syslog-ng Premium Edition 7.0.19 - Administration Guide

Unsetting message fields

You can unset a macro or a field of the message, including any user-defined macros created using parsers (for details, see parser: Parse and segment structured messages and Processing message content with a pattern database). Hard macros cannot be modified. For details on hard and soft macros, see Hard versus soft macros). Note that the unset operation completely deletes any previous value of the field that you apply it on. Use the following syntax:

Declaration

rewrite <name_of_the_rule> {
    unset(value("<field name>"));
};

rewrite r_rewrite_unset{unset(value("HOST"));};

To unset a group of fields, you can use the groupunset() rewrite rule.

Declaration

rewrite <name_of_the_rule> {
    groupunset(values("<expression-for-field-names>"));
};

rewrite r_rewrite_unset_SDATA{ groupunset(values(".SDATA.*"));};

Creating custom SDATA fields

If you use RFC5424-formatted (IETF-syslog) messages, you can also create custom fields in the SDATA part of the message (For details on the SDATA message part, see The STRUCTURED-DATA message part). According to RFC5424, the name of the field (its SD-ID) must not contain the @ character for reserved SD-IDs. Custom SDATA fields must be in the following format: .SDATA.group-name@<private enterprise number>.field-name, for example, .SDATA.mySDATA-field-group@18372.4.mySDATA-field. (18372.4 is the private enterprise number of One Identity LLC, the developer of syslog-ng PE.)

rewrite r_sd {
    set("55555" value(".SDATA.meta.sequenceId"));
};

rewrite r_rewrite_set {
    set("${SOURCEIP}" value(".SDATA.custom@18372.4.sourceip"));
};

Setting multiple message fields to specific values

The groupset() rewrite rule allows you to modify the value of multiple message fields at once, for example, to change the value of sensitive fields extracted using patterndb, or received in a JSON format.

• The first parameter is the new value of the modified fields. This can be a simple string, a macro, or a template (which can include template functions as well).

• The second parameter (values()) specifies the fields to modify. You can explicitly list the macros or fields (a space-separated list with the values enclosed in double-quotes), or use wildcards and glob expressions to select multiple fields.

• Note that groupset() does not create new fields, it only modifies existing fields.

• You can refer to the old value of the field using the $_ macro. This is resolved to the value of the current field, and is available only in groupset() rules.

Declaration

rewrite <name_of_the_rule> {
    groupset("<new-value-of-the-fields>", values("<field-name-or-glob>" ["<another-field-name-or-glob>"]));
};

Conditional rewrites

Starting with 4 F1, 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.