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. Any AxoSyslog 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 AxoSyslog 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.conf configuration file, reference the block, and use it in a logpath:

   @version: 4.9.0
    @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 AxoSyslog 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);
        };
    };

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 AxoSyslog 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 AxoSyslog 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));
    };