You can set the PRI value of a BSD or IETF syslog message with the set-pri() rewrite function by specifying a template string. This is useful, for example, if incoming messages do not have a PRI value specified by default, but a PRI value is required for filtering purposes.
When configured, the set-pri() function will only rewrite the PRI value of the message field.
NOTE: If the specified parameter value is not a valid value, the function ignores it and sends a debug message. However, the syslog-ng Open Source Edition (syslog-ng OSE) application will still send the message.
Declaration
rewrite <rule-name> {
set-pri("template-string");
};
Parameters
The set-pri() rewrite function expects a template string as its only parameter, for example:
Accepted values
The template string specified for the set-pri() rewrite function must expand to a natural number in the interval of 0–1023, inclusive. This means that if you, for example, extract the value from a syslog <PRI> header (such as <42>), then you need to remove the opening and closing brackets (< >) in the specified template string.
Example: Temporarily raising the priority of an application
In the following example, the set-pri() rewrite function is used to temporarily raise the priority of the application myprogram:
log {
source { system(); };
if (program("myprogram")){
rewrite { set-pri("92"); };
};
destination { file("/var/log/mail.log"); };
flags(flow-control);
}
Example: Changing the priority of an application log message in JSON format
In the following example, an application sends log messages in the following JSON format:
{
"time": "2003-10-11T22:14:15.003Z",
"host": "mymachine",
"priority": "165",
"message": "An application event log entry."
}
You can parse these logs with the JSON parser function:
{
parser p_json {
json-parser (prefix(".json."));
}
As the application message contains a valid priority field, you can use the set-pri() rewrite function to modify the priority of the message:
set-pri("$.json.priority");
Match macros ($1, $2, ... $255) are temporary variables. You can use them for general purposes when operating with list-like items. For example, the match() filter stores capture group results in match variables when the store-matches flag is set, or the JSON parser produces match variables if the parsed JSON data is an array.
It is possible to set match variables in a single operation with the set-matches() rewrite function. set-matches() uses syslog-ng OSE list expressions to set $1, $2, ... $255, so it can be considered as a conversion function between syslog-ng OSE lists and match variables.
NOTE: To convert match variables into a syslog-ng OSE list, use the $* macro, which can be further manipulated using List manipulation, or turned into a list in type-aware destinations.
TIP: To reset match variables to be empty, use the unset-matches() rewrite rule.
Declaration
rewrite <name_of_the_rule> {
set-matches("<list-expression or list-based template function>");
};
Example usage for the set-matches() rewrite function
In the following two examples, $1, $2, and $3 will be set to foo, bar, and baz, respectively.
Example using string:
rewrite {
set-matches("foo,bar,baz");
};
Example using a list template function:
rewrite {
set-matches("$(explode ':' 'foo:bar:baz')");
};
You can unset macros or fields of the message, including any user-defined macros created using parsers (for details, see parser: Parse and segment structured messages and db-parser: Process message content with a pattern database (patterndb)). 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>"));
};
Example: Unsetting a message field
The following example unsets the HOST field of the message.
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>"));
};
Example: Unsetting a group of fields
The following rule clears all SDATA fields:
rewrite r_rewrite_unset_SDATA{
groupunset(values(".SDATA.*"));
};
If you want to change the name of a field of a message, you can use rename() rewrite rules. This can be also achieved via using set() and unset() but those require extra conditions and two operation instead of one.
The rename() rewrite rule uses positional arguments and they are both required. It supports condition rewrite. For more information, see Conditional rewrites.
Declaration
rewrite r_rewrite_rename {
rename("<string1>" "<string2>");
};
Example usage for the rename() rewrite function
The following example renames the .app.name into .container if the .app.name exists. Otherwise, it does nothing.
rewrite r_rewrite_rename {
rename(".app.name" ".container");
};
