syslog-ng Open Source Edition 3.16 - Administration Guide

How relaying log messages works

Depending on your exact needs about relaying log messages, there are many scenarios and syslog-ng OSE options that influence how the log message will look like on the logserver. Some of the most common cases are summarized in the following example.

Consider the following example: client-host > syslog-ng-relay > syslog-ng-server, where the IP address of client-host is 192.168.1.2. The client-host device sends a syslog message to syslog-ng-relay. Depending on the settings of syslog-ng-relay, the following can happen.

• By default, the keep-hostname() option is disabled, so syslog-ng-relay writes the IP address of the sender host (in this case, 192.168.1.2) to the HOST field of the syslog message, discarding any IP address or hostname that was originally in the message.

• If the keep-hostname() option is enabled on syslog-ng-relay, but name resolution is disabled (the use-dns() option is set to no), syslog-ng-relay uses the HOST field of the message as-is, which is probably 192.168.1.2.

• To resolve the 192.168.1.2 IP address to a hostname on syslog-ng-relay using a DNS server, use the keep-hostname(no) and use-dns(yes) options. If the DNS server is properly configured and reverse DNS lookup is available for the 192.168.1.2 address, syslog-ng OSE will rewrite the HOST field of the log message to client-host.

  NOTE: It is also possible to resolve IP addresses locally, without relying on the DNS server. For details on local name resolution, see Resolving hostnames locally.

• The above points apply to the syslog-ng OSE server (syslog-ng-server) as well, so if syslog-ng-relay is configured properly, use the keep-hostname(yes) option on syslog-ng-server to retain the proper HOST field. Setting keep-hostname(no) on syslog-ng-server would result in syslog-ng OSE rewriting the HOST field to the address of the host that sent the message to syslog-ng-server, which is syslog-ng-relay in this case.

• If you cannot or do not want to resolve the 192.168.1.2 IP address on syslog-ng-relay, but want to store your log messages on syslog-ng-server using the IP address of the original host (that is, client-host), you can enable the spoof-source() option on syslog-ng-relay. However, spoof-source() works only under the following conditions:

  • The syslog-ng OSE binary has been compiled with the --enable-spoof-source option.

  • The log messages are sent using the highly unreliable UDP transport protocol. (Extremely unrecommended.)

The syslog-ng OSE configuration file

Location of the syslog-ng configuration file

To configure syslog-ng OSE, edit the syslog-ng.conf file with any regular text editor application. The location of the configuration file depends on how you installed syslog-ng OSE. Native packages of a platform (like the ones downloaded from Linux repositories) typically place the configuration file under the /etc/syslog-ng/ directory.

The configuration syntax in detail

Every syslog-ng configuration file must begin with a line containing the version information of syslog-ng. For syslog-ng version 3.16, this line looks like:

@version: 3.16

Versioning the configuration file was introduced in syslog-ng 3.0. If the configuration file does not contain the version information, syslog-ng assumes that the file is for syslog-ng 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 will correctly operate with configuration files of version 2.x, but the default values of certain parameters have changed since 3.0.

@version: 3.16
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 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 configuration file are case sensitive.

    TIP: 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.

    NOTE: 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 syslog-ng OSE should use DNS resolution to resolve IP addresses. Global options are detailed in Global options of syslog-ng OSE.

  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));
  };