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QUIC: Update SSL_shutdown manpage
Fixes https://github.com/openssl/project/issues/138 Reviewed-by: Tomas Mraz <tomas@openssl.org> Reviewed-by: Matt Caswell <matt@openssl.org> (Merged from https://github.com/openssl/openssl/pull/21618)
This commit is contained in:
Hugo Landau
parent
1875642c89
commit
51b2a670c8
@ -93,6 +93,14 @@ are buffered bytes available for immediate retrieval.
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In this case the read function can be called without blocking or actually
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receiving new data from the underlying socket.
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When used with a QUIC SSL object, calling an I/O function such as SSL_read()
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allows internal network event processing to be performed. It is important that
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this processing is performed regularly. If an application is not using thread
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assisted mode, an application should ensure that an I/O function such as
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SSL_read() is called regularly, or alternatively ensure that SSL_handle_events()
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is called regularly. See L<openssl-quic(7)> and L<SSL_handle_events(3)> for more
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information.
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=head1 RETURN VALUES
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SSL_read_ex() and SSL_peek_ex() will return 1 for success or 0 for failure.
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@ -21,155 +21,284 @@ SSL_shutdown, SSL_shutdown_ex - shut down a TLS/SSL or QUIC connection
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=head1 DESCRIPTION
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SSL_shutdown() shuts down an active TLS/SSL connection. It sends the
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close_notify shutdown alert to the peer.
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SSL_shutdown() tries to send the close_notify shutdown alert to the peer.
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Whether the operation succeeds or not, the SSL_SENT_SHUTDOWN 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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Note that SSL_shutdown() must not be called if a previous fatal error has
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occurred on a connection i.e. if SSL_get_error() has returned SSL_ERROR_SYSCALL
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or SSL_ERROR_SSL.
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The shutdown procedure consists of two steps: sending of the close_notify
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shutdown alert, and reception of the peer's close_notify shutdown alert.
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The order of those two steps depends on the application.
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It is acceptable for an application to only send its shutdown alert and
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then close the underlying connection without waiting for the peer's response.
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This way resources can be saved, as the process can already terminate or
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serve another connection.
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This should only be done when it is known that the other side will not send more
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data, otherwise there is a risk of a truncation attack.
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When a client only writes and never reads from the connection, and the server
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has sent a session ticket to establish a session, the client might not be able
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to resume the session because it did not received and process the session ticket
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from the server.
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In case the application wants to be able to resume the session, it is recommended to
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do a complete shutdown procedure (bidirectional close_notify alerts).
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When the underlying connection shall be used for more communications, the
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complete shutdown procedure must be performed, so that the peers stay
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synchronized.
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SSL_shutdown() only closes the write direction.
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It is not possible to call SSL_write() after calling SSL_shutdown().
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The read direction is closed by the peer.
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The behaviour of SSL_shutdown() additionally depends on the underlying BIO.
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If the underlying BIO is B<blocking>, SSL_shutdown() will only return once the
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handshake step has been finished or an error occurred.
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If the underlying BIO is B<nonblocking>, SSL_shutdown() will also return
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when the underlying BIO could not satisfy the needs of SSL_shutdown()
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to continue the handshake. In this case a call to SSL_get_error() with the
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return value of SSL_shutdown() will yield B<SSL_ERROR_WANT_READ> or
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B<SSL_ERROR_WANT_WRITE>. The calling process then must repeat the call after
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taking appropriate action to satisfy the needs of SSL_shutdown().
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The action depends on the underlying BIO. When using a nonblocking socket,
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nothing is to be done, but select() can be used to check for the required
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condition. When using a buffering BIO, like a BIO pair, data must be written
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into or retrieved out of the BIO before being able to continue.
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After SSL_shutdown() returned 0, it is possible to call SSL_shutdown() again
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to wait for the peer's close_notify alert.
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SSL_shutdown() will return 1 in that case.
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However, it is recommended to wait for it using SSL_read() instead.
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SSL_shutdown() can be modified to only set the connection to "shutdown"
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state but not actually send the close_notify alert messages,
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see L<SSL_CTX_set_quiet_shutdown(3)>.
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When "quiet shutdown" is enabled, SSL_shutdown() will always succeed
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and return 1.
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Note that this is not standard compliant behaviour.
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It should only be done when the peer has a way to make sure all
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data has been received and doesn't wait for the close_notify alert
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message, otherwise an unexpected EOF will be reported.
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There are implementations that do not send the required close_notify alert.
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If there is a need to communicate with such an implementation, and it's clear
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that all data has been received, do not wait for the peer's close_notify alert.
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Waiting for the close_notify alert when the peer just closes the connection
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will result in an error being generated.
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The error can be ignored using the B<SSL_OP_IGNORE_UNEXPECTED_EOF>.
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For more information see L<SSL_CTX_set_options(3)>.
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SSL_shutdown() shuts down an active connection represented by an SSL object.
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SSL_shutdown_ex() is an extended version of SSL_shutdown(). If non-NULL, I<args>
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must point to a B<SSL_SHUTDOWN_EX_ARGS> structure and I<args_len> must be set to
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C<sizeof(SSL_SHUTDOWN_EX_ARGS)>. The B<SSL_SHUTDOWN_EX_ARGS> structure must be
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zero-initialized. If I<args> is NULL, the behaviour is the same as passing a
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zero-initialised B<SSL_SHUTDOWN_EX_ARGS> structure. When used with a non-QUIC
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SSL object, the arguments are ignored and the call functions identically to
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SSL_shutdown().
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zero-initialised B<SSL_SHUTDOWN_EX_ARGS> structure. Currently, all extended
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arguments relate to usage with QUIC, therefore this call functions identically
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to SSL_shutdown() when not being used with QUIC.
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When used with a QUIC connection SSL object, SSL_shutdown_ex() initiates a QUIC
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immediate close. The I<quic_error_code> field can be used to specify a 62-bit
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application error code to be signalled via QUIC. The value specified must be in
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the range [0, 2**62-1], else this call fails. I<quic_reason> may optionally
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specify a zero-terminated reason string to be signalled to the peer. If a reason
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is not specified, a zero-length string is used as the reason. The reason string
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is copied and need not remain allocated after the call to the function returns.
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Reason strings are bounded by the path MTU and may be silently truncated if they
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are too long to fit in a QUIC packet. The arguments are only used on the first
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call to SSL_shutdown_ex() for a given QUIC connection SSL object.
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While the general operation of SSL_shutdown() is common between protocols, the
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exact nature of how a shutdown is performed depends on the underlying protocol
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being used. See the section below pertaining to each protocol for more
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information.
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When using QUIC, how an application uses SSL_shutdown() or SSL_shutdown_ex() has
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implications for whether QUIC closes a connection in an RFC-compliant manner.
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For discussion of these issues, and for discussion of the I<flags> argument, see
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B<QUIC-SPECIFIC SHUTDOWN CONSIDERATIONS> below.
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In general, calling SSL_shutdown() in nonblocking mode will initiate the
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shutdown process and return 0 to indicate that the shutdown process has not yet
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completed. Once the shutdown process has completed, subsequent calls to
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SSL_shutdown() will return 1. See the RETURN VALUES section for more
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information.
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=head2 First to close the connection
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SSL_shutdown() should not be called if a previous fatal error has occurred on a
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connection; i.e., if L<SSL_get_error(3)> has returned B<SSL_ERROR_SYSCALL> or
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B<SSL_ERROR_SSL>.
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When the application is the first party to send the close_notify
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alert, SSL_shutdown() will only send the alert and then set the
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SSL_SENT_SHUTDOWN flag (so that the session is considered good and will
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be kept in the cache).
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If successful, SSL_shutdown() will return 0.
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=head1 TLS AND DTLS-SPECIFIC CONSIDERATIONS
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If a unidirectional shutdown is enough (the underlying connection shall be
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closed anyway), this first successful call to SSL_shutdown() is sufficient.
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Shutdown for SSL/TLS and DTLS is implemented in terms of the SSL/TLS/DTLS
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close_notify alert message. The shutdown process for SSL/TLS and DTLS
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consists of two steps:
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In order to complete the bidirectional shutdown handshake, the peer needs
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to send back a close_notify alert.
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The SSL_RECEIVED_SHUTDOWN flag will be set after receiving and processing
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it.
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=over 4
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The peer is still allowed to send data after receiving the close_notify
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event.
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When it is done sending data, it will send the close_notify alert.
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SSL_read() should be called until all data is received.
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SSL_read() will indicate the end of the peer data by returning <= 0
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and SSL_get_error() returning SSL_ERROR_ZERO_RETURN.
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=item *
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=head2 Peer closes the connection
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A close_notify shutdown alert message is sent to the peer.
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If the peer already sent the close_notify alert B<and> it was
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already processed implicitly inside another function
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(L<SSL_read(3)>), the SSL_RECEIVED_SHUTDOWN flag is set.
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SSL_read() will return <= 0 in that case, and SSL_get_error() will return
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SSL_ERROR_ZERO_RETURN.
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SSL_shutdown() will send the close_notify alert, set the SSL_SENT_SHUTDOWN
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flag.
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If successful, SSL_shutdown() will return 1.
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=item *
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Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the
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SSL_get_shutdown() (see also L<SSL_set_shutdown(3)> call.
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A close_notify shutdown alert message is received from the peer.
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=back
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These steps can occur in either order depending on whether the connection
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shutdown process was first initiated by the local application or by the peer.
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=head2 Locally-Initiated Shutdown
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Calling SSL_shutdown() on a SSL/TLS or DTLS SSL object initiates the shutdown
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process and causes OpenSSL to try to send a close_notify shutdown alert to the
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peer. The shutdown process will then be considered completed once the peer
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responds in turn with a close_notify shutdown alert message.
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Calling SSL_shutdown() only closes the write direction of the connection; the
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read direction is closed by the peer. Once SSL_shutdown() is called,
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L<SSL_write(3)> can no longer be used, but L<SSL_read(3)> may still be used
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until the peer decides to close the connection in turn. The peer might
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continue sending data for some period of time before handling the local
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application's shutdown indication.
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SSL_shutdown() does not affect an underlying network connection such as a TCP
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connection, which remains open.
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=head2 Remotely-Initiated Shutdown
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If the peer was the first to initiate the shutdown process by sending a
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close_notify alert message, an application will be notified of this as an EOF
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condition when calling
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L<SSL_read(3)> (i.e., L<SSL_read(3)> will fail and L<SSL_get_error(3)> will
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return B<SSL_ERROR_ZERO_RETURN>), after all application data sent by the peer
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prior to initiating the shutdown has been read. An application should handle
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this condition by calling SSL_shutdown() to respond with a close_notify alert in
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turn, completing the shutdown process, though it may choose to write additional
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application data using L<SSL_write(3)> before doing so. If an application does
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not call SSL_shutdown() in this case, a close_notify alert will not be sent and
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the behaviour will not be fully standards compliant.
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=head2 Shutdown Lifecycle
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Regardless of whether a shutdown was initiated locally or by the peer, if the
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underlying BIO is blocking, a call to SSL_shutdown() will return firstly once a
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close_notify alert message is written to the peer (returning 0), and upon a
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second and subsequent call, once a corresponding message is received from the
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peer (returning 1 and completing the shutdown process). Calls to SSL_shutdown()
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with a blocking underlying BIO will also return if an error occurs.
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If the underlying BIO is nonblocking and the shutdown process is not yet
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complete (for example, because a close_notify alert message has not yet been
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received from the peer, or because a close_notify alert message needs to be sent
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but would currently block), SSL_shutdown() returns 0 to indicate that the
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shutdown process is still ongoing; in this case, a call to L<SSL_get_error(3)>
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will yield B<SSL_ERROR_WANT_READ> or B<SSL_ERROR_WANT_WRITE>.
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An application can then detect completion of the shutdown process by calling
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SSL_shutdown() again repeatedly until it returns 1, indicating that the shutdown
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process is complete (with a close_notify alert having both been sent and
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received).
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However, the preferred method of waiting for the shutdown to complete is to use
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L<SSL_read(3)> until L<SSL_get_error(3)> indicates EOF by returning
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B<SSL_ERROR_ZERO_RETURN>. This ensures any data received immediately before the
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peer's close_notify alert is still provided to the application. It also ensures
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any final handshake-layer messages received are processed (for example, messages
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issuing new session tickets).
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If this approach is not used, the second call to SSL_shutdown() (to complete the
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shutdown by confirming receipt of the peer's close_notify message) will fail if
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it is called when the application has not read all pending application data
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sent by the peer using L<SSL_read(3)>.
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When calling SSL_shutdown(), the B<SSL_SENT_SHUTDOWN> flag is set once an
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attempt is made to send a close_notify alert, regardless of whether the attempt
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was successful. The B<SSL_RECEIVED_SHUTDOWN> flag is set once a close_notify
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alert is received, which may occur during any call which processes incoming data
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from the network, such as L<SSL_read(3)> or SSL_shutdown(). These flags
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may be checked using L<SSL_get_shutdown(3)>.
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=head2 Fast Shutdown
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Alternatively, it is acceptable for an application to call SSL_shutdown() once
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(such that it returns 0) and then close the underlying connection without
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waiting for the peer's response. This allows for a more rapid shutdown process
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if the application does not wish to wait for the peer.
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This alternative "fast shutdown" approach should only be done if it is known
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that the peer will not send more data, otherwise there is a risk of an
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application exposing itself to a truncation attack. The full SSL_shutdown()
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process, in which both parties send close_notify alerts and SSL_shutdown()
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returns 1, provides a cryptographically authenticated indication of the end of a
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connection.
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This approach of a single SSL_shutdown() call without waiting is preferable to
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simply calling L<SSL_free(3)> or L<SSL_clear(3)> as calling SSL_shutdown()
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beforehand makes an SSL session eligible for subsequent reuse and notifies the
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peer of connection shutdown.
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The fast shutdown approach can only be used if there is no intention to reuse
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the underlying connection (e.g. a TCP connection) for further communication; in
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this case, the full shutdown process must be performed to ensure
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synchronisation.
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=head2 Effects on Session Reuse
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Calling SSL_shutdown() sets the SSL_SENT_SHUTDOWN flag (see
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L<SSL_set_shutdown(3)>), regardless of whether the transmission of the
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close_notify alert was successful or not. This makes the SSL session eligible
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for reuse; the SSL session is considered properly closed and can be reused for
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future connections.
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=head2 Quiet Shutdown
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SSL_shutdown() can be modified to set the connection to the "shutdown"
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state without actually sending a close_notify alert message; see
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L<SSL_CTX_set_quiet_shutdown(3)>. When "quiet shutdown" is enabled,
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SSL_shutdown() will always succeed and return 1 immediately.
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This is not standards-compliant behaviour. It should only be done when the
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application protocol in use enables the peer to ensure that all data has been
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received, such that it doesn't need to wait for a close_notify alert, otherwise
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application data may be truncated unexpectedly.
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=head2 Non-Compliant Peers
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There are SSL/TLS implementations that never send the required close_notify
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alert message but simply close the underlying transport (e.g. a TCP connection)
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instead. This will ordinarily result in an error being generated.
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If compatibility with such peers is desired, the option
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B<SSL_OP_IGNORE_UNEXPECTED_EOF> can be set. For more information, see
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L<SSL_CTX_set_options(3)>.
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Note that use of this option means that the EOF condition for application data
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does not receive cryptographic protection, and therefore renders an application
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potentially vulnerable to truncation attacks. Thus, this option must only be
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used in conjunction with an application protocol which indicates unambiguously
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when all data has been received.
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An alternative approach is to simply avoid calling L<SSL_read(3)> if it is known
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that no more data is going to be sent. This requires an application protocol
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which indicates unambiguously when all data has been sent.
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=head2 Session Ticket Handling
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If a client application only writes to a SSL/TLS or DTLS connection and never
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reads, OpenSSL may never process new SSL/TLS session tickets sent by the server.
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This is because OpenSSL ordinarily processes handshake messages received from a
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peer during calls to L<SSL_read(3)> by the application.
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Therefore, client applications which only write and do not read but which wish
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to benefit from session resumption are advised to perform a complete shutdown
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procedure by calling SSL_shutdown() until it returns 1, as described above. This
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will ensure there is an opportunity for SSL/TLS session ticket messages to be
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received and processed by OpenSSL.
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=head1 QUIC-SPECIFIC SHUTDOWN CONSIDERATIONS
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When using QUIC, SSL_shutdown() or SSL_shutdown_ex() causes any data written to
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a stream which has not yet been sent to the peer to be written before the
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shutdown process is considered complete. An exception to this is streams which
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terminated in a non-normal fashion, for example due to a stream reset; only
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streams which are non-terminated or which terminated in a normal fashion have
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their pending send buffers flushed in this manner. This behaviour can be skipped
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by setting the B<SSL_SHUTDOWN_FLAG_NO_STREAM_FLUSH> flag; in this case, data
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remaining in stream send buffers may not be transmitted to the peer. This flag
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may be used when a non-normal application condition has occurred and the
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delivery of data written to streams via L<SSL_write(3)> is no longer relevant.
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When used with a QUIC connection SSL object, SSL_shutdown() initiates a QUIC
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immediate close using QUIC B<CONNECTION_CLOSE> frames.
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SSL_shutdown() cannot be used on QUIC stream SSL objects. To conclude a stream
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normally, see L<SSL_stream_conclude(3)>; to perform a non-normal stream
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termination, see L<SSL_stream_reset(3)>.
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SSL_shutdown_ex() may be used instead of SSL_shutdown() by an application to
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provide additional information to the peer on the reason why a connection is
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being shut down. The information which can be provided is as follows:
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=over 4
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=item I<quic_error_code>
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An optional 62-bit application error code to be signalled to the peer. The value
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must be in the range [0, 2**62-1], else the call to SSL_shutdown_ex() fails. If
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not provided, an error code of 0 is used by default.
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=item I<quic_reason>
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An optional zero-terminated (UTF-8) reason string to be signalled to the peer.
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The application is responsible for providing a valid UTF-8 string and OpenSSL
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will not validate the string. If a reason is not provided, or SSL_shutdown() is
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used, a zero-length string is used as the reason. If provided, the reason string
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is copied and stored inside the QUIC connection SSL object and need not remain
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allocated after the call to SSL_shutdown_ex() returns. Reason strings are
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bounded by the path MTU and may be silently truncated if they are too long to
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fit in a QUIC packet.
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Reason strings are intended for human diagnostic purposes only, and should not
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be used for application signalling.
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=back
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The arguments to SSL_shutdown_ex() are used only on the first call to
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SSL_shutdown_ex() (or SSL_shutdown()) for a given QUIC connection SSL object.
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These arguments are ignored on subsequent calls.
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These functions do not affect an underlying network BIO or the resource it
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represents; for example, a UDP datagram provided to a QUIC connection as the
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network BIO will remain open.
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|
||||
Note that when using QUIC, an application must call SSL_shutdown() 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(), 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() 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() is called regularly, or alternatively ensure that
|
||||
SSL_handle_events() is called regularly. See L<openssl-quic(7)> and
|
||||
L<SSL_handle_events(3)> for more information.
|
||||
|
||||
=head2 Application Data Drainage Behaviour
|
||||
|
||||
When using QUIC, SSL_shutdown() 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() 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 B<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
|
||||
L<SSL_write(3)> is no longer relevant.
|
||||
|
||||
=head2 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
|
||||
@ -255,7 +384,8 @@ Call L<SSL_get_error(3)> with the return value B<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().
|
||||
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.
|
||||
|
||||
|
||||
@ -66,6 +66,14 @@ operation is considered completed. The bytes are sent and a new write call with
|
||||
a new buffer (with the already sent bytes removed) must be started. A partial
|
||||
write is performed with the size of a message block, which is 16kB.
|
||||
|
||||
When used with a QUIC SSL object, calling an I/O function such as SSL_write()
|
||||
allows internal network event processing to be performed. It is important that
|
||||
this processing is performed regularly. If an application is not using thread
|
||||
assisted mode, an application should ensure that an I/O function such as
|
||||
SSL_write() is called regularly, or alternatively ensure that SSL_handle_events()
|
||||
is called regularly. See L<openssl-quic(7)> and L<SSL_handle_events(3)> for more
|
||||
information.
|
||||
|
||||
=head1 WARNINGS
|
||||
|
||||
When a write function call has to be repeated because L<SSL_get_error(3)>
|
||||
|
||||
Loading…
Reference in New Issue
Block a user