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12.2. Parsing messages with comma-separated and similar values

The syslog-ng PE application can separate parts of log messages (that is, the contents of the ${MSG} macro) at delimiter characters or strings to named fields (columns). One way to achieve this is to use a csv (comma-separated-values) parser (for other methods and possibilities, see the other sections of Chapter 12, Parsers and segmenting structured messages. The parsed fields act as user-defined macros that can be referenced in message templates, file- and tablenames, and so on.

Parsers are similar to filters: they must be defined in the syslog-ng PE configuration file and used in the log statement. You can also define the parser inline in the log path.


The order of filters, rewriting rules, and parsers in the log statement is important, as they are processed sequentially.

To create a csv-parser(), you have to define the columns of the message, the separator characters or strings (also called delimiters, for example, semicolon or tabulator), and optionally the characters that are used to escape the delimiter characters (quote-pairs()).


parser <parser_name> {
            columns(column1, column2, ...)
            delimiters(chars("<delimiter_characters>"), strings("<delimiter_strings>"))

Column names work like macros.

Names starting with a dot (for example, .example) are reserved for use by syslog-ng PE. If you use such a macro name as the name of a parsed value, it will attempt to replace the original value of the macro (note that only soft macros can be overwritten, see Section 11.1.4, Hard vs. soft macros for details). To avoid such problems, use a prefix when naming the parsed values, for example, prefix(my-parsed-data.)

Example 12.2. Segmenting hostnames separated with a dash

The following example separates hostnames like example-1 and example-2 into two parts.

parser p_hostname_segmentation {
    csv-parser(columns("HOSTNAME.NAME", "HOSTNAME.ID")
destination d_file { file("/var/log/messages-${HOSTNAME.NAME:-examplehost}"); };
log { source(s_local); parser(p_hostname_segmentation); destination(d_file);};
Example 12.3. Parsing Apache log files

The following parser processes the log of Apache web servers and separates them into different fields. Apache log messages can be formatted like:

"%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\" %T %v"

Here is a sample message: - - [31/Dec/2007:00:17:10 +0100] "GET /cgi-bin/example.cgi HTTP/1.1" 200 2708 "-" "curl/7.15.5 (i4 86-pc-linux-gnu) libcurl/7.15.5 OpenSSL/0.9.8c zlib/1.2.3 libidn/0.6.5" 2 example.balabit

To parse such logs, the delimiter character is set to a single whitespace (delimiters(" ")). Whitespaces between quotes and brackets are ignored (quote-pairs('""[]')).

parser p_apache {
         delimiters(" ")

The results can be used for example to separate log messages into different files based on the APACHE.USER_NAME field. If the field is empty, the nouser name is assigned.

log { source(s_local);
    parser(p_apache); destination(d_file);};
destination d_file { file("/var/log/messages-${APACHE.USER_NAME:-nouser}"); };
Example 12.4. Segmenting a part of a message

Multiple parsers can be used to split a part of an already parsed message into further segments. The following example splits the timestamp of a parsed Apache log message into separate fields.

parser p_apache_timestamp {
    delimiters("/: ")
log { source(s_local); parser(p_apache); parser(p_apache_timestamp); destination(d_file);

Further examples: