SSL_SHUTDOWN(3) BSD Library Functions Manual SSL_SHUTDOWN(3)NAMESSL_shutdown — shut down a TLS/SSL connection
SYNOPSIS
#include <openssl/ssl.h>
int
SSL_shutdown(SSL *ssl);
DESCRIPTIONSSL_shutdown() shuts down an active TLS/SSL connection. It sends the
“close notify” shutdown alert to the peer.
NOTESSSL_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.
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 con‐
nection). When the underlying connection shall be used for more communi‐
cations, 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 behavior.
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 0. If a unidirec‐
tional shutdown is enough (the underlying connection shall be closed any‐
way), this first call to SSL_shutdown() is sufficient. In order to com‐
plete 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 1.
If the peer already sent the “close notify” alert and it was already pro‐
cessed implicitly inside another function (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 the SSL_set_shutdown(3) call).
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 blocking, SSL_shutdown() will only return once
the handshake step has been finished or an error occurred.
If the underlying BIO is 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(3) with the
return value of SSL_shutdown() will yield SSL_ERROR_WANT_READ or
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(2) 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
SSL_CTX_set_quiet_shutdown(3). When “quiet shutdown” is enabled,
SSL_shutdown() will always succeed and return 1.
RETURN VALUES
The following return values can occur:
0 The shutdown is not yet finished. Call SSL_shutdown() for a sec‐
ond time, if a bidirectional shutdown shall be performed. The
output of SSL_get_error(3) may be misleading, as an erroneous
SSL_ERROR_SYSCALL may be flagged even though no error occurred.
1 The shutdown was successfully completed. The “close notify”
alert was sent and the peer's “close notify” alert was received.
−1 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 SSL_get_error(3) with the return value
ret to find out the reason.
SEE ALSObio(3), ssl(3), SSL_accept(3), SSL_clear(3), SSL_connect(3),
SSL_CTX_set_quiet_shutdown(3), SSL_free(3), SSL_get_error(3),
SSL_set_shutdown(3)BSD April 24, 2024 BSD
