OpenSSL

Cryptography and SSL/TLS Toolkit

SSL_CONF_cmd

NAME

SSL_CONF_cmd_value_type, 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);

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.

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 TLSv1.2 and TLSv1.3. 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 or signature_scheme. 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. signature_scheme is one of the signature schemes defined in TLSv1.3, specified using the IETF name, e.g., ecdsa_secp256r1_sha256, ed25519, or rsa_pss_pss_sha256.

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

Note: algorithms which specify a PKCS#1 v1.5 signature scheme (either by using RSA as the algorithm or by using one of the rsa_pkcs1_* identifiers) are ignored in TLSv1.3 and will not be negotiated.

-client_sigalgs

This sets the supported signature algorithms associated with client authentication for TLSv1.2 and TLSv1.3. For servers the value is used in the signature_algorithms field of a CertificateRequest message. For clients it is used to determine which signature algorithm to use 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.

-groups

This sets the supported groups. For clients, the groups are sent using the supported groups extension. For servers, it is used to determine which group to use. This setting affects groups used for signatures (in TLSv1.2 and earlier) and key exchange. The first group listed will also be used for the key_share sent by a client in a TLSv1.3 ClientHello.

The value argument is a colon separated list of groups. The group can be either the NIST name (e.g. P-256), some other commonly used name where applicable (e.g. X25519) or an OpenSSL OID name (e.g prime256v1). Group names are case sensitive. The list should be in order of preference with the most preferred group first.

-curves

This is a synonym for the "-groups" command.

-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 TLSv1.2 and below ciphersuite list to value. This list will be combined with any configured TLSv1.3 ciphersuites. Note: syntax checking of value is currently not performed unless a SSL or SSL_CTX structure is associated with cctx.

-ciphersuites

Sets the available ciphersuites for TLSv1.3 to value. This is a simple colon (":") separated list of TLSv1.3 ciphersuite names in order of preference. This list will be combined any configured TLSv1.2 and below ciphersuites. See ciphers(1) for more information.

-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 unless the flag SSL_CONF_FLAG_REQUIRE_PRIVATE is set.

-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.

-record_padding

Attempts to pad TLSv1.3 records so that they are a multiple of value in length on send. A value of 0 or 1 turns off padding. Otherwise, the value must be >1 or <=16384.

-no_renegotiation

Disables all attempts at renegotiation in TLSv1.2 and earlier, same as setting SSL_OP_NO_RENEGOTIATION.

-min_protocol, -max_protocol

Sets the minimum and maximum supported protocol. Currently supported protocol values are SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3 for TLS and DTLSv1, DTLSv1.2 for DTLS, and None for no limit. If either bound is not specified then only the other bound applies, if specified. To restrict the supported protocol versions use these commands rather than the deprecated alternative commands below.

-no_ssl3, -no_tls1, -no_tls1_1, -no_tls1_2, -no_tls1_3

Disables protocol support for SSLv3, TLSv1.0, TLSv1.1, TLSv1.2 or TLSv1.3 by setting the corresponding options SSL_OP_NO_SSLv3, SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1, SSL_OP_NO_TLSv1_2 and SSL_OP_NO_TLSv1_3 respectively. These options are deprecated, instead use -min_protocol and -max_protocol.

-bugs

Various bug workarounds are set, same as setting SSL_OP_ALL.

-comp

Enables support for SSL/TLS compression, same as clearing SSL_OP_NO_COMPRESSION. This command was introduced in OpenSSL 1.1.0. As of OpenSSL 1.1.0, compression is off by default.

-no_comp

Disables support for SSL/TLS compression, same as setting SSL_OP_NO_COMPRESSION. As of OpenSSL 1.1.0, compression is off by default.

-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.

-prioritize_chacha

Prioritize ChaCha ciphers when the client has a ChaCha20 cipher at the top of its preference list. This usually indicates a client without AES hardware acceleration (e.g. mobile) is in use. Equivalent to SSL_OP_PRIORITIZE_CHACHA. Only used by servers. Requires -serverpref.

-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.

-allow_no_dhe_kex

In TLSv1.3 allow a non-(ec)dhe based key exchange mode on resumption. This means that there will be no forward secrecy for the resumed session.

-strict

enables strict mode protocol handling. Equivalent to setting SSL_CERT_FLAG_TLS_STRICT.

-anti_replay, -no_anti_replay

Switches replay protection, on or off respectively. With replay protection on, OpenSSL will automatically detect if a session ticket has been used more than once, TLSv1.3 has been negotiated, and early data is enabled on the server. A full handshake is forced if a session ticket is used a second or subsequent time. Anti-Replay is on by default unless overridden by a configuration file and is only used by servers. Anti-replay measures are required for compliance with the TLSv1.3 specification. Some applications may be able to mitigate the replay risks in other ways and in such cases the built-in OpenSSL functionality is not required. Switching off anti-replay is equivalent to SSL_OP_NO_ANTI_REPLAY.

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 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 ciphersuite list for TLSv1.2 and below to value. This list will be combined with any configured TLSv1.3 ciphersuites. Note: syntax checking of value is currently not performed unless an SSL or SSL_CTX structure is associated with cctx.

Ciphersuites

Sets the available ciphersuites for TLSv1.3 to value. This is a simple colon (":") separated list of TLSv1.3 ciphersuite names in order of preference. This list will be combined any configured TLSv1.2 and below ciphersuites. See ciphers(1) for more information.

Certificate

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.

PrivateKey

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 PrivateKey option is set then a private key is not loaded unless the SSL_CONF_FLAG_REQUIRE_PRIVATE is set.

ChainCAFile, ChainCAPath, VerifyCAFile, VerifyCAPath

These options indicate a file or directory used for building certificate chains or verifying certificate chains. These options are only supported if certificate operations are permitted.

RequestCAFile

This option indicates a file containing a set of certificates in PEM form. The subject names of the certificates are sent to the peer in the certificate_authorities extension for TLS 1.3 (in ClientHello or CertificateRequest) or in a certificate request for previous versions or TLS.

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.

RecordPadding

Attempts to pad TLSv1.3 records so that they are a multiple of value in length on send. A value of 0 or 1 turns off padding. Otherwise, the value must be >1 or <=16384.

NoRenegotiation

Disables all attempts at renegotiation in TLSv1.2 and earlier, same as setting SSL_OP_NO_RENEGOTIATION.

SignatureAlgorithms

This sets the supported signature algorithms for TLSv1.2 and TLSv1.3. 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 or signature_scheme. 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. signature_scheme is one of the signature schemes defined in TLSv1.3, specified using the IETF name, e.g., ecdsa_secp256r1_sha256, ed25519, or rsa_pss_pss_sha256.

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

Note: algorithms which specify a PKCS#1 v1.5 signature scheme (either by using RSA as the algorithm or by using one of the rsa_pkcs1_* identifiers) are ignored in TLSv1.3 and will not be negotiated.

ClientSignatureAlgorithms

This sets the supported signature algorithms associated with client authentication for TLSv1.2 and TLSv1.3. For servers the value is used in the signature_algorithms field of a CertificateRequest message. For clients it is used to determine which signature algorithm to use with the client certificate. If a server does not request a certificate this option has no effect.

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

Groups

This sets the supported groups. For clients, the groups are sent using the supported groups extension. For servers, it is used to determine which group to use. This setting affects groups used for signatures (in TLSv1.2 and earlier) and key exchange. The first group listed will also be used for the key_share sent by a client in a TLSv1.3 ClientHello.

The value argument is a colon separated list of groups. The group can be either the NIST name (e.g. P-256), some other commonly used name where applicable (e.g. X25519) or an OpenSSL OID name (e.g prime256v1). Group names are case sensitive. The list should be in order of preference with the most preferred group first.

Curves

This is a synonym for the "Groups" command.

MinProtocol

This sets the minimum supported SSL, TLS or DTLS version.

Currently supported protocol values are SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3, DTLSv1 and DTLSv1.2. The value None will disable the limit.

MaxProtocol

This sets the maximum supported SSL, TLS or DTLS version.

Currently supported protocol values are SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3, DTLSv1 and DTLSv1.2. The value None will disable the limit.

Protocol

This can be used to enable or disable certain versions of the SSL, TLS or DTLS protocol.

The value argument is a comma separated list of supported protocols to enable or disable. If a protocol is preceded by - that version is disabled.

All protocol versions are enabled by default. You need to disable at least one protocol version for this setting have any effect. Only enabling some protocol versions does not disable the other protocol versions.

Currently supported protocol values are SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3, DTLSv1 and DTLSv1.2. The special value ALL refers to all supported versions.

This can't enable protocols that are disabled using MinProtocol or MaxProtocol, but can disable protocols that are still allowed by them.

The Protocol command is fragile and deprecated; do not use it. Use MinProtocol and MaxProtocol instead. If you do use Protocol, make sure that the resulting range of enabled protocols has no "holes", e.g. if TLS 1.0 and TLS 1.2 are both enabled, make sure to also leave TLS 1.1 enabled.

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(3) 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.

PrioritizeChaCha: prioritizes ChaCha ciphers when the client has a ChaCha20 cipher at the top of its preference list. This usually indicates a mobile client is in use. Equivalent to SSL_OP_PRIORITIZE_CHACHA. 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.

EncryptThenMac: use encrypt-then-mac extension, enabled by default. Inverse of SSL_OP_NO_ENCRYPT_THEN_MAC: that is, -EncryptThenMac is the same as setting SSL_OP_NO_ENCRYPT_THEN_MAC.

AllowNoDHEKEX: In TLSv1.3 allow a non-(ec)dhe based key exchange mode on resumption. This means that there will be no forward secrecy for the resumed session. Equivalent to SSL_OP_ALLOW_NO_DHE_KEX.

MiddleboxCompat: If set then dummy Change Cipher Spec (CCS) messages are sent in TLSv1.3. This has the effect of making TLSv1.3 look more like TLSv1.2 so that middleboxes that do not understand TLSv1.3 will not drop the connection. This option is set by default. A future version of OpenSSL may not set this by default. Equivalent to SSL_OP_ENABLE_MIDDLEBOX_COMPAT.

AntiReplay: If set then OpenSSL will automatically detect if a session ticket has been used more than once, TLSv1.3 has been negotiated, and early data is enabled on the server. A full handshake is forced if a session ticket is used a second or subsequent time. This option is set by default and is only used by servers. Anti-replay measures are required to comply with the TLSv1.3 specification. Some applications may be able to mitigate the replay risks in other ways and in such cases the built-in OpenSSL functionality is not required. Disabling anti-replay is equivalent to setting SSL_OP_NO_ANTI_REPLAY.

VerifyMode

The value argument is a comma separated list of flags to set.

Peer enables peer verification: for clients only.

Request requests but does not require a certificate from the client. Servers only.

Require requests and requires a certificate from the client: an error occurs if the client does not present a certificate. Servers only.

Once requests a certificate from a client only on the initial connection: not when renegotiating. Servers only.

RequestPostHandshake configures the connection to support requests but does not require a certificate from the client post-handshake. A certificate will not be requested during the initial handshake. The server application must provide a mechanism to request a certificate post-handshake. Servers only. TLSv1.3 only.

RequiresPostHandshake configures the connection to support requests and requires a certificate from the client post-handshake: an error occurs if the client does not present a certificate. A certificate will not be requested during the initial handshake. The server application must provide a mechanism to request a certificate post-handshake. Servers only. TLSv1.3 only.

ClientCAFile, ClientCAPath

A file or directory of certificates in PEM format whose names are used as the set of acceptable names for client CAs. Servers only. This option is only supported if certificate operations are permitted.

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.

SSL_CONF_TYPE_NONE

The value string is not used e.g. a command line option which doesn't take an argument.

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");

SSLv3 is always disabled and attempt to override this by the user are ignored.

By checking the return code of SSL_CONF_cmd() it is possible to query if a given cmd is recognised, this is useful if SSL_CONF_cmd() values are mixed with additional application specific operations.

For example an application might call SSL_CONF_cmd() and if it returns -2 (unrecognised command) continue with processing of application specific commands.

Applications can also use SSL_CONF_cmd() to process command lines though the utility function SSL_CONF_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_CONF_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");

There are various ways to select the supported protocols.

This set the minimum protocol version to TLSv1, and so disables SSLv3. This is the recommended way to disable protocols.

 SSL_CONF_cmd(ctx, "MinProtocol", "TLSv1");

The following also disables SSLv3:

 SSL_CONF_cmd(ctx, "Protocol", "-SSLv3");

The following will first enable all protocols, and then disable SSLv3. If no protocol versions were disabled before this has the same effect as "-SSLv3", but if some versions were disables this will re-enable them before disabling SSLv3.

 SSL_CONF_cmd(ctx, "Protocol", "ALL,-SSLv3");

Only enable TLSv1.2:

 SSL_CONF_cmd(ctx, "MinProtocol", "TLSv1.2");
 SSL_CONF_cmd(ctx, "MaxProtocol", "TLSv1.2");

This also only enables TLSv1.2:

 SSL_CONF_cmd(ctx, "Protocol", "-ALL,TLSv1.2");

Disable TLS session tickets:

 SSL_CONF_cmd(ctx, "Options", "-SessionTicket");

Enable compression:

 SSL_CONF_cmd(ctx, "Options", "Compression");

Set supported curves to P-256, P-384:

 SSL_CONF_cmd(ctx, "Curves", "P-256:P-384");

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.

SEE ALSO

SSL_CONF_CTX_new(3), SSL_CONF_CTX_set_flags(3), SSL_CONF_CTX_set1_prefix(3), SSL_CONF_CTX_set_ssl_ctx(3), SSL_CONF_cmd_argv(3), SSL_CTX_set_options(3)

HISTORY

SSL_CONF_cmd() was first added to OpenSSL 1.0.2

SSL_OP_NO_SSL2 doesn't have effect since 1.1.0, but the macro is retained for backwards compatibility.

SSL_CONF_TYPE_NONE was first added to OpenSSL 1.1.0. In earlier versions of OpenSSL passing a command which didn't take an argument would return SSL_CONF_TYPE_UNKNOWN.

MinProtocol and MaxProtocol where added in OpenSSL 1.1.0.

AllowNoDHEKEX and PrioritizeChaCha were added in OpenSSL 1.1.1.

Copyright 2012-2018 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at https://www.openssl.org/source/license.html.