The syslog-ng application supports including external files in its configuration file, so parts of its configuration can be managed separately. To include the contents of a file in the syslog-ng configuration, use the following syntax:

@include "<filename>"

This imports the entire file into the configuration of syslog-ng OSE, at the location of the include statement. The <filename> can be one of the following:

A filename, optionally with full path. The filename (not the path) can include UNIX-style wildcard characters (*, ?). When using wildcard characters, syslog-ng OSE will include every matching file. For details on using wildcard characters, see Options of regular expressions.
A directory. When including a directory, syslog-ng OSE will try to include every file from the directory, except files beginning with a ~ (tilde) or a . (dot) character. Including a directory is not recursive. The files are included in alphabetic order, first files beginning with uppercase characters, then files beginning with lowercase characters. For example, if the directory contains the a.conf, B. conf, c.conf, D.conf files, they will be included in the following order: B.conf, D. conf, a.conf, c.conf.

When including configuration files, consider the following points:

The default path where syslog-ng OSE looks for the file depends on where syslog-ng OSE is installed. The syslog-ng --version command displays this path as Include-Path.
Defining an object twice is not allowed, unless you use the @define allow-config-dups 1 definition in the configuration file. If an object is defined twice (for example, the original syslog-ng configuration file and the file imported into this configuration file both define the same option, source, or other object), then the object that is defined later in the configuration file will be effective. For example, if you set a global option at the beginning of the configuration file, and later include a file that defines the same option with a different value, then the option defined in the imported file will be used.
Files can be embedded into each other: the included files can contain include statements as well, up to a maximum depth of 15 levels.
You cannot include complete configuration files into each other, only configuration snippets can be included. This means that the included file cannot have a @version statement.
Include statements can only be used at top level of the configuration file. For example, the following is correct:
```
@version: 3.38
@include "example.conf"
```
But the following is not:
```
source s_example {
    @include "example.conf"
};
```

Caution:

The syslog-ng application will not start if it cannot find a file that is to be included in its configuration. Always double-check the filenames, paths, and access rights when including configuration files, and use the --syntax-only command-line option to check your configuration.

Reusing configuration blocks

The syslog-ng OSE configuration file > Managing complex syslog-ng configurations > Reusing configuration blocks

To create a reusable configuration snippet and reuse parts of a configuration file, you have to define the block (for example, a source) once, and reference it later. (Such reusable blocks are sometimes called a Source Configuration Library, or SCL.) Any syslog-ng object can be a block. Use the following syntax to define a block:

block type name() {<contents of the block>};

Type must be one of the following: destination, filter, log, options, parser, rewrite, root, source. The root blocks can be used in the "root" context of the configuration file, that is, outside any other statements.

Note that options can be used in blocks only in version 3.22 and later.

Blocks may be nested into each other, so for example, a block can be built from other blocks. Blocks are somewhat similar to C++ templates.

The type and name combination of each block must be unique, that is, two blocks can have the same name if their type is different.

To use a block in your configuration file, you have to do two things:

Include the file defining the block in the syslog-ng.conf file — or a file already included into syslog-ng.conf. Version 3.7 and newer automatically includes the *.conf files from the <directory-where-syslog-ng-is-installed>/scl/*/ directories.
Reference the name of the block in your configuration file. This will insert the block into your configuration. For example, to use a block called myblock, include the following line in your configuration:
```
myblock()
```
Blocks may have parameters, but even if they do not, the reference must include opening and closing parentheses like in the previous example.

The contents of the block will be inserted into the configuration when syslog-ng OSE is started or reloaded.

Example: Reusing configuration blocks

Suppose you are running an application on your hosts that logs into the /opt/var/myapplication.log file. Create a file (for example, myblocks.conf) that stores a source describing this file and how it should be read:

block source myappsource() {
        file("/opt/var/myapplication.log" follow-freq(1) default-facility(syslog)); };

Include this file in your main syslog-ng configuration file, reference the block, and use it in a logpath:

@version: 3.38
@include "<correct/path>/myblocks.conf"
source s_myappsource { myappsource(); };
...
log { source(s_myappsource); destination(...); };

To define a block that defines more than one object, use root as the type of the block, and reference the block from the main part of the syslog-ng OSE configuration file.

Example: Defining blocks with multiple elements

The following example defines a source, a destination, and a log path to connect them.

block root mylogs() {
    source s_file {
        file("/var/log/mylogs.log" follow-freq(1));
    };
    destination d_local {
        file("/var/log/messages");
    };
    log {
        source(s_file); destination(d_local);
    };
};

TIP: Since the block is inserted into the syslog-ng OSE configuration when syslog-ng OSE is started, the block can be generated dynamically using an external script if needed. This is useful when you are running syslog-ng OSE on different hosts and you want to keep the main configuration identical.

If you want to reuse more than a single configuration object, for example, a logpath and the definitions of its sources and destinations, use the include feature to reuse the entire snippet. For details, see Including configuration files.

Mandatory parameters

You can express in block definitons that a parameter is mandatory by defining it with empty brackets (). In this case, the parameter must be overridden in the reference block. Failing to do so will result in an error message and initialization failure.

To make a parameter expand into nothing (for example, because it has no default value, like hook-commands() or tls()), insert a pair of double quote marks inside the empty brackets: ("")

Example: Mandatory parameters

The following example defines a TCP source that can receive the following parameters: the port where syslog-ng OSE listens (localport), and optionally source flags (flags).

block source my_tcp_source(localport() flags("")) {
    network(port(`localport`) transport(tcp) flags(`flags`));
};

Because localport is defined with empty brackets (), it is a mandatory parameter. However, the flags parameter is not mandatory, because it is defined with an empty double quote bracket pair (""). If you do not enter a specific value when referencing this parameter, the value will be an empty string. This means that in this case

my_tcp_source(localport(8080))

will be expanded to:

network(port(8080) transport(tcp) flags());

Passing arguments to configuration blocks

Configuration blocks can receive arguments as well. The parameters the block can receive must be specified when the block is defined, using the following syntax:

block type block_name(argument1(<default-value-of-the-argument>) argument2(<default-value-of-the-argument>) argument3())

If an argument does not have a default value, use an empty double quote bracket pair ("") after the name of the argument. To refer the value of the argument in the block, use the name of the argument between backticks (for example, `argument1`).

Example: Passing arguments to blocks

The following sample defines a file source block, which can receive the name of the file as a parameter. If no parameter is set, it reads messages from the /var/log/messages file.

block source s_logfile (filename("messages")) {
    file("/var/log/`filename`" );
};

source s_example {
    s_logfile(filename("logfile.log"));
};

If you reference the block with more arguments then specified in its definition, you can use these additional arguments as a single argument-list within the block. That way, you can use a variable number of optional arguments in your block. This can be useful when passing arguments to a template, or optional arguments to an underlying driver.

The three dots (…) at the end of the argument list refer to any additional parameters. It tells syslog-ng OSE that this macro accepts `__VARARGS__`, therefore any name-value pair can be passed without validation. To reference this argument-list, insert `__VARARGS__` to the place in the block where you want to insert the argument-list. Note that you can use this only once in a block.

The following definition extends the logfile block from the previous example, and passes the optional arguments (follow-freq(1) flags(no-parse)) to the file() source.

block source s_logfile(filename("messages") ...) {
    file("/var/log/`filename`" `__VARARGS__`);
};

source s_example {
    s_logfile(
        filename("logfile.log")
        follow-freq(1)
        flags(no-parse)
    );
};

Example: Using arguments in blocks

The following example is the code of the pacct() source driver, which is actually a block that can optionally receive two arguments.

block source pacct(file("/var/log/account/pacct") follow-freq(1) ...) {
    file("`file`" follow-freq(`follow-freq`) format("pacct") tags(".pacct") `__VARARGS__`);
};

Example: Defining global options in blocks

The following example defines a block called setup-dns() to set DNS-related settings at a single place.

block options setup-dns(use-dns()) {
    keep-hostname(no);
    use-dns(`use-dns`);
    use-fqdn(`use-dns`);
dns-cache(`use-dns`);
};

options {
    setup-dns(use-dns(yes));
};

Generating configuration blocks from a script

The syslog-ng OSE configuration file > Managing complex syslog-ng configurations > Generating configuration blocks from a script

Purpose:

The syslog-ng OSE application can automatically execute scripts when it is started, and can include the output of such script in the configuration file. To create and use a script that generates a part of the syslog-ng OSE configuration file (actually, a configuration block), complete the following steps. The steps include examples for collecting Apache access log files (access.log) from subdirectories, but you can create any script that creates a valid syslog-ng OSE configuration snippet.

Steps:

Navigate to the directory where you have installed syslog-ng OSE (for example, /opt/syslog-ng/share/include/scl/), and create a new directory, for example, apache-access-logs. The name of the directory will be used in the syslog-ng OSE configuration file as well, so use a descriptive name.
Create a file called plugin.conf in this new directory.
Edit the plugin.conf file and add the following line:
```
@module confgen context(source) name(<directory-name>) exec("`scl-root`/<directory-name>/<my-script>")
```
Replace <directory-name> with the name of the directory (for example, apache-access-logs), and <my-script> with the filename of your script (for example, apache-access-logs.sh). You can reference the script in your syslog-ng OSE configuration file as a configuration block using the value name option.

The context option determines the type of the configuration snippet that the script generates, and must be one of the following: destination, filter, log, parser, rewrite, root, source. The root blocks can be used in the "root" context of the configuration file, that is, outside any other statements. In the example, context(source) means that the output of the script will be used within a source statement.

You can pass parameters to the script. In the script these parameters are available as environment variables, and have the confgen_ prefix. For example, passing the --myparameter parameter becomes available in the script as the confgen_myparameter environment variable.
Write a script that generates the output you need, and formats it to a configuration snippet that syslog-ng OSE can use. The filename of the script must match with the filename used in plugin.conf, for example, apache-access-logs.sh.

The following example checks the /var/log/apache2/ directory and its subdirectories, and creates a source driver for every directory that contains an access.log file.
```
#!/bin/bash
for i in `find /var/log/apache2/ -type d`; do
    echo "file(\"$i/access.log\" flags(no-parse) program-override(\"apache2\"));";
done;
```
The script generates an output similar to this one, where service* is the actual name of a subdirectory:
```
file("/var/log/apache2/service1/access.log" flags(no-parse) program-override("apache2"));
file("/var/log/apache2/service2/access.log" flags(no-parse) program-override("apache2"));
```
Include the plugin.conf file in the syslog-ng.conf file — or a file already included into syslog-ng.conf. Version 3.7 and newer automatically includes the *.conf files from the <directory-where-syslog-ng-is-installed>/scl/*/ directories. For details on including configuration files, see Including configuration files.
Add the block you defined in the plugin.conf file to your syslog-ng OSE configuration file. You can reference the block using the value of the name option from the plugin.conf file, followed by parentheses, for example, apache-access-logs(). Make sure to use the block in the appropriate context of the configuration file, for example, within a source statement if the value of the context option in the plugin.conf file is source.
```
@include "scl.conf"
...
source s_apache {
    file("/var/log/apache2/access.log" flags(no-parse) program-override("apache2"));
    file("/var/log/apache2/error.log" flags(no-parse) program-override("apache2"));
    file("/var/log/apache2/ssl.log" flags(no-parse) program-override("apache2"));
    apache-access-logs();
};

log {
    source(s_apache); destination(d_central);
};
...
```
Check if your modified syslog-ng OSE configuration file is syntactically correct using the syslog-ng --syntax-only command.
If your modified configuration is syntactically correct, load the new configuration file using the syslog-ng-ctl reload command.

Python code in external files

The syslog-ng OSE configuration file > Python code in external files

You can extend and customize syslog-ng OSE easily by writing destinations, parsers, template functions, and sources in Python.

Instead of writing Python code into your syslog-ng OSE configuration file, you can store the Python code for your Python object in an external file. That way, it is easier to write, maintain, and debug the code. You can store the Python code in any directory in your system, but make sure to include it in your Python path.

When referencing a Python class from an external file in the class() option of a Python block in the syslog-ng OSE configuration file, the class name must include the name of the Python file containing the class, without the path and the .py extension. For example, if the MyDestination class is available in the /etc/syslog-ng/etc/pythonexample.py file, use class("pythonexample.MyDestination"):

destination d_python_to_file {
    python(
        class("pythonexample.MyDestination")
    );
};
log {
    source(src);
    destination(d_python_to_file);
};

NOTE: Starting with 3.26, syslog-ng OSE assigns a persist name to Python sources and destinations. The persist name is generated from the class name. If you want to use the same Python class multiple times in your syslog-ng OSE configuration, add a unique persist-name() to each source or destination, otherwise syslog-ng OSE will not start. For example:

log {
    source { python(class(PyNetworkSource) options("port" "8080") persist-name("<unique-string>); };
    source { python(class(PyNetworkSource) options("port" "8081")); };
};

Alternatively, you can include the following line in the Python package: @staticmethod generate_persist_name. For example:

from syslogng import LogSource
  class PyNetworSource(LogSource):
    @staticmethod
    def generate_persist_name(options):
        return options["port"]
    def run(self):
        pass
    def request_exit(self):
        pass

If you store the Python code in a separate Python file and only include it in the syslog-ng OSE configuration file, make sure that the PYTHON_PATH environment variable includes the path to the Python file, and export the PYTHON_PATH environment variable. For example, if you start syslog-ng OSE manually from a terminal and you store your Python files in the /opt/syslog-ng/etc directory, use the following command: export PYTHONPATH=/opt/syslog-ng/etc.

In production, when syslog-ng OSE starts on boot, you must configure your startup script to include the Python path. The exact method depends on your operating system. For recent Red Hat Enterprise Linux, Fedora, and CentOS distributions that use systemd, the systemctl command sources the /etc/sysconfig/syslog-ng file before starting syslog-ng OSE. (On openSUSE and SLES, /etc/sysconfig/syslog file.) Append the following line to the end of this file: PYTHONPATH="<path-to-your-python-file>", for example, PYTHONPATH="/opt/syslog-ng/etc".

To help debugging and troubleshooting your Python code, you can send log messages to the internal() source of syslog-ng OSE. For details, see Logging from your Python code.

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