|
|
|
@@ -13,27 +13,36 @@ SSL_shutdown - shut down a TLS/SSL connection
|
|
|
|
|
=head1 DESCRIPTION
|
|
|
|
|
|
|
|
|
|
SSL_shutdown() shuts down an active TLS/SSL connection. It sends the
|
|
|
|
|
"close notify" shutdown alert to the peer.
|
|
|
|
|
close_notify shutdown alert to the peer.
|
|
|
|
|
|
|
|
|
|
=head1 NOTES
|
|
|
|
|
|
|
|
|
|
SSL_shutdown() tries to send 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.
|
|
|
|
|
The shutdown procedure consists of two steps: sending of the close_notify
|
|
|
|
|
shutdown alert, and reception of the peer's close_notify shutdown alert.
|
|
|
|
|
The order of those two steps depends on the application.
|
|
|
|
|
|
|
|
|
|
SSL_shutdown() supports both uni- and bidirectional shutdown by its 2 step
|
|
|
|
|
behaviour.
|
|
|
|
|
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.
|
|
|
|
|
This should only be done when it is known that the other side will not send more
|
|
|
|
|
data, otherwise there is a risk of a truncation attack.
|
|
|
|
|
|
|
|
|
|
When a client only writes and never reads from the connection, and the server
|
|
|
|
|
has sent a session ticket to establish a session, the client might not be able
|
|
|
|
|
to resume the session because it did not received and process the session ticket
|
|
|
|
|
from the server.
|
|
|
|
|
In case the application wants to be able to resume the session, it is recommended to
|
|
|
|
|
do a complete shutdown procedure (bidirectional close_notify alerts).
|
|
|
|
|
|
|
|
|
|
When the underlying connection shall be used for more communications, the
|
|
|
|
|
complete shutdown procedure must be performed, so that the peers stay
|
|
|
|
|
synchronized.
|
|
|
|
|
|
|
|
|
|
SSL_shutdown() only closes the write direction.
|
|
|
|
|
It is not possible to call SSL_write() after calling SSL_shutdown().
|
|
|
|
@@ -41,45 +50,43 @@ The read direction is closed by the peer.
|
|
|
|
|
|
|
|
|
|
=head2 First to close the connection
|
|
|
|
|
|
|
|
|
|
When the application is the first party to send the "close notify"
|
|
|
|
|
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 the cache).
|
|
|
|
|
SSL_shutdown() will then return with 0.
|
|
|
|
|
If successful, SSL_shutdown() will return 0.
|
|
|
|
|
|
|
|
|
|
If a unidirectional shutdown is enough (the underlying connection shall be
|
|
|
|
|
closed anyway), this first call to SSL_shutdown() is sufficient.
|
|
|
|
|
closed anyway), this first successful call to SSL_shutdown() is sufficient.
|
|
|
|
|
|
|
|
|
|
In order to complete the bidirectional shutdown handshake, the peer needs
|
|
|
|
|
to send back a "close notify" alert.
|
|
|
|
|
to send back a close_notify alert.
|
|
|
|
|
The SSL_RECEIVED_SHUTDOWN flag will be set after receiving and processing
|
|
|
|
|
it.
|
|
|
|
|
SSL_shutdown() will return 1 when it has been received.
|
|
|
|
|
|
|
|
|
|
The peer is still allowed to send data after receiving the "close notify"
|
|
|
|
|
The peer is still allowed to send data after receiving the close_notify
|
|
|
|
|
event.
|
|
|
|
|
If the peer did send data it needs to be processed by calling SSL_read()
|
|
|
|
|
before calling SSL_shutdown() a second time.
|
|
|
|
|
When it is done sending data, it will send the close_notify alert.
|
|
|
|
|
SSL_read() should be called until all data is received.
|
|
|
|
|
SSL_read() will indicate the end of the peer data by returning <= 0
|
|
|
|
|
and SSL_get_error() returning SSL_ERROR_ZERO_RETURN.
|
|
|
|
|
It is recommended to call SSL_read() between SSL_shutdown() calls.
|
|
|
|
|
|
|
|
|
|
=head2 Peer closes the connection
|
|
|
|
|
|
|
|
|
|
If the peer already sent the "close notify" alert B<and> it was
|
|
|
|
|
If the peer already sent the close_notify alert B<and> it was
|
|
|
|
|
already processed implicitly inside another function
|
|
|
|
|
(L<SSL_read(3)>), the SSL_RECEIVED_SHUTDOWN flag is set.
|
|
|
|
|
SSL_read() will return <= 0 in that case, and SSL_get_error() will return
|
|
|
|
|
SSL_ERROR_ZERO_RETURN.
|
|
|
|
|
SSL_shutdown() will send the "close notify" alert, set the SSL_SENT_SHUTDOWN
|
|
|
|
|
flag and will immediately return with 1.
|
|
|
|
|
SSL_shutdown() will send the close_notify alert, set the SSL_SENT_SHUTDOWN
|
|
|
|
|
flag.
|
|
|
|
|
If successful, SSL_shutdown() will return 1.
|
|
|
|
|
|
|
|
|
|
Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the
|
|
|
|
|
SSL_get_shutdown() (see also L<SSL_set_shutdown(3)> call.
|
|
|
|
|
|
|
|
|
|
=head1 NOTES
|
|
|
|
|
|
|
|
|
|
It is recommended to do a bidirectional shutdown by checking the return value
|
|
|
|
|
of SSL_shutdown() and call it again until it returns 1 or a fatal error.
|
|
|
|
|
|
|
|
|
|
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.
|
|
|
|
@@ -95,8 +102,13 @@ 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.
|
|
|
|
|
|
|
|
|
|
After SSL_shutdown() returned 0, it is possible to call SSL_shutdown() again
|
|
|
|
|
to wait for the peer's close_notify alert.
|
|
|
|
|
SSL_shutdown() will return 1 in that case.
|
|
|
|
|
However, it is recommended to wait for it using SSL_read() instead.
|
|
|
|
|
|
|
|
|
|
SSL_shutdown() can be modified to only set the connection to "shutdown"
|
|
|
|
|
state but not actually send the "close notify" alert messages,
|
|
|
|
|
state but not actually send the close_notify alert messages,
|
|
|
|
|
see L<SSL_CTX_set_quiet_shutdown(3)>.
|
|
|
|
|
When "quiet shutdown" is enabled, SSL_shutdown() will always succeed
|
|
|
|
|
and return 1.
|
|
|
|
@@ -109,16 +121,16 @@ The following return values can occur:
|
|
|
|
|
|
|
|
|
|
=item Z<>0
|
|
|
|
|
|
|
|
|
|
The shutdown is not yet finished: the "close notify" was send but the peer
|
|
|
|
|
The shutdown is not yet finished: the close_notify was sent but the peer
|
|
|
|
|
did not send it back yet.
|
|
|
|
|
Call SSL_shutdown() again to do a bidirectional shutdown.
|
|
|
|
|
Call SSL_read() to do a bidirectional shutdown.
|
|
|
|
|
The output of L<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.
|
|
|
|
|
The shutdown was successfully completed. The close_notify alert was sent
|
|
|
|
|
and the peer's close_notify alert was received.
|
|
|
|
|
|
|
|
|
|
=item E<lt>0
|
|
|
|
|
|
|
|
|
|