The configuration syntax in detail

Every syslog-ng.conf configuration file must begin with a line containing version information. For syslog-ng version 4.9, this line looks like:

   @version: 4.9.0

• If the configuration file does not contain the version information, syslog-ng assumes that the file is for version 2.x. In this case it interprets the configuration and sends warnings about the parts of the configuration that should be updated. Version 3.0 and later can operate with configuration files of version 2.x, but the default values of certain parameters have changed since 3.0.
• @version: current sets the configuration version to the currently installed version.

Example: A simple configuration file

The following is a very simple configuration file for syslog-ng: it collects the internal messages of syslog-ng and the messages from /dev/log into the /var/log/messages_syslog-ng.log file.

   @version: 4.9.0
    source s_local {
        unix-dgram("/dev/log"); internal();
    };
    destination d_file {
        file("/var/log/messages_syslog-ng.log");
    };
    log {
        source(s_local); destination(d_file);
    };

As a syslog-ng user described on a mailing list:

Alan McKinnon

The syslog-ng’s config file format was written by programmers for programmers to be understood by programmers. That may not have been the stated intent, but it is how things turned out. The syntax is exactly that of C, all the way down to braces and statement terminators.

• The main body of the configuration file consists of object definitions: sources, destinations, logpaths define which log message are received and where they are sent. All identifiers, option names and attributes, and any other strings used in the syslog-ng.conf configuration file are case sensitive. Object definitions (also called statements) have the following syntax:

  type-of-the-object identifier-of-the-object {<parameters>};
  

  • Type of the object: One of source, destination, log, filter, parser, rewrite rule, or template.

  • Identifier of the object: A unique name identifying the object. When using a reserved word as an identifier, enclose the identifier in quotation marks.

    All identifiers, attributes, and any other strings used in the syslog-ng.conf configuration file are case sensitive.

    • Use identifiers that refer to the type of the object they identify. For example, prefix source objects with s_, destinations with d_, and so on.
    • Repeating a definition of an object (that is, defining the same object with the same id more than once) is not allowed, unless you use the @define allow-config-dups 1 definition in the configuration file.

  • Parameters: The parameters of the object, enclosed in braces {parameters}.

  • Semicolon: Object definitions end with a semicolon (;).

  For example, the following line defines a source and calls it s_internal.

  source s_internal {
      internal();
  };
  

  The object can be later referenced in other statements using its ID, for example, the previous source is used as a parameter of the following log statement:

  log {
      source(s_internal); destination(d_file);
  };
  

• The parameters and options within a statement are similar to function calls of the C programming language: the name of the option followed by a list of its parameters enclosed within brackets and terminated with a semicolon.

  option(parameter1, parameter2); option2(parameter1, parameter2);
  

  For example, the file() driver in the following source statement has three options: the filename (/var/log/apache/access.log), follow-freq(), and flags(). The follow-freq() option also has a parameter, while the flags() option has two parameters.

  source s_tail {
      file("/var/log/apache/access.log" follow-freq(1) flags(no-parse, validate-utf8));
  };
  

  Objects may have required and optional parameters. Required parameters are positional, meaning that they must be specified in a defined order. Optional parameters can be specified in any order using the option(value) format. If a parameter (optional or required) is not specified, its default value is used. The parameters and their default values are listed in the reference section of the particular object.

  Example: Using required and optional parameters

  The unix-stream() source driver has a single required argument: the name of the socket to listen on. Optional parameters follow the socket name in any order, so the following source definitions have the same effect:

  source s_demo_stream1 {
      unix-stream("<path-to-socket>" max-connections(10) group(log));
  };
  source s_demo_stream2 {
      unix-stream("<path-to-socket>" group(log) max-connections(10));
  };
  

• Some options are global options, or can be set globally, for example, whether AxoSyslog should use DNS resolution to resolve IP addresses. Global options are detailed in Global options.

  options {
      use-dns(no);
  };
  

• Objects can be used before definition.

• Objects can be defined inline as well. This is useful if you use the object only once (for example, a filter). For details, see Defining configuration objects inline.

• To add comments to the configuration file, start a line with # and write your comments. These lines are ignored by syslog-ng.

  # Comment: This is a stream source
  source s_demo_stream {
      unix-stream("<path-to-socket>" max-connections(10) group(log));
  };