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SSL_CONF_cmd

Section: OpenSSL (3)
Updated: 2017-05-25
Index Return to Main Contents

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 framework 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 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.

-cert

Attempts to use the file value as the certificate for the appropriate 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 supported if certificate operations are permitted.

-key

Attempts to use the file value as the private key for the appropriate context. This option is only supported if certificate operations 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 parameters 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_PREFERENCE. 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 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.

Certificate

PrivateKey

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 parameters for the appropriate context. This option is only supported if certificate operations are permitted.

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. 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_RESUMPTION_ON_RENEGOTIATION flag. 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.

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 application 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 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.

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 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.

SSL_CONF_finish() returns 1 for success and 0 for failure.