xref: /openssl/doc/man3/SSL_CTX_set_mode.pod (revision f5afac4b) 
1=pod
2
3=head1 NAME
4
5SSL_CTX_set_mode, SSL_CTX_clear_mode, SSL_set_mode, SSL_clear_mode, SSL_CTX_get_mode, SSL_get_mode - manipulate SSL engine mode
6
7=head1 SYNOPSIS
8
9 #include <openssl/ssl.h>
10
11 long SSL_CTX_set_mode(SSL_CTX *ctx, long mode);
12 long SSL_CTX_clear_mode(SSL_CTX *ctx, long mode);
13 long SSL_set_mode(SSL *ssl, long mode);
14 long SSL_clear_mode(SSL *ssl, long mode);
15
16 long SSL_CTX_get_mode(SSL_CTX *ctx);
17 long SSL_get_mode(SSL *ssl);
18
19=head1 DESCRIPTION
20
21SSL_CTX_set_mode() adds the mode set via bit-mask in B<mode> to B<ctx>.
22Options already set before are not cleared.
23SSL_CTX_clear_mode() removes the mode set via bit-mask in B<mode> from B<ctx>.
24
25SSL_set_mode() adds the mode set via bit-mask in B<mode> to B<ssl>.
26Options already set before are not cleared.
27SSL_clear_mode() removes the mode set via bit-mask in B<mode> from B<ssl>.
28
29SSL_CTX_get_mode() returns the mode set for B<ctx>.
30
31SSL_get_mode() returns the mode set for B<ssl>.
32
33=head1 NOTES
34
35The following mode changes are available:
36
37=over 4
38
39=item SSL_MODE_ENABLE_PARTIAL_WRITE
40
41Allow SSL_write_ex(..., n, &r) to return with 0 < r < n (i.e. report success
42when just a single record has been written). This works in a similar way for
43SSL_write(). When not set (the default), SSL_write_ex() or SSL_write() will only
44report success once the complete chunk was written. Once SSL_write_ex() or
45SSL_write() returns successful, B<r> bytes have been written and the next call
46to SSL_write_ex() or SSL_write() must only send the n-r bytes left, imitating
47the behaviour of write().
48
49=item SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER
50
51Make it possible to retry SSL_write_ex() or SSL_write() with changed buffer
52location (the buffer contents must stay the same). This is not the default to
53avoid the misconception that nonblocking SSL_write() behaves like
54nonblocking write().
55
56=item SSL_MODE_AUTO_RETRY
57
58During normal operations, non-application data records might need to be sent or
59received that the application is not aware of.
60If a non-application data record was processed,
61L<SSL_read_ex(3)> and L<SSL_read(3)> can return with a failure and indicate the
62need to retry with B<SSL_ERROR_WANT_READ>.
63If such a non-application data record was processed, the flag
64B<SSL_MODE_AUTO_RETRY> causes it to try to process the next record instead of
65returning.
66
67In a nonblocking environment applications must be prepared to handle
68incomplete read/write operations.
69Setting B<SSL_MODE_AUTO_RETRY> for a nonblocking B<BIO> will process
70non-application data records until either no more data is available or
71an application data record has been processed.
72
73In a blocking environment, applications are not always prepared to
74deal with the functions returning intermediate reports such as retry
75requests, and setting the B<SSL_MODE_AUTO_RETRY> flag will cause the functions
76to only return after successfully processing an application data record or a
77failure.
78
79Turning off B<SSL_MODE_AUTO_RETRY> can be useful with blocking B<BIO>s in case
80they are used in combination with something like select() or poll().
81Otherwise the call to SSL_read() or SSL_read_ex() might hang when a
82non-application record was sent and no application data was sent.
83
84=item SSL_MODE_RELEASE_BUFFERS
85
86When we no longer need a read buffer or a write buffer for a given SSL,
87then release the memory we were using to hold it.
88Using this flag can
89save around 34k per idle SSL connection.
90This flag has no effect on SSL v2 connections, or on DTLS connections.
91
92=item SSL_MODE_SEND_FALLBACK_SCSV
93
94Send TLS_FALLBACK_SCSV in the ClientHello.
95To be set only by applications that reconnect with a downgraded protocol
96version; see draft-ietf-tls-downgrade-scsv-00 for details.
97
98DO NOT ENABLE THIS if your application attempts a normal handshake.
99Only use this in explicit fallback retries, following the guidance
100in draft-ietf-tls-downgrade-scsv-00.
101
102=item SSL_MODE_ASYNC
103
104Enable asynchronous processing. TLS I/O operations may indicate a retry with
105SSL_ERROR_WANT_ASYNC with this mode set if an asynchronous capable engine is
106used to perform cryptographic operations. See L<SSL_get_error(3)>.
107
108=item SSL_MODE_DTLS_SCTP_LABEL_LENGTH_BUG
109
110Older versions of OpenSSL had a bug in the computation of the label length
111used for computing the endpoint-pair shared secret. The bug was that the
112terminating zero was included in the length of the label. Setting this option
113enables this behaviour to allow interoperability with such broken
114implementations. Please note that setting this option breaks interoperability
115with correct implementations. This option only applies to DTLS over SCTP.
116
117=back
118
119All modes are off by default except for SSL_MODE_AUTO_RETRY which is on by
120default since 1.1.1.
121
122=head1 RETURN VALUES
123
124SSL_CTX_set_mode() and SSL_set_mode() return the new mode bit-mask
125after adding B<mode>.
126
127SSL_CTX_get_mode() and SSL_get_mode() return the current bit-mask.
128
129=head1 SEE ALSO
130
131L<ssl(7)>, L<SSL_read_ex(3)>, L<SSL_read(3)>, L<SSL_write_ex(3)> or
132L<SSL_write(3)>, L<SSL_get_error(3)>
133
134=head1 HISTORY
135
136SSL_MODE_ASYNC was added in OpenSSL 1.1.0.
137
138=head1 COPYRIGHT
139
140Copyright 2001-2021 The OpenSSL Project Authors. All Rights Reserved.
141
142Licensed under the Apache License 2.0 (the "License").  You may not use
143this file except in compliance with the License.  You can obtain a copy
144in the file LICENSE in the source distribution or at
145L<https://www.openssl.org/source/license.html>.
146
147=cut
148