SSL_SHUTDOWN(3ossl) OpenSSL SSL_SHUTDOWN(3ossl)
NAME
SSL_shutdown, SSL_shutdown_ex - shut down a TLS/SSL or QUIC connection
SYNOPSIS
#include <openssl/ssl.h>
int SSL_shutdown(SSL *ssl);
typedef struct ssl_shutdown_ex_args_st {
uint64_t quic_error_code;
const char *quic_reason;
} SSL_SHUTDOWN_EX_ARGS;
__owur int SSL_shutdown_ex(SSL *ssl, uint64_t flags,
const SSL_SHUTDOWN_EX_ARGS *args,
size_t args_len);
DESCRIPTION
SSL_shutdown(3) shuts down an active connection represented by an SSL
object.
SSL_shutdown(3). If
non-NULL, args must point to a SSL_SHUTDOWN_EX_ARGS structure and
args_len must be set to sizeof(SSL_SHUTDOWN_EX_ARGS). The
SSL_SHUTDOWN_EX_ARGS structure must be zero-initialized. If args is
NULL, the behaviour is the same as passing a zero-initialised
SSL_SHUTDOWN_EX_ARGS structure. Currently, all extended arguments
relate to usage with QUIC, therefore this call functions identically to
SSL_shutdown(3) when not being used with QUIC.
While the general operation of SSL_shutdown(3) is common between
protocols, the exact nature of how a shutdown is performed depends on
the underlying protocol being used. See the section below pertaining to
each protocol for more information.
In general, calling SSL_shutdown(3) in nonblocking mode will initiate
the shutdown process and return 0 to indicate that the shutdown process
has not yet completed. Once the shutdown process has completed,
subsequent calls to SSL_shutdown(3) will return 1. See the RETURN VALUES
section for more information.
SSL_shutdown(3) should not be called if a previous fatal error has
occurred on a connection; i.e., if SSL_get_error(3) has returned
SSL_ERROR_SYSCALL or SSL_ERROR_SSL.
TLS AND DTLS-SPECIFIC CONSIDERATIONS
Shutdown for SSL/TLS and DTLS is implemented in terms of the
SSL/TLS/DTLS close_notify alert message. The shutdown process for
SSL/TLS and DTLS consists of two steps:
o A close_notify shutdown alert message is sent to the peer.
o A close_notify shutdown alert message is received from the peer.
These steps can occur in either order depending on whether the
connection shutdown process was first initiated by the local
application or by the peer.
Locally-Initiated Shutdown
Calling SSL_shutdown(3) on a SSL/TLS or DTLS SSL object initiates the
shutdown process and causes OpenSSL to try to send a close_notify
shutdown alert to the peer. The shutdown process will then be
considered completed once the peer responds in turn with a close_notify
shutdown alert message.
Calling SSL_shutdown(3) only closes the write direction of the
connection; the read direction is closed by the peer. Once
SSL_shutdown(3) is called, SSL_write(3) can no longer be used, but
SSL_read(3) may still be used until the peer decides to close the
connection in turn. The peer might continue sending data for some
period of time before handling the local application's shutdown
indication.
SSL_shutdown(3) does not affect an underlying network connection such as
a TCP connection, which remains open.
Remotely-Initiated Shutdown
If the peer was the first to initiate the shutdown process by sending a
close_notify alert message, an application will be notified of this as
an EOF condition when calling SSL_read(3) (i.e., SSL_read(3) will fail
and SSL_get_error(3) will return SSL_ERROR_ZERO_RETURN), after all
application data sent by the peer prior to initiating the shutdown has
been read. An application should handle this condition by calling
SSL_shutdown(3) to respond with a close_notify alert in turn, completing
the shutdown process, though it may choose to write additional
application data using SSL_write(3) before doing so. If an application
does not call SSL_shutdown(3) in this case, a close_notify alert will
not be sent and the behaviour will not be fully standards compliant.
Shutdown Lifecycle
Regardless of whether a shutdown was initiated locally or by the peer,
if the underlying BIO is blocking, a call to SSL_shutdown(3) will return
firstly once a close_notify alert message is written to the peer
(returning 0), and upon a second and subsequent call, once a
corresponding message is received from the peer (returning 1 and
completing the shutdown process). Calls to SSL_shutdown(3) with a
blocking underlying BIO will also return if an error occurs.
If the underlying BIO is nonblocking and the shutdown process is not
yet complete (for example, because a close_notify alert message has not
yet been received from the peer, or because a close_notify alert
message needs to be sent but would currently block), SSL_shutdown(3)
returns 0 to indicate that the shutdown process is still ongoing; in
this case, a call to SSL_get_error(3) will yield SSL_ERROR_WANT_READ or
SSL_ERROR_WANT_WRITE.
An application can then detect completion of the shutdown process by
calling SSL_shutdown(3) again repeatedly until it returns 1, indicating
that the shutdown process is complete (with a close_notify alert having
both been sent and received).
However, the preferred method of waiting for the shutdown to complete
is to use SSL_read(3) until SSL_get_error(3) indicates EOF by returning
SSL_ERROR_ZERO_RETURN. This ensures any data received immediately
before the peer's close_notify alert is still provided to the
application. It also ensures any final handshake-layer messages
received are processed (for example, messages issuing new session
tickets).
If this approach is not used, the second call to SSL_shutdown(3) (to
complete the shutdown by confirming receipt of the peer's close_notify
message) will fail if it is called when the application has not read
all pending application data sent by the peer using SSL_read(3).
When calling SSL_shutdown(3), the SSL_SENT_SHUTDOWN flag is set once an
attempt is made to send a close_notify alert, regardless of whether the
attempt was successful. The SSL_RECEIVED_SHUTDOWN flag is set once a
close_notify alert is received, which may occur during any call which
processes incoming data from the network, such as SSL_read(3) or
SSL_shutdown(3). These flags may be checked using SSL_get_shutdown(3).
Fast Shutdown
Alternatively, it is acceptable for an application to call
SSL_shutdown(3) once (such that it returns 0) and then close the
underlying connection without waiting for the peer's response. This
allows for a more rapid shutdown process if the application does not
wish to wait for the peer.
This alternative "fast shutdown" approach should only be done if it is
known that the peer will not send more data, otherwise there is a risk
of an application exposing itself to a truncation attack. The full
SSL_shutdown(3) process, in which both parties send close_notify alerts
and SSL_shutdown(3) returns 1, provides a cryptographically
authenticated indication of the end of a connection.
This approach of a single SSL_shutdown(3) call without waiting is
preferable to simply calling SSL_free(3) or SSL_clear(3) as calling
SSL_shutdown(3) beforehand makes an SSL session eligible for subsequent
reuse and notifies the peer of connection shutdown.
The fast shutdown approach can only be used if there is no intention to
reuse the underlying connection (e.g. a TCP connection) for further
communication; in this case, the full shutdown process must be
performed to ensure synchronisation.
Effects on Session Reuse
Calling SSL_shutdown(3) sets the SSL_SENT_SHUTDOWN flag (see
SSL_set_shutdown(3)), regardless of whether the transmission of the
close_notify alert was successful or not. This makes the SSL session
eligible for reuse; the SSL session is considered properly closed and
can be reused for future connections.
Quiet Shutdown
SSL_shutdown(3) can be modified to set the connection to the "shutdown"
state without actually sending a close_notify alert message; see
SSL_CTX_set_quiet_shutdown(3). When "quiet shutdown" is enabled,
SSL_shutdown(3) will always succeed and return 1 immediately.
This is not standards-compliant behaviour. It should only be done when
the application protocol in use enables the peer to ensure that all
data has been received, such that it doesn't need to wait for a
close_notify alert, otherwise application data may be truncated
unexpectedly.
Non-Compliant Peers
There are SSL/TLS implementations that never send the required
close_notify alert message but simply close the underlying transport
(e.g. a TCP connection) instead. This will ordinarily result in an
error being generated.
If compatibility with such peers is desired, the option
SSL_OP_IGNORE_UNEXPECTED_EOF can be set. For more information, see
SSL_CTX_set_options(3).
Note that use of this option means that the EOF condition for
application data does not receive cryptographic protection, and
therefore renders an application potentially vulnerable to truncation
attacks. Thus, this option must only be used in conjunction with an
application protocol which indicates unambiguously when all data has
been received.
An alternative approach is to simply avoid calling SSL_read(3) if it is
known that no more data is going to be sent. This requires an
application protocol which indicates unambiguously when all data has
been sent.
Session Ticket Handling
If a client application only writes to a SSL/TLS or DTLS connection and
never reads, OpenSSL may never process new SSL/TLS session tickets sent
by the server. This is because OpenSSL ordinarily processes handshake
messages received from a peer during calls to SSL_read(3) by the
application.
Therefore, client applications which only write and do not read but
which wish to benefit from session resumption are advised to perform a
complete shutdown procedure by calling SSL_shutdown(3) until it returns
1, as described above. This will ensure there is an opportunity for
SSL/TLS session ticket messages to be received and processed by
OpenSSL.
QUIC-SPECIFIC SHUTDOWN CONSIDERATIONS
When used with a QUIC connection SSL object, SSL_shutdown(3) initiates a
QUIC immediate close using QUIC CONNECTION_CLOSE frames.
SSL_shutdown(3) cannot be used on QUIC stream SSL objects. To conclude a
stream normally, see SSL_stream_conclude(3); to perform a non-normal
stream termination, see SSL_stream_reset(3).
SSL_shutdown(3) by an
application to provide additional information to the peer on the reason
why a connection is being shut down. The information which can be
provided is as follows:
quic_error_code
An optional 62-bit application error code to be signalled to the
peer. The value must be in the range [0, 2**62-1], else the call to
SSL_shutdown_ex() fails. If not provided, an error code of 0 is
used by default.
quic_reason
An optional zero-terminated (UTF-8) reason string to be signalled
to the peer. The application is responsible for providing a valid
UTF-8 string and OpenSSL will not validate the string. If a reason
is not provided, or SSL_shutdown(3) is used, a zero-length string is
used as the reason. If provided, the reason string is copied and
stored inside the QUIC connection SSL object and need not remain
allocated after the call to SSL_shutdown_ex() returns. Reason
strings are bounded by the path MTU and may be silently truncated
if they are too long to fit in a QUIC packet.
Reason strings are intended for human diagnostic purposes only, and
should not be used for application signalling.
The arguments to SSL_shutdown_ex() are used only on the first call to
SSL_shutdown(3)) for a given QUIC connection SSL
object. These arguments are ignored on subsequent calls.
These functions do not affect an underlying network BIO or the resource
it represents; for example, a UDP datagram provided to a QUIC
connection as the network BIO will remain open.
Note that when using QUIC, an application must call SSL_shutdown(3) if
it wants to ensure that all transmitted data was received by the peer.
This is unlike a TLS/TCP connection, where reliable transmission of
buffered data is the responsibility of the operating system. If an
application calls SSL_free() on a QUIC connection SSL object or exits
before completing the shutdown process using SSL_shutdown(3), data which
was written by the application using SSL_write(), but could not yet be
transmitted, or which was sent but lost in the network, may not be
received by the peer.
When using QUIC, calling SSL_shutdown(3) allows internal network event
processing to be performed. It is important that this processing is
performed regularly, whether during connection usage or during
shutdown. If an application is not using thread assisted mode, an
application conducting shutdown should either ensure that
SSL_shutdown(3) is called regularly, or alternatively ensure that
SSL_handle_events() is called regularly. See openssl-quic(7) and
SSL_handle_events(3) for more information.
Application Data Drainage Behaviour
When using QUIC, SSL_shutdown(3) or SSL_shutdown_ex() ordinarily waits
until all data written to a stream by an application has been
acknowledged by the peer. In other words, the shutdown process waits
until all data written by the application has been sent to the peer,
and until the receipt of all such data is acknowledged by the peer.
Only once this process is completed is the shutdown considered
complete.
An exception to this is streams which terminated in a non-normal
fashion, for example due to a stream reset; only streams which are
non-terminated at the time SSL_shutdown(3) is called, or which
terminated in a normal fashion, have their pending send buffers flushed
in this manner.
This behaviour of flushing streams during the shutdown process can be
skipped by setting the SSL_SHUTDOWN_FLAG_NO_STREAM_FLUSH flag in a call
to SSL_shutdown_ex(); in this case, data remaining in stream send
buffers may not be transmitted to the peer. This flag may be used when
a non-normal application condition has occurred and the delivery of
data written to streams via SSL_write(3) is no longer relevant.
Shutdown Mode
Aspects of how QUIC handles connection closure must be taken into
account by applications. Ordinarily, QUIC expects a connection to
continue to be serviced for a substantial period of time after it is
nominally closed. This is necessary to ensure that any connection
closure notification sent to the peer was successfully received.
However, a consequence of this is that a fully RFC-compliant QUIC
connection closure process could take of the order of seconds. This may
be unsuitable for some applications, such as short-lived processes
which need to exit immediately after completing an application-layer
transaction.
As such, there are two shutdown modes available to users of QUIC
connection SSL objects:
RFC compliant shutdown mode
This is the default behaviour. The shutdown process may take a
period of time up to three times the current estimated RTT to the
peer. It is possible for the closure process to complete much
faster in some circumstances but this cannot be relied upon.
In blocking mode, the function will return once the closure process
is complete. In nonblocking mode, SSL_shutdown_ex() should be
called until it returns 1, indicating the closure process is
complete and the connection is now fully shut down.
Rapid shutdown mode
In this mode, the peer is notified of connection closure on a best
effort basis by sending a single QUIC packet. If that QUIC packet
is lost, the peer will not know that the connection has terminated
until the negotiated idle timeout (if any) expires.
This will generally return 0 on success, indicating that the
connection has not yet been fully shut down (unless it has already
done so, in which case it will return 1).
If SSL_SHUTDOWN_FLAG_RAPID is specified in flags, a rapid shutdown is
performed, otherwise an RFC-compliant shutdown is performed.
If an application calls SSL_shutdown_ex() with SSL_SHUTDOWN_FLAG_RAPID,
an application can subsequently change its mind about performing a
rapid shutdown by making a subsequent call to SSL_shutdown_ex() without
the flag set.
Peer-Initiated Shutdown
In some cases, an application may wish to wait for a shutdown initiated
by the peer rather than triggered locally. To do this, call
SSL_shutdown_ex() with SSL_SHUTDOWN_FLAG_WAIT_PEER specified in flags.
In blocking mode, this waits until the peer initiates a shutdown or the
connection otherwise becomes terminated for another reason. In
nonblocking mode it exits immediately with either success or failure
depending on whether a shutdown has occurred.
If a locally initiated shutdown has already been triggered or the
connection has started terminating for another reason, this flag has no
effect.
SSL_SHUTDOWN_FLAG_WAIT_PEER implies SSL_SHUTDOWN_FLAG_NO_STREAM_FLUSH,
as stream data cannot be flushed after a peer closes the connection.
Stream data may still be sent to the peer in any time spent waiting
before the peer closes the connection, though there is no guarantee of
this.
Nonblocking Mode
SSL_shutdown(3) and SSL_shutdown_ex() block if the connection is
configured in blocking mode. This may be overridden by specifying
SSL_SHUTDOWN_FLAG_NO_BLOCK in flags when calling SSL_shutdown_ex(),
which causes the call to operate as though in nonblocking mode.
RETURN VALUES
For both SSL_shutdown(3) and SSL_shutdown_ex() the following return
values can occur:
0 The shutdown process is ongoing and has not yet completed.
For TLS and DTLS, this means that a close_notify alert has been
sent but the peer has not yet replied in turn with its own
close_notify.
For QUIC connection SSL objects, a CONNECTION_CLOSE frame may have
been sent but the connection closure process has not yet completed.
Unlike most other functions, returning 0 does not indicate an
error. SSL_get_error(3) should not be called; it may misleadingly
indicate an error even though no error occurred.
1 The shutdown was successfully completed.
For TLS and DTLS, this means that a close_notify alert was sent and
the peer's close_notify alert was received.
For QUIC connection SSL objects, this means that the connection
closure process has completed.
<0 The shutdown was not successful. Call SSL_get_error(3) with the
return value ret to find out the reason. It can occur if an action
is needed to continue the operation for nonblocking BIOs.
It can also occur when not all data was read using SSL_read(), or
if called on a QUIC stream SSL object.
This value is also returned when called on QUIC stream SSL objects.
SEE ALSO
SSL_get_error(3), SSL_connect(3), SSL_accept(3), SSL_set_shutdown(3),
SSL_CTX_set_quiet_shutdown(3), SSL_CTX_set_options(3) SSL_clear(3),
SSL_free(3), ssl(7), bio(7)
HISTORY
The SSL_shutdown_ex() function was added in OpenSSL 3.2.
COPYRIGHT
Copyright 2000-2023 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file LICENSE in the source distribution or at
<https://www.openssl.org/source/license.html>.
3.3.2 2024-09-04 SSL_SHUTDOWN(3ossl)
openssl 3.3.2 - Generated Fri Sep 27 16:18:14 CDT 2024