235 lines
12 KiB
Groff
235 lines
12 KiB
Groff
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.\" ========================================================================
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.\"
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.IX Title "SSL_shutdown 3"
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.TH SSL_shutdown 3 "2013-02-11" "1.0.1e" "OpenSSL"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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SSL_shutdown \- shut down a TLS/SSL connection
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <openssl/ssl.h>
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\&
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\& int SSL_shutdown(SSL *ssl);
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.Ve
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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\&\fISSL_shutdown()\fR shuts down an active \s-1TLS/SSL\s0 connection. It sends the
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\&\*(L"close notify\*(R" shutdown alert to the peer.
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.SH "NOTES"
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.IX Header "NOTES"
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\&\fISSL_shutdown()\fR tries to send the \*(L"close notify\*(R" shutdown alert to the peer.
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Whether the operation succeeds or not, the \s-1SSL_SENT_SHUTDOWN\s0 flag is set and
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a currently open session is considered closed and good and will be kept in the
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session cache for further reuse.
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.PP
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The shutdown procedure consists of 2 steps: the sending of the \*(L"close notify\*(R"
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shutdown alert and the reception of the peer's \*(L"close notify\*(R" shutdown
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alert. According to the \s-1TLS\s0 standard, it is acceptable for an application
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to only send its shutdown alert and then close the underlying connection
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without waiting for the peer's response (this way resources can be saved,
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as the process can already terminate or serve another connection).
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When the underlying connection shall be used for more communications, the
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complete shutdown procedure (bidirectional \*(L"close notify\*(R" alerts) must be
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performed, so that the peers stay synchronized.
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.PP
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\&\fISSL_shutdown()\fR supports both uni\- and bidirectional shutdown by its 2 step
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behaviour.
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.ie n .IP "When the application is the first party to send the ""close notify"" 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 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. In order to complete the bidirectional shutdown handshake, \fISSL_shutdown()\fR must be called again. The second call will make \fISSL_shutdown()\fR wait for the peer's ""close notify"" shutdown alert. On success, the second call to \fISSL_shutdown()\fR will return with 1." 4
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.el .IP "When the application is the first party to send the ``close notify'' 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 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. In order to complete the bidirectional shutdown handshake, \fISSL_shutdown()\fR must be called again. The second call will make \fISSL_shutdown()\fR wait for the peer's ``close notify'' shutdown alert. On success, the second call to \fISSL_shutdown()\fR will return with 1." 4
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.IX Item "When the application is the first party to send the close notify alert, SSL_shutdown() 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). SSL_shutdown() will then return with 0. If a unidirectional shutdown is enough (the underlying connection shall be closed anyway), this first call to SSL_shutdown() is sufficient. In order to complete the bidirectional shutdown handshake, SSL_shutdown() must be called again. The second call will make SSL_shutdown() wait for the peer's close notify shutdown alert. On success, the second call to SSL_shutdown() will return with 1."
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.PD 0
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.ie n .IP "If the peer already sent the ""close notify"" 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_shutdown()\fR will send the ""close notify"" 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." 4
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.el .IP "If the peer already sent the ``close notify'' 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_shutdown()\fR will send the ``close notify'' 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." 4
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.IX Item "If the peer already sent the close notify alert and it was already processed implicitly inside another function (SSL_read), the SSL_RECEIVED_SHUTDOWN flag is set. SSL_shutdown() 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 SSL_get_shutdown() (see also SSL_set_shutdown call."
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.PD
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.PP
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It is therefore recommended, to check the return value of \fISSL_shutdown()\fR
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and call \fISSL_shutdown()\fR again, if the bidirectional shutdown is not yet
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complete (return value of the first call is 0). As the shutdown is not
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specially handled in the SSLv2 protocol, \fISSL_shutdown()\fR will succeed on
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the first call.
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.PP
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The behaviour of \fISSL_shutdown()\fR additionally depends on the underlying \s-1BIO\s0.
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.PP
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If the underlying \s-1BIO\s0 is \fBblocking\fR, \fISSL_shutdown()\fR will only return once the
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handshake step has been finished or an error occurred.
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.PP
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If the underlying \s-1BIO\s0 is \fBnon-blocking\fR, \fISSL_shutdown()\fR will also return
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when the underlying \s-1BIO\s0 could not satisfy the needs of \fISSL_shutdown()\fR
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to continue the handshake. In this case a call to \fISSL_get_error()\fR with the
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return value of \fISSL_shutdown()\fR will yield \fB\s-1SSL_ERROR_WANT_READ\s0\fR or
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\&\fB\s-1SSL_ERROR_WANT_WRITE\s0\fR. The calling process then must repeat the call after
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taking appropriate action to satisfy the needs of \fISSL_shutdown()\fR.
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The action depends on the underlying \s-1BIO\s0. When using a non-blocking socket,
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nothing is to be done, but \fIselect()\fR can be used to check for the required
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condition. When using a buffering \s-1BIO\s0, like a \s-1BIO\s0 pair, data must be written
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into or retrieved out of the \s-1BIO\s0 before being able to continue.
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.PP
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\&\fISSL_shutdown()\fR can be modified to only set the connection to \*(L"shutdown\*(R"
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state but not actually send the \*(L"close notify\*(R" alert messages,
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see \fISSL_CTX_set_quiet_shutdown\fR\|(3).
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When \*(L"quiet shutdown\*(R" is enabled, \fISSL_shutdown()\fR will always succeed
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and return 1.
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.SH "RETURN VALUES"
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.IX Header "RETURN VALUES"
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The following return values can occur:
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.IP "1." 4
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The shutdown was successfully completed. The \*(L"close notify\*(R" alert was sent
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and the peer's \*(L"close notify\*(R" alert was received.
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.IP "2." 4
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The shutdown is not yet finished. Call \fISSL_shutdown()\fR for a second time,
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if a bidirectional shutdown shall be performed.
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The output of \fISSL_get_error\fR\|(3) may be misleading, as an
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erroneous \s-1SSL_ERROR_SYSCALL\s0 may be flagged even though no error occurred.
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.IP "3." 4
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\&\-1
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.Sp
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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
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action is need to continue the operation for non-blocking BIOs.
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Call \fISSL_get_error\fR\|(3) with the return value \fBret\fR
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to find out the reason.
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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\&\fISSL_get_error\fR\|(3), \fISSL_connect\fR\|(3),
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\&\fISSL_accept\fR\|(3), \fISSL_set_shutdown\fR\|(3),
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\&\fISSL_CTX_set_quiet_shutdown\fR\|(3),
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\&\fISSL_clear\fR\|(3), \fISSL_free\fR\|(3),
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\&\fIssl\fR\|(3), \fIbio\fR\|(3)
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.SH "POD ERRORS"
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.IX Header "POD ERRORS"
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Hey! \fBThe above document had some coding errors, which are explained below:\fR
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.IP "Around line 100:" 4
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.IX Item "Around line 100:"
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You have '=item 0' instead of the expected '=item 2'
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.IP "Around line 107:" 4
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.IX Item "Around line 107:"
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Expected '=item 3'
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