syslog-ng Open Source Edition 3.25 - Administration Guide

Adding and deleting tags

To add or delete a tag, you can use rewrite rules. To add a tag, use the following syntax:

rewrite <name_of_the_rule> {
    set-tag("<tag-to-add>");
};

To delete a tag, use the following syntax:

rewrite <name_of_the_rule> {
    clear-tag("<tag-to-delete>");
};

You cannot use macros in the tags.

Rewrite the timezone of a message

Starting with version 3.24 of the syslog-ng Open Source Edition (syslog-ng OSE) application, you can manipulate the timezone information of messages using rewrite rules. You can:

• Set a timezone to a specific value

• Fix a timezone if it was improperly parsed

• Assuming the sender is sending messages in near-real-time, syslog-ng OSE can guess the timezone

By default, these operations modify the date-related macros of the message that correspond to the date the message was sent (that is, the S_ macros). You can modify the dates when syslog-ng OSE has received the messages (that is, the R_ macros), but this is rarely needed. To do so, include the time-stamp(recvd) option in the operation, for example:

rewrite { fix-time-zone("EST5EDT" time-stamp(recvd)); };

rewrite { fix-time-zone("EST5EDT"); };

Anonymizing credit card numbers

Log messages of banking and e-commerce applications might include credit card numbers (Primary Account Number or PAN). According to privacy best practices and the requirements of the Payment Card Industry Data Security Standards (PCI-DSS), PAN must be rendered unreadable. The syslog-ng OSE application uses a regular expression to detect credit card numbers, and provides two ways to accomplish this: you can either mask the credit card numbers, or replace them with a hash. To mask the credit card numbers, use the credit-card-mask() or the credit-card-hash() rewrite rules in a log path.

@include "scl/rewrite/cc-mask.conf"

rewrite {
    credit-card-mask(value("<message-field-to-process>"));
};

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.