syslog-ng Open Source Edition 3.18 - Administration Guide

Regular expressions

Filters and substitution rewrite rules can use regular expressions. In regular expressions, the characters ()[].*?+^$|\ are used as special symbols. Depending on how you want to use these characters and which quotation mark you use, these characters must be used differently, as summarized below.

• Strings between single quotes ('string') are treated literally and are not interpreted at all, you do not have to escape special characters. For example the output of '\x41' is \x41 (characters as follows: backslash, x(letter), 4(number), 1(number)). This makes writing and reading regular expressions much more simple: it is recommended to use single quotes when writing regular expressions.

• When enclosing strings between double-quotes ("string"), the string is interpreted and you have to escape special characters, that is, to precede them with a backslash (\) character if they are meant literally. For example the output of the "\x41" is simply the letter a. Therefore special characters like \(backslash) or "(quotation mark) must be escaped (\\ and \"). The following expressions are interpreted: \a, \n, \r, \t, \v. For example, the \$40 expression matches the $40 string. Backslashes have to be escaped as well if they are meant literally, for example, the \\d expression matches the \d string.

  TIP: If you use single quotes, you do not need to escape the backslash, for example match("\\.") is equivalent to match('\.').

• Enclosing alphanumeric strings between double-quotes ("string") is not necessary, you can just omit the double-quotes. For example when writing filters, match("sometext") and match(sometext) will both match for the sometext string.

  NOTE: Only strings containing alphanumerical characters can be used without quotes or double quotes. If the string contains whitespace or any special characters (()[].*?+^$|\ or ;:#), you must use quotes or double quotes. When using the ;:# characters, you must use quotes or double quotes, but escaping them is not required.

By default, all regular expressions are case sensitive. To disable the case sensitivity of the expression, add the flags(ignore-case) option to the regular expression.

filter demo_regexp_insensitive {
    host("system" flags(ignore-case));
};

The regular expressions can use up to 255 regexp matches (${1} ... ${255}), but only from the last filter and only if the flags("store-matches") flag was set for the filter. For case-insensitive searches, use the flags("ignore-case") option.

Types and options of regular expressions

By default, syslog-ng uses PCRE-style regular expressions. To use other expression types, add the type() option after the regular expression.

The syslog-ng OSE application supports the following expression types:

• Perl Compatible Regular Expressions (PCRE)

• Literal string searches

• Glob patterns without regular expression support

rewrite r_rewrite_subst {
    subst("a*", "?", value("MESSAGE") flags("utf8" "global"));  
};

Optimizing regular expressions

The host(), match(), and program() filter functions and some other syslog-ng objects accept regular expressions as parameters. But evaluating general regular expressions puts a high load on the CPU, which can cause problems when the message traffic is very high. Often the regular expression can be replaced with simple filter functions and logical operators. Using simple filters and logical operators, the same effect can be achieved at a much lower CPU load.

xntpd[1567]: time error -1159.777379 is too large (set clock manually);

filter f_demo_regexp {
    program("demo_program") and
    match("time error .* is too large .* set clock manually");
};

filter f_demo_optimized_regexp {
    program("demo_program") and
    match("time error") and
    match("is too large") and
    match("set clock manually");
};

parser: Parse and segment structured messages

The filters and default macros of syslog-ng work well on the headers and metainformation of the log messages, but are rather limited when processing the content of the messages. Parsers can segment the content of the messages into name-value pairs, and these names can be used as user-defined macros. Subsequent filtering or other type of processing of the message can use these custom macros to refer to parts of the message. Parsers are global objects most often used together with filters and rewrite rules.

The syslog-ng OSE application provides the following possibilities to parse the messages, or parts of the messages:

• By default, syslog-ng OSE parses every message as a syslog message. To disable message parsing, use the flags(no-parse) option of the source. To explicitly parse a message as a syslog message, use the syslog parser. For details, see Parsing syslog messages.

• To segment a message into columns using a CSV-parser, see Parsing messages with comma-separated and similar values.

• To segment a message consisting of whitespace or comma-separated key=value pairs (for example, Postfix log messages), see Parsing key=value pairs.

• To parse JSON-formatted messages, see The JSON parser The JSON parser.

• To parse XML-formatted messages, see The XML parser.

• To parse a specially-formatted date or timestamp, see Parsing dates and timestamps.

• To write a custom parser in Python, see The Python Parser.

• To identify and parse the messages using a pattern database, see db-parser: Process message content with a pattern database (patterndb).

The syslog-ng OSE application provides built-in parsers for the following application logs:

• Apache HTTP server access logs. For details, see The Apache Access Log Parser.

• Cisco devices. For details, see The Cisco Parser.

• Linux Audit (auditd) logs. For details, see The Linux Audit Parser.

• osquery result logs. For details, see osquery: Collect and parse osquery result logs.

• SNMP traps of the Net-SNMP's snmptrapd application. For details, see snmptrap: Read Net-SNMP traps.