[Bro] How "priority" keyword works?

Siwek, Jon jsiwek at illinois.edu
Fri Oct 31 06:41:08 PDT 2014

> On Oct 31, 2014, at 3:48 AM, Vito Logrillo <vitologrillo at gmail.com> wrote:
> ------snippet code.bro------
> event dns_message(c: connection, is_orig: bool, msg: dns_msg, len: count) &priority=5
> {
>  if(c$id$orig_p == 138/udp)
> {
> ...do something and write in my custom log file...
> }
> }
> -------------------------------------
> My custom event dns_message overrides the standard event?

The body of it just gets executed before any other dns_message event handlers with a lower priority.

> The standard event is executed or not in this case?

It still executes.

> Should i find the same packet logged in dns.log and in my custom log or not?

In both (technically not in your custom log if the condition you show isn’t true).

> It depends on priority keyword?

No, &priority just changes the order that the event handlers execute (highest goes first).

>  And what happens if i set priority = -5?

The logic in your event handler runs after other event handlers that have priority greater than -5 (if no &priority is given, it defaults to 0).  If the priority of two event handlers is the same, the order is not well-defined.

- Jon

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