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SSL_CONF_cmd(3)

NAME
SYNOPSIS
DESCRIPTION
SUPPORTED COMMAND LINE COMMANDS
SUPPORTED CONFIGURATION FILE COMMANDS
NOTES
EXAMPLES
RETURN VALUES
SEE ALSO
HISTORY

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);

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 framework for command line options or configuration files.

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 and 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 algorithm+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 sensitive.

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.

-no_ssl2, -no_ssl3, -no_tls1, -no_tls1_1, -no_tls1_2

Disables protocol support for SSLv2, SSLv3, TLS 1.0, TLS 1.1 or TLS 1.2 by setting the corresponding options SSL_OP_NO_SSL2, SSL_OP_NO_SSL3, SSL_OP_NO_TLS1, SSL_OP_NO_TLS1_1 and SSL_OP_NO_TLS1_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_PREFERENCE. Only used by servers.

-legacyrenegotiation

permits the use of unsafe legacy renegotiation. Equivalent to setting 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 protocol behaviour for testing purposes: it should NEVER be used in anything 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.

SignatureAlgorithms

If this option is not set then all signature algorithms supported by the OpenSSL library are permissible.

ClientSignatureAlgorithms

The syntax of value is identical to SignatureAlgorithms. If not set then the value set for SignatureAlgorithms will be used instead.

Curves

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 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.

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. All versions are enabled by default, though applications may choose to explicitly disable some. Currently supported protocol values are SSLv2, SSLv3, TLSv1, TLSv1.1 and TLSv1.2. The special value ALL refers to all supported versions.

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.

UnsafeLegacyRenegotiation permits the use of unsafe legacy renegotiation. Equivalent to SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION.

UnsafeLegacyServerConnect permits the use of unsafe legacy renegotiation for OpenSSL clients only. Equivalent to SSL_OP_LEGACY_SERVER_CONNECT. Set by default.

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 application calls:

 SSL_CONF_cmd(ctx, "Protocol", "-SSLv2");
 SSL_CONF_cmd(ctx, userparam, uservalue);

it will disable SSLv2 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", "-SSLv2");

SSLv2 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 specific 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.

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 processing 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 provide additional information.

HISTORY

SSL_CONF_cmd() was first added to OpenSSL 1.0.2