OPENSSL.CNF(5) File Formats Manual OPENSSL.CNF(5)

openssl.cnfOpenSSL configuration files

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 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.

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 . 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

oid_section = new_oids

... new oids here ...

The features of each configuration module are described below.

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:

some_new_oid =
some_other_oid =

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,

standard configuration file

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

# This is the default section.

[ 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 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:

# The above value is used if TMP isn't in the environment
# The above value is used if TEMP isn't in the environment

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

# Configuration module list
oid_section = new_oids

# New OID, just short name
newoid1 =
# New OID shortname and long name
newoid2 = New OID 2 long name,

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:

will output:

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

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

openssl(1), CONF_modules_load_file(3), OPENSSL_config(3), x509v3.cnf(5)

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"

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.

November 19, 2023 OpenBSD 7.5