdown is not yet finished: the \*(L"close notify\*(R" was send but the peer
did not send it back yet.
Call \fISSL_shutdown()\fR again to do a bidirectional shutdown.
The output of \fISSL_get_error\fR\|(3) may be misleading, as an
erroneous \s-1SSL_ERROR_SYSCALL\s0 may be flagged even though no error occurred.
.IP "1" 4
.IX Item "1"
The shutdown was successfully completed. The \*(L"close notify\*(R" alert was sent
and the peer's \*(L"close notify\*(R" alert was received.
.IP "<0" 4
.IX Item "<0"
The shutdown was not successful.
Call \fISSL_get_error\fR\|(3) with the return value \fBret\fR to find out the reason.
It can occur if an action is needed to continue the operation for non-blocking
BIOs.
.Sp
It can also occur when not all data was read using \fISSL_read()\fR.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fISSL_get_error\fR\|(3), \fISSL_connect\fR\|(3),
\&\fISSL_accept\fR\|(3), \fISSL_set_shutdown\fR\|(3),
\&\fISSL_CTX_set_quiet_shutdown\fR\|(3),
\&\fISSL_clear\fR\|(3), \fISSL_free\fR\|(3),
\&\fIssl\fR\|(7), \fIbio\fR\|(7)
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2000\-2018 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (the \*(L"License\*(R"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.
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