NAME

openssl.cnf —

OpenSSL configuration files

DESCRIPTION

The OpenSSL CONF library can be used to read configuration files; see CONF_modules_load_file(3). It is used for the OpenSSL master configuration file /etc/ssl/openssl.cnf and in a few other places like SPKAC files and certificate extension files for the openssl(1) x509 utility. OpenSSL applications can also use the CONF library for their own purposes.

A configuration file is divided into a number of sections. Each section starts with a line [section_name] and ends when a new section is started or the end of the file is reached. A section name can consist of alphanumeric characters and underscores.

The first section of a configuration file is special and is referred to as the “default section”. It is usually unnamed and extends from the start of file to the first named section. When a name is being looked up, it is first looked up in a named section (if any) and then in the default section.

The environment is mapped onto a section called ENV.

Comments can be included by preceding them with the ‘#’ character.

Each section in a configuration file consists of a number of name and value pairs of the form name=value.

The name string can contain any alphanumeric characters as well as a few punctuation symbols such as ‘.’ ‘,’ ‘;’ and ‘_’.

The value string consists of the string following the ‘=’ character until the end of the line with any leading and trailing whitespace removed.

The value string undergoes variable expansion. This can be done by including substrings of the form $name or ${name}: this will substitute the value of the named variable in the current section. It is also possible to substitute a value from another section using the syntax $section::name or ${section::name}. By using the form $ENV::name, environment variables can be substituted. It is also possible to assign values to environment variables by using the name ENV::name. This will work if the program looks up environment variables using the CONF library instead of calling getenv(3) directly. The value string must not exceed 64k in length after variable expansion or an error will occur.

It is possible to escape certain characters by using any kind of quote or the ‘\’ character. By making the last character of a line a ‘\’, a value string can be spread across multiple lines. In addition the sequences ‘\n’, ‘\r’, ‘\b’, and ‘\t’ are recognized.

OPENSSL LIBRARY CONFIGURATION

Applications can automatically configure certain aspects of OpenSSL using the master OpenSSL configuration file, or optionally an alternative configuration file. The openssl(1) utility includes this functionality: any sub command uses the master OpenSSL configuration file unless an option is used in the sub command to use an alternative configuration file.

To enable library configuration, the default section needs to contain an appropriate line which points to the main configuration section. The default name is openssl_conf, which is used by the openssl(1) utility. Other applications may use an alternative name such as myapplication_conf. All library configuration lines appear in the default section at the start of the configuration file.

The configuration section should consist of a set of name value pairs which contain specific module configuration information. The name represents the name of the configuration module. The meaning of the value is module specific: it may, for example, represent a further configuration section containing configuration module specific information. For example:

# The following line must be in the default section.
openssl_conf = openssl_init

[openssl_init]
oid_section = new_oids
engines = engine_section

[new_oids]
... new oids here ...

[engine_section]
... engine stuff here ...

The features of each configuration module are described below.

ASN1 Object Configuration Module

This module has the name oid_section. The value of this variable points to a section containing name value pairs of OIDs: the name is the OID short and long name, and the value is the numerical form of the OID. Although some of the openssl(1) utility subcommands already have their own ASN1 OBJECT section functionality, not all do. By using the ASN1 OBJECT configuration module, all the openssl(1) utility subcommands can see the new objects as well as any compliant applications. For example:

[new_oids]
some_new_oid = 1.2.3.4
some_other_oid = 1.2.3.5

It is also possible to set the value to the long name followed by a comma and the numerical OID form. For example:

shortName = some object long name,
  1.2.3.4

Engine Configuration Module

This ENGINE configuration module has the name engines. The value of this variable points to a section containing further ENGINE configuration information.

The section pointed to by engines is a table of engine names (though see engine_id below) and further sections containing configuration information specific to each ENGINE.

Each ENGINE specific section is used to set default algorithms, load dynamic ENGINEs, perform initialization and send ctrls. The actual operation performed depends on the command name which is the name of the name value pair. The currently supported commands are listed below.

For example:

[engine_section]
# Configure ENGINE named "foo"
foo = foo_section
# Configure ENGINE named "bar"
bar = bar_section

[foo_section]
... foo ENGINE specific commands ...

[bar_section]
... "bar" ENGINE specific commands ...

The command engine_id is used to give the ENGINE name. If used this command must be first. For example:

[engine_section]
# This would normally handle an ENGINE named "foo"
foo = foo_section

[foo_section]
# Override default name and use "myfoo" instead.
engine_id = myfoo

The command dynamic_path loads and adds an ENGINE from the given path. It is equivalent to sending the ctrls SO_PATH with the path argument followed by LIST_ADD with value 2 and LOAD to the dynamic ENGINE. If this is not the required behaviour then alternative ctrls can be sent directly to the dynamic ENGINE using ctrl commands.

The command init determines whether to initialize the ENGINE. If the value is 0, the ENGINE will not be initialized. If it is 1, an attempt is made to initialized the ENGINE immediately. If the init command is not present, then an attempt will be made to initialize the ENGINE after all commands in its section have been processed.

The command default_algorithms sets the default algorithms an ENGINE will supply using the functions ENGINE_set_default_string(3).

If the name matches none of the above command names it is assumed to be a ctrl command which is sent to the ENGINE. The value of the command is the argument to the ctrl command. If the value is the string EMPTY, then no value is sent to the command.

For example:

[engine_section]
# Configure ENGINE named "foo"
foo = foo_section

[foo_section]
# Load engine from DSO
dynamic_path = /some/path/fooengine.so
# A foo specific ctrl.
some_ctrl = some_value
# Another ctrl that doesn't take a value.
other_ctrl = EMPTY
# Supply all default algorithms
default_algorithms = ALL

FILES

/etc/ssl/openssl.cnf: standard configuration file

EXAMPLES

Here is a sample configuration file using some of the features mentioned above:

# This is the default section.
HOME=/temp
RANDFILE= ${ENV::HOME}/.rnd
configdir=$ENV::HOME/config

[ section_one ]
# We are now in section one.

# Quotes permit leading and trailing whitespace
any = " any variable name "

other = A string that can \
cover several lines \
by including \\ characters

message = Hello World\n

[ section_two ]
greeting = $section_one::message

This next example shows how to expand environment variables safely.

Suppose you want a variable called tmpfile to refer to a temporary filename. The directory it is placed in can determined by the TEMP or TMP environment variables but they may not be set to any value at all. If you just include the environment variable names and the variable doesn't exist then this will cause an error when an attempt is made to load the configuration file. By making use of the default section both values can be looked up with TEMP taking priority and /tmp used if neither is defined:

TMP=/tmp
# The above value is used if TMP isn't in the environment
TEMP=$ENV::TMP
# The above value is used if TEMP isn't in the environment
tmpfile=${ENV::TEMP}/tmp.filename

More complex OpenSSL library configuration. Add OID:

# Default appname: should match "appname" parameter (if any)
# supplied to CONF_modules_load_file et al.
openssl_conf = openssl_conf_section

[openssl_conf_section]
# Configuration module list
alg_section = evp_sect
oid_section = new_oids

[new_oids]
# New OID, just short name
newoid1 = 1.2.3.4.1
# New OID shortname and long name
newoid2 = New OID 2 long name, 1.2.3.4.2

The above examples can be used with any application supporting library configuration if "openssl_conf" is modified to match the appropriate "appname".

For example if the second sample file above is saved to "example.cnf" then the command line:

OPENSSL_CONF=example.cnf openssl
  asn1parse -genstr OID:1.2.3.4.1

will output:

0:d=0 hl=2 l= 4 prim: OBJECT
  :newoid1

showing that the OID "newoid1" has been added as "1.2.3.4.1".

CAVEATS

If a configuration file attempts to expand a variable that doesn't exist, then an error is flagged and the file will not load. This can also happen if an attempt is made to expand an environment variable that doesn't exist. For example, in a previous version of OpenSSL the default OpenSSL master configuration file used the value of HOME which may not be defined on non Unix systems and would cause an error.

This can be worked around by including a default section to provide a default value: then if the environment lookup fails, the default value will be used instead. For this to work properly, the default value must be defined earlier in the configuration file than the expansion. See the EXAMPLES section for an example of how to do this.

If the same variable is defined more than once in the same section, then all but the last value will be silently ignored. In certain circumstances such as with DNs, the same field may occur multiple times. This is usually worked around by ignoring any characters before an initial ‘.’, for example:

1.OU="My first OU"
2.OU="My Second OU"

BUGS

Currently there is no way to include characters using the octal \nnn form. Strings are all NUL terminated, so NUL bytes cannot form part of the value.

The escaping isn't quite right: if you want to use sequences like ‘\n’, you can't use any quote escaping on the same line.

Files are loaded in a single pass. This means that a variable expansion will only work if the variables referenced are defined earlier in the file.