SSL_CONF_cmd(3)
NAME
SSL_CONF_cmd - send configuration command
SYNOPSIS
#include <openssl/ssl.h>
int SSL_CONF_cmd(SSL_CONF_CTX *cctx, const char *cmd, const char *value);
int SSL_CONF_cmd_value_type(SSL_CONF_CTX *cctx, const char *cmd);
int SSL_CONF_finish(SSL_CONF_CTX *cctx);
DESCRIPTION
The function SSL_CONF_cmd() performs configuration operation cmd with
optional parameter value on ctx. Its purpose is to simplify application
configuration of SSL_CTX or SSL structures by providing a common frame-
work for command line options or configuration files.
SSL_CONF_cmd_value_type() returns the type of value that cmd refers to.
The function SSL_CONF_finish() must be called after all configuration
operations have been completed. It is used to finalise any operations
or to process defaults.
SUPPORTED COMMAND LINE COMMANDS
Currently supported cmd names for command lines (i.e. when the flag
SSL_CONF_CMDLINE is set) are listed below. Note: all cmd names are case
sensitive. Unless otherwise stated commands can be used by both clients
and servers and the value parameter is not used. The default prefix for
command line commands is - and that is reflected below.
-sigalgs
This sets the supported signature algorithms for TLS v1.2. For
clients this value is used directly for the supported signature
algorithms extension. For servers it is used to determine which
signature algorithms to support.
The value argument should be a colon separated list of signature
algorithms in order of decreasing preference of the form algo-
rithm+hash. algorithm is one of RSA, DSA or ECDSA and hash is a
supported algorithm OID short name such as SHA1, SHA224, SHA256,
SHA384 of SHA512. Note: algorithm and hash names are case sensi-
tive.
If this option is not set then all signature algorithms supported
by the OpenSSL library are permissible.
-client_sigalgs
This sets the supported signature algorithms associated with client
authentication for TLS v1.2. For servers the value is used in the
supported signature algorithms field of a certificate request. For
clients it is used to determine which signature algorithm to with
the client certificate. If a server does not request a certificate
this option has no effect.
The syntax of value is identical to -sigalgs. If not set then the
value set for -sigalgs will be used instead.
-curves
This sets the supported elliptic curves. For clients the curves are
sent using the supported curves extension. For servers it is used
to determine which curve to use. This setting affects curves used
for both signatures and key exchange, if applicable.
The value argument is a colon separated list of curves. The curve
can be either the NIST name (e.g. P-256) or an OpenSSL OID name
(e.g prime256v1). Curve names are case sensitive.
-named_curve
This sets the temporary curve used for ephemeral ECDH modes. Only
used by servers
The value argument is a curve name or the special value auto which
picks an appropriate curve based on client and server preferences.
The curve can be either the NIST name (e.g. P-256) or an OpenSSL
OID name (e.g prime256v1). Curve names are case sensitive.
-cipher
Sets the cipher suite list to value. Note: syntax checking of value
is currently not performed unless a SSL or SSL_CTX structure is
associated with cctx.
-cert
Attempts to use the file value as the certificate for the appropri-
ate context. It currently uses SSL_CTX_use_certificate_chain_file()
if an SSL_CTX structure is set or SSL_use_certificate_file() with
filetype PEM if an SSL structure is set. This option is only sup-
ported if certificate operations are permitted.
-key
Attempts to use the file value as the private key for the appropri-
ate context. This option is only supported if certificate opera-
tions are permitted. Note: if no -key option is set then a private
key is not loaded: it does not currently use the -cert file.
-dhparam
Attempts to use the file value as the set of temporary DH parame-
ters for the appropriate context. This option is only supported if
certificate operations are permitted.
-no_ssl2, -no_ssl3, -no_tls1, -no_tls1_1, -no_tls1_2
Disables protocol support for SSLv2, SSLv3, TLSv1.0, TLSv1.1 or
TLSv1.2 by setting the corresponding options SSL_OP_NO_SSLv2,
SSL_OP_NO_SSLv3, SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1 and
SSL_OP_NO_TLSv1_2 respectively.
-bugs
Various bug workarounds are set, same as setting SSL_OP_ALL.
-no_comp
Disables support for SSL/TLS compression, same as setting
SSL_OP_NO_COMPRESS.
-no_ticket
Disables support for session tickets, same as setting
SSL_OP_NO_TICKET.
-serverpref
Use server and not client preference order when determining which
cipher suite, signature algorithm or elliptic curve to use for an
incoming connection. Equivalent to SSL_OP_CIPHER_SERVER_PREFER-
ENCE. Only used by servers.
-no_resumption_on_reneg
set SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION flag. Only used
by servers.
-legacyrenegotiation
permits the use of unsafe legacy renegotiation. Equivalent to set-
ting SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION.
-legacy_server_connect, -no_legacy_server_connect
permits or prohibits the use of unsafe legacy renegotiation for
OpenSSL clients only. Equivalent to setting or clearing
SSL_OP_LEGACY_SERVER_CONNECT. Set by default.
-strict
enables strict mode protocol handling. Equivalent to setting
SSL_CERT_FLAG_TLS_STRICT.
-debug_broken_protocol
disables various checks and permits several kinds of broken proto-
col behaviour for testing purposes: it should NEVER be used in any-
thing other than a test environment. Only supported if OpenSSL is
configured with -DOPENSSL_SSL_DEBUG_BROKEN_PROTOCOL.
SUPPORTED CONFIGURATION FILE COMMANDS
Currently supported cmd names for configuration files (i.e. when the
flag SSL_CONF_FLAG_FILE is set) are listed below. All configuration
file cmd names and are case insensitive so signaturealgorithms is
recognised as well as SignatureAlgorithms. Unless otherwise stated the
value names are also case insensitive.
Note: the command prefix (if set) alters the recognised cmd values.
CipherString
Sets the cipher suite list to value. Note: syntax checking of value
is currently not performed unless an SSL or SSL_CTX structure is
associated with cctx.
Certificate
Attempts to use the file value as the certificate for the appropri-
ate context. It currently uses SSL_CTX_use_certificate_chain_file()
if an SSL_CTX structure is set or SSL_use_certificate_file() with
filetype PEM if an SSL structure is set. This option is only sup-
ported if certificate operations are permitted.
PrivateKey
Attempts to use the file value as the private key for the appropri-
ate context. This option is only supported if certificate opera-
tions are permitted. Note: if no -key option is set then a private
key is not loaded: it does not currently use the Certificate file.
ServerInfoFile
Attempts to use the file value in the "serverinfo" extension using
the function SSL_CTX_use_serverinfo_file.
DHParameters
Attempts to use the file value as the set of temporary DH parame-
ters for the appropriate context. This option is only supported if
certificate operations are permitted.
SignatureAlgorithms
This sets the supported signature algorithms for TLS v1.2. For
clients this value is used directly for the supported signature
algorithms extension. For servers it is used to determine which
signature algorithms to support.
The value argument should be a colon separated list of signature
algorithms in order of decreasing preference of the form algo-
rithm+hash. algorithm is one of RSA, DSA or ECDSA and hash is a
supported algorithm OID short name such as SHA1, SHA224, SHA256,
SHA384 of SHA512. Note: algorithm and hash names are case sensi-
tive.
If this option is not set then all signature algorithms supported
by the OpenSSL library are permissible.
ClientSignatureAlgorithms
This sets the supported signature algorithms associated with client
authentication for TLS v1.2. For servers the value is used in the
supported signature algorithms field of a certificate request. For
clients it is used to determine which signature algorithm to with
the client certificate.
The syntax of value is identical to SignatureAlgorithms. If not set
then the value set for SignatureAlgorithms will be used instead.
Curves
This sets the supported elliptic curves. For clients the curves are
sent using the supported curves extension. For servers it is used
to determine which curve to use. This setting affects curves used
for both signatures and key exchange, if applicable.
The value argument is a colon separated list of curves. The curve
can be either the NIST name (e.g. P-256) or an OpenSSL OID name
(e.g prime256v1). Curve names are case sensitive.
ECDHParameters
This sets the temporary curve used for ephemeral ECDH modes. Only
used by servers
The value argument is a curve name or the special value Automatic
which picks an appropriate curve based on client and server prefer-
ences. The curve can be either the NIST name (e.g. P-256) or an
OpenSSL OID name (e.g prime256v1). Curve names are case sensitive.
Protocol
The supported versions of the SSL or TLS protocol.
The value argument is a comma separated list of supported protocols
to enable or disable. If an protocol is preceded by - that version
is disabled. Currently supported protocol values are SSLv2, SSLv3,
TLSv1, TLSv1.1 and TLSv1.2. All protocol versions other than SSLv2
are enabled by default. To avoid inadvertent enabling of SSLv2,
when SSLv2 is disabled, it is not possible to enable it via the
Protocol command.
Options
The value argument is a comma separated list of various flags to
set. If a flag string is preceded - it is disabled. See the
SSL_CTX_set_options function for more details of individual
options.
Each option is listed below. Where an operation is enabled by
default the -flag syntax is needed to disable it.
SessionTicket: session ticket support, enabled by default. Inverse
of SSL_OP_NO_TICKET: that is -SessionTicket is the same as setting
SSL_OP_NO_TICKET.
Compression: SSL/TLS compression support, enabled by default.
Inverse of SSL_OP_NO_COMPRESSION.
EmptyFragments: use empty fragments as a countermeasure against a
SSL 3.0/TLS 1.0 protocol vulnerability affecting CBC ciphers. It is
set by default. Inverse of SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS.
Bugs: enable various bug workarounds. Same as SSL_OP_ALL.
DHSingle: enable single use DH keys, set by default. Inverse of
SSL_OP_DH_SINGLE. Only used by servers.
ECDHSingle enable single use ECDH keys, set by default. Inverse of
SSL_OP_ECDH_SINGLE. Only used by servers.
ServerPreference use server and not client preference order when
determining which cipher suite, signature algorithm or elliptic
curve to use for an incoming connection. Equivalent to
SSL_OP_CIPHER_SERVER_PREFERENCE. Only used by servers.
NoResumptionOnRenegotiation set SSL_OP_NO_SESSION_RESUMP-
TION_ON_RENEGOTIATION flag. Only used by servers.
UnsafeLegacyRenegotiation permits the use of unsafe legacy renego-
tiation. Equivalent to SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION.
UnsafeLegacyServerConnect permits the use of unsafe legacy renego-
tiation for OpenSSL clients only. Equivalent to
SSL_OP_LEGACY_SERVER_CONNECT. Set by default.
SUPPORTED COMMAND TYPES
The function SSL_CONF_cmd_value_type() currently returns one of the
following types:
SSL_CONF_TYPE_UNKNOWN
The cmd string is unrecognised, this return value can be use to
flag syntax errors.
SSL_CONF_TYPE_STRING
The value is a string without any specific structure.
SSL_CONF_TYPE_FILE
The value is a file name.
SSL_CONF_TYPE_DIR
The value is a directory name.
NOTES
The order of operations is significant. This can be used to set either
defaults or values which cannot be overridden. For example if an appli-
cation calls:
SSL_CONF_cmd(ctx, "Protocol", "-SSLv3");
SSL_CONF_cmd(ctx, userparam, uservalue);
it will disable SSLv3 support by default but the user can override it.
If however the call sequence is:
SSL_CONF_cmd(ctx, userparam, uservalue);
SSL_CONF_cmd(ctx, "Protocol", "-SSLv3");
then SSLv3 is always disabled and attempt to override this by the user
are ignored.
By checking the return code of SSL_CTX_cmd() it is possible to query if
a given cmd is recognised, this is useful is SSL_CTX_cmd() values are
mixed with additional application specific operations.
For example an application might call SSL_CTX_cmd() and if it returns
-2 (unrecognised command) continue with processing of application spe-
cific commands.
Applications can also use SSL_CTX_cmd() to process command lines though
the utility function SSL_CTX_cmd_argv() is normally used instead. One
way to do this is to set the prefix to an appropriate value using
SSL_CONF_CTX_set1_prefix(), pass the current argument to cmd and the
following argument to value (which may be NULL).
In this case if the return value is positive then it is used to skip
that number of arguments as they have been processed by SSL_CTX_cmd().
If -2 is returned then cmd is not recognised and application specific
arguments can be checked instead. If -3 is returned a required argument
is missing and an error is indicated. If 0 is returned some other error
occurred and this can be reported back to the user.
The function SSL_CONF_cmd_value_type() can be used by applications to
check for the existence of a command or to perform additional syntax
checking or translation of the command value. For example if the return
value is SSL_CONF_TYPE_FILE an application could translate a relative
pathname to an absolute pathname.
EXAMPLES
Set supported signature algorithms:
SSL_CONF_cmd(ctx, "SignatureAlgorithms", "ECDSA+SHA256:RSA+SHA256:DSA+SHA256");
Enable all protocols except SSLv3 and SSLv2:
SSL_CONF_cmd(ctx, "Protocol", "ALL,-SSLv3,-SSLv2");
Only enable TLSv1.2:
SSL_CONF_cmd(ctx, "Protocol", "-ALL,TLSv1.2");
Disable TLS session tickets:
SSL_CONF_cmd(ctx, "Options", "-SessionTicket");
Set supported curves to P-256, P-384:
SSL_CONF_cmd(ctx, "Curves", "P-256:P-384");
Set automatic support for any elliptic curve for key exchange:
SSL_CONF_cmd(ctx, "ECDHParameters", "Automatic");
RETURN VALUES
SSL_CONF_cmd() returns 1 if the value of cmd is recognised and value is
NOT used and 2 if both cmd and value are used. In other words it
returns the number of arguments processed. This is useful when process-
ing command lines.
A return value of -2 means cmd is not recognised.
A return value of -3 means cmd is recognised and the command requires a
value but value is NULL.
A return code of 0 indicates that both cmd and value are valid but an
error occurred attempting to perform the operation: for example due to
an error in the syntax of value in this case the error queue may pro-
vide additional information.
SSL_CONF_finish() returns 1 for success and 0 for failure.
SEE ALSO
SSL_CONF_CTX_new(3), SSL_CONF_CTX_set_flags(3), SSL_CONF_CTX_set1_pre-
fix(3), SSL_CONF_CTX_set_ssl_ctx(3), SSL_CONF_cmd_argv(3)
HISTORY
SSL_CONF_cmd() was first added to OpenSSL 1.0.2
1.0.2t 2019-09-10 SSL_CONF_cmd(3)
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