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.

Using conditional rewrite

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);
    };

To configure condtional rewrite

1. The log path receives a message from the source (s1).

2. The rewrite rule (r_rewrite_set) evaluates the condition. If the message matches the condition (the PROGRAM field of the message is “myapplication”), AxoSyslog rewrites the log message (sets the value of the HOST field to “myhost”), otherwise it is not modified.

3. The next element of the log path processes the message (d1).

Example: Using conditional rewriting

The following example sets the HOST field of the message to myhost only if the message was sent by the myapplication program.

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

The following example is identical to the previous one, except that the condition references an existing filter template.

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