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Improve SSL_shutdown() documentation

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Reviewed-by: Ben Kaduk <kaduk@mit.edu>
GH: #7188
This commit is contained in:
Kurt Roeckx 2018-09-11 23:39:25 +02:00
parent cd92d1fdd3
commit 8e593f0a0d
4 changed files with 52 additions and 40 deletions
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4
doc/man3/SSL_CTX_set_quiet_shutdown.pod
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@ -33,7 +33,7 @@ SSL_get_quiet_shutdown() returns the "quiet shutdown" setting of B<ssl>.
=head1 NOTES
Normally when a SSL connection is finished, the parties must send out
"close notify" alert messages using L<SSL_shutdown(3)>
close_notify alert messages using L<SSL_shutdown(3)>
for a clean shutdown.
When setting the "quiet shutdown" flag to 1, L<SSL_shutdown(3)>
@ -41,7 +41,7 @@ will set the internal flags to SSL_SENT_SHUTDOWN|SSL_RECEIVED_SHUTDOWN.
(L<SSL_shutdown(3)> then behaves like
L<SSL_set_shutdown(3)> called with
SSL_SENT_SHUTDOWN|SSL_RECEIVED_SHUTDOWN.)
The session is thus considered to be shutdown, but no "close notify" alert
The session is thus considered to be shutdown, but no close_notify alert
is sent to the peer. This behaviour violates the TLS standard.
The default is normal shutdown behaviour as described by the TLS standard.

2
doc/man3/SSL_get_error.pod
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@ -39,7 +39,7 @@ if and only if B<ret E<gt> 0>.
=item SSL_ERROR_ZERO_RETURN
The TLS/SSL peer has closed the connection for writing by sending the
"close notify" alert.
close_notify alert.
No more data can be read.
Note that B<SSL_ERROR_ZERO_RETURN> does not necessarily
indicate that the underlying transport has been closed.

8
doc/man3/SSL_set_shutdown.pod
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@ -30,12 +30,12 @@ No shutdown setting, yet.
=item SSL_SENT_SHUTDOWN
A "close notify" shutdown alert was sent to the peer, the connection is being
A close_notify shutdown alert was sent to the peer, the connection is being
considered closed and the session is closed and correct.
=item SSL_RECEIVED_SHUTDOWN
A shutdown alert was received form the peer, either a normal "close notify"
A shutdown alert was received form the peer, either a normal close_notify
or a fatal error.
=back
@ -47,13 +47,13 @@ the ssl session. If the session is still open, when
L<SSL_clear(3)> or L<SSL_free(3)> is called,
it is considered bad and removed according to RFC2246.
The actual condition for a correctly closed session is SSL_SENT_SHUTDOWN
(according to the TLS RFC, it is acceptable to only send the "close notify"
(according to the TLS RFC, it is acceptable to only send the close_notify
alert but to not wait for the peer's answer, when the underlying connection
is closed).
SSL_set_shutdown() can be used to set this state without sending a
close alert to the peer (see L<SSL_shutdown(3)>).
If a "close notify" was received, SSL_RECEIVED_SHUTDOWN will be set,
If a close_notify was received, SSL_RECEIVED_SHUTDOWN will be set,
for setting SSL_SENT_SHUTDOWN the application must however still call
L<SSL_shutdown(3)> or SSL_set_shutdown() itself.

78
doc/man3/SSL_shutdown.pod
View File

@ -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