Conditional rewrites

Starting with 3.2, it is possible to apply a rewrite rule to a message only if certain conditions are met. The condition() option effectively embeds a filter expression into the rewrite rule: the message is modified only if the message passes the filter. If the condition is not met, the message is passed to the next element of the log path (that is, the element following the rewrite rule in the log statement, for example, the destination). Any filter expression normally used in filters can be used as a rewrite condition. Existing filter statements can be referenced using the filter() function within the condition. For details on filters, see Filters.

How conditional rewriting works

Purpose:

The following procedure summarizes how conditional rewrite rules (rewrite rules that have the condition() parameter set) work. The following configuration snippet is used to illustrate the procedure:

rewrite r_rewrite_set{
    set(
        "myhost",
        value("HOST")
        condition(program("myapplication"))
    );
};
log {
    source(s1);
    rewrite(r_rewrite_set);
    destination(d1);
};

rewrite r_rewrite_set{set("myhost", value("HOST") condition(program("myapplication")));};

filter f_rewritefilter {program("myapplication");};
rewrite r_rewrite_set{set("myhost", value("HOST") condition(filter(f_rewritefilter)));};

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.

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.