130 lines
5.1 KiB
Plaintext
130 lines
5.1 KiB
Plaintext
=pod
|
|
|
|
=head1 NAME
|
|
|
|
SSL_shutdown - shut down a TLS/SSL connection
|
|
|
|
=head1 SYNOPSIS
|
|
|
|
#include <openssl/ssl.h>
|
|
|
|
int SSL_shutdown(SSL *ssl);
|
|
|
|
=head1 DESCRIPTION
|
|
|
|
SSL_shutdown() shuts down an active TLS/SSL connection. It sends the
|
|
"close notify" shutdown alert to the peer.
|
|
|
|
=head1 NOTES
|
|
|
|
SSL_shutdown() 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.
|
|
|
|
Note that SSL_shutdown() must not be called if a previous fatal error has
|
|
occurred on a connection i.e. if SSL_get_error() has returned SSL_ERROR_SYSCALL
|
|
or SSL_ERROR_SSL.
|
|
|
|
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.
|
|
|
|
SSL_shutdown() supports both uni- and bidirectional shutdown by its 2 step
|
|
behaviour.
|
|
|
|
=over 4
|
|
|
|
=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.
|
|
|
|
=item If the peer already sent the "close notify" alert B<and> it was
|
|
already processed implicitly inside another function
|
|
(L<SSL_read(3)|SSL_read(3)>), 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 L<SSL_set_shutdown(3)|SSL_set_shutdown(3)> call.
|
|
|
|
=back
|
|
|
|
It is therefore recommended, to check the return value of SSL_shutdown()
|
|
and call SSL_shutdown() 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, SSL_shutdown() will succeed on
|
|
the first call.
|
|
|
|
The behaviour of SSL_shutdown() additionally depends on the underlying BIO.
|
|
|
|
If the underlying BIO is B<blocking>, SSL_shutdown() will only return once the
|
|
handshake step has been finished or an error occurred.
|
|
|
|
If the underlying BIO is B<non-blocking>, SSL_shutdown() will also return
|
|
when the underlying BIO could not satisfy the needs of SSL_shutdown()
|
|
to continue the handshake. In this case a call to SSL_get_error() with the
|
|
return value of SSL_shutdown() will yield B<SSL_ERROR_WANT_READ> or
|
|
B<SSL_ERROR_WANT_WRITE>. The calling process then must repeat the call after
|
|
taking appropriate action to satisfy the needs of SSL_shutdown().
|
|
The action depends on the underlying BIO. When using a non-blocking socket,
|
|
nothing is to be done, but select() 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.
|
|
|
|
SSL_shutdown() can be modified to only set the connection to "shutdown"
|
|
state but not actually send the "close notify" alert messages,
|
|
see L<SSL_CTX_set_quiet_shutdown(3)|SSL_CTX_set_quiet_shutdown(3)>.
|
|
When "quiet shutdown" is enabled, SSL_shutdown() will always succeed
|
|
and return 1.
|
|
|
|
=head1 RETURN VALUES
|
|
|
|
The following return values can occur:
|
|
|
|
=over 4
|
|
|
|
=item Z<>0
|
|
|
|
The shutdown is not yet finished. Call SSL_shutdown() for a second time,
|
|
if a bidirectional shutdown shall be performed.
|
|
The output of L<SSL_get_error(3)|SSL_get_error(3)> may be misleading, as an
|
|
erroneous SSL_ERROR_SYSCALL may be flagged even though no error occurred.
|
|
|
|
=item Z<>1
|
|
|
|
The shutdown was successfully completed. The "close notify" alert was sent
|
|
and the peer's "close notify" alert was received.
|
|
|
|
=item E<lt>0
|
|
|
|
The shutdown was not successful because a fatal error occurred either
|
|
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 L<SSL_get_error(3)|SSL_get_error(3)> with the return value B<ret>
|
|
to find out the reason.
|
|
|
|
=back
|
|
|
|
=head1 SEE ALSO
|
|
|
|
L<SSL_get_error(3)|SSL_get_error(3)>, L<SSL_connect(3)|SSL_connect(3)>,
|
|
L<SSL_accept(3)|SSL_accept(3)>, L<SSL_set_shutdown(3)|SSL_set_shutdown(3)>,
|
|
L<SSL_CTX_set_quiet_shutdown(3)|SSL_CTX_set_quiet_shutdown(3)>,
|
|
L<SSL_clear(3)|SSL_clear(3)>, L<SSL_free(3)|SSL_free(3)>,
|
|
L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)>
|
|
|
|
=cut
|