SSL_SHUTDOWN(3) | Library Functions Manual | SSL_SHUTDOWN(3)

# NAME

`SSL_shutdown` — shut down a TLS/SSL connection

# SYNOPSIS

library “libssl” libcrypto
`#include <openssl/ssl.h>`

int
`SSL_shutdown`(SSL *ssl);

# DESCRIPTION

`SSL_shutdown`() shuts down an active TLS/SSL connection. It sends the “close notify” shutdown alert to the peer.

`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.

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 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 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 1.

If the peer already sent the “close notify” alert and it was already processed 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).

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 second 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 ALSO

BIO_new(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)

# HISTORY

`SSL_shutdown`() first appeared in SSLeay 0.8.0 and has been available since OpenBSD 2.4.

June 8, 2025 | Debian