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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Sets the minimum and maximum supported protocol.
Currently supported protocol values are SSLv3, TLSv1, TLSv1.1,
TLSv1.2, TLSv1.3 for TLS; DTLSv1, DTLSv1.2 for DTLS, and None
for no limit.
If either the lower or upper bound is not specified then only the other bound
applies, if specified.
If your application supports both TLS and DTLS you can specify any of these
options twice, once with a bound for TLS and again with an appropriate bound
for DTLS.
To restrict the supported protocol versions use these commands rather than the
deprecated alternative commands below.
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.
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.
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 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 SSL and TLS bounds apply only to TLS-based contexts, while the DTLS bounds
apply only to DTLS-based contexts.
The command can be repeated with one instance setting a TLS bound, and the
other setting a DTLS bound.
The value None applies to both types of contexts and disables the limits.
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 SSL and TLS bounds apply only to TLS-based contexts, while the DTLS bounds
apply only to DTLS-based contexts.
The command can be repeated with one instance setting a TLS bound, and the
other setting a DTLS bound.
The value None applies to both types of contexts and disables the limits.
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.
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.
NoRenegotiation: disables all attempts at renegotiation in TLSv1.2 and
earlier, same as setting SSL_OP_NO_RENEGOTIATION.
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.
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.
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.
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:
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.
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.
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.
The SSL_CONF_cmd() function was added in OpenSSL 1.0.2.
The SSL_OP_NO_SSL2 option doesn't have effect since 1.1.0, but the macro
is retained for backwards compatibility.
The SSL_CONF_TYPE_NONE was added in 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 2012-2020 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.