Configuration File Format

The server configuration files all share a standard format. A common format ensures that it is simple to read, edit, and understand the configuration.

When the server starts, it loads all of the configurations files. These files are usually kept separate on disk for organization and management purpose. However, the files are all part of the global "server configuration". That is, the organization of the file names is completely independent of the file contents.

The configuration files also include the unlang policies. While those policies have a similar format as described here, the formats ar not exactly the same.

Comments and Blank Lines

The configuration files may contain comments and blank lines. Comments are text which has meaning to the administrator, but which is not used to configure the server.

Comments begin with the character, and can be placed anywhere on a line. All text after the character is ignored by the server.

Syntax

 # this is a comment

The server ignores all blank lines and comments.

Named Variables

The configuration files can set named variables which are given a value. These variables are also referred to as configuration items.

Syntax

name = value

name: The name of the configuration variable.

The name is usually a descriptive English word. It can be composed of letters (upper and lower case), numbers, and underscores.

The allowed names change depending on the context of the variable.
operator: The = character is the only permitted operator for configuration variables.
value: The value of the configuration variable.

The value of a variable is a value which is assigned to name. Types of values are unquoted text, single-quoted strings, and double-quoted strings. Back-tick strings are not permitted.

The interpretation of the value depends on the meaning of name. The parser is generally forgiving, and will accept anything that can be interpreted as being in the correct format. For example, when a configuration variable is documented as taking an IP address, all of the following examples result in the same configuration:

Example 1. Equivalent Configurations

ipaddr = 192.0.2.2
ipaddr = '192.0.2.2'
ipaddr = "  192.0.2.2"

Unquoted Strings

Unquoted strings delimited by spaces, or by other tokens. This unquoted text is interpreted as a simple string and are generally equivalent to placing the string in single quotes.

Example 2. Unquoted Strings

ipaddr = 192.168.0.2
delay = 1
locking = yes
filename = /path/

Single-Quoted Strings

A single-quoted string allow it to contain spaces, among other special characters. The single quote character can be placed in such a string by escaping it with a backslash.

Example 3. Single-Quoted Strings

message = 'Hello there'
filter = 'yes \` is allowed`

Double-Quoted Strings

A double-quoted string allows escape sequences and variable references. As with single-quoted strings, text within double quotes can include spaces.

The main difference between double quoted strings and other types is that the double quoted strings can expand variable references. Please see the Variable References section for more information.

The contents of a double-quoted can be interpreted as a string, a number, or an IP address, depending on the configuration varible.

Example 4. Double-Quoted Strings

message = "Hello there"
filter = "yes \" is allowed"

Escape sequences

Escape sequences in double-quoted strings allow characters which may be otherwise difficult to represent.

Table 1. Escape sequences and their descriptions
Escape sequence	Character represented
`\\`	Literal backslash (0x5c)
`\r`	Carriage return (0x0d)
`\n`	Line feed (0x0a)
`\t`	Horizontal tab (0x09)
`\"`	Double quote (0x22)
`\x<hex><hex>`	A byte whose numerical value is given by `<hex><hex>` interpreted as a hexadecimal number.
`\x<oct><oct><oct>`	A byte whose numerical value is given by `<oct><oct><oct>` interpreted as an octal number.

Configuration Sections

A configuration section is allows named variables to be collected togther in a logical group. Configuration sections can also contain other configuration sections, to any depth of nesting.

A configuration section begins with a section name, followed on the same line by an open bracket {. A section ends with a close bracket }.

Syntax

<name> [<instance>] {
    [ statements ]
}

<name>: The name of this configuration section. The set of allowed names depends on the context.
<instance>: An optional secton name.
[ statements ]: Zero or more statements, including comments, variable assignments, and other sections.

Example 5. Configuration Section

group {
    foo = bar
    baz = hello
    subgroup {
        bug = gone
    }
}

In some circumstances, configuration section can have a second name following the first one. The situations where this is permitted depend on the context. Please see the configuration file contents for information on when and where these second names are allowed.

We also use the word instance or instance names when referring to the second name of a section.

Example 6. Configuration Section

group mine {
    yours = bob
    theirs = no
}

Additional Functionality

The configuration file format has additional functionality which is described here.

Long Lines

Long lines can be broken up via continuations, using \\ as the last character of the line. When one line is continued to another, the result is treated as if all of the "continued" text was on one line.

Example 7. Text Broken across multiple lines

foo = "blah \
blah \
blah"

This example will set the value of the variable foo to blah blah blah. Any CR or LF is not turned into a space, but all other whitespace is preserved in the final value.

Including other files

As noted above, the configuration is logically one global "server configuration", but is split up into multiple files on disk. This splitting is done by the use of a $INCLUDE statement.

Syntax

$INCLUDE <filename>
-$INCLUDE <filename>

$INCLUDE: Key word to load the <filename>, and to fail if the file does not exist, or cannot be parsed
-$INCLUDE: Key word to load the <filename>, and to silently do nothing if the file does not exist. If the file does exist, this keyword behaves exactly the same as $INCLUDE.
<filename>: The filename to include

If <filename> begins with a / character, then it is assumed to be a full path to the file, starting at the "root" directory of the file system.

If <filename> does not begin with a / character, then it is assumed to be a path relative to the current file being read.

Example 8. Including other files

foo = bar
$INCLUDE other.conf

Configuration as a Database

The configuration file parser is independent of the server configuration. This means that you can put almost anything into the configuration file. So long as it is properly formatted, the server will start.

When the server parses the configuration file, it looks only for those configurations it understands. Extra configuration items are ignored. This "feature" can be (ab)used in certain interesting ways.

For example, you can "annotate" a configuration section with additional variables which will be accepted, but ignored. These variables can be used as a simple read-only key-value store.

Variable References

When the value of a configuration variable is a double-quoted string, the parser looks in the input string for variable references. If a reference is found to another variable, then the reference is replaced with the value of that variable. The value can also be just an unquoted variable reference.

These references are evaluated when the configuration file is loaded, which means that there is no run-time cost associated with them. This feature is most useful for turning long, repeated pieces of text into short ones.

Variables are referenced by ${variable_name}, as in the following examples.

Example 9. Simple Variable References

foo = bar       # set variable 'foo' to value 'bar'
who = ${foo}    # sets variable 'who' to value of variable 'foo'
my = "${foo} a" # sets variable 'my' to "bar a"

If the variable exists in a section or subsection, it can be referenced as ${section.subsection.variable}. Forward references are not allowed.

Relative references are allowed, by prepending the name with one or more periods.

Example 10. References in the Current Section

foo = bar
blogs = ${.foo}  # set 'blogs' to 'bar'
ergo = "${foo}"  # set 'ergo' to 'bar'

Example 11. Parent References

group {
    foo = bar
    subgroup {
        blogs = ${..foo}
    }
}

Will set variable blogs to the value bar. That value is taken from the variable foo, which is contained in the a parent section of the section containing blogs

Example 12. References to Other Sections

blogs = ${modules.detail.filename}

Will set variable blogs to the value of variable filename, of the detail module, which is in the modules section of the configuration file.

Properties of anonymous parent sections may also be referenced, by using a : after the section reference. Currently :name and :instance are supported.

Example 13. References to the Name of a Section

modules {
    example foo {
        file = ${.:name}
    }
}

Will set variable file to the name of the containing section, in this case, example.

Example 14. References to the Instance of a Section

modules {
    example foo {
        file = ${.:instance}
    }
}

Will set variable file to the instance name of the containing section, in this case foo.

Example 15. Combining References

modules {
    example foo {
        file = ${..:name}
    }
}

Will set variable file to the name of the parent of the containing section, in this case modules.

A string can contain multiple references, which will all be expanded.

Example 16. Multiple References

foo = bar
baz = bug
blogs = "this ${foo} is ${baz}"

Will set variable blogs to the string this bar is bug.