[Bro] How to modify dns.log

[fasf safas]

Hi,
i want to introduce two new fields in dns.log: i've tried to use a code
like this:

-----script.bro------
redef record DNS::Info += {
    foo: bool &optional &log;
};

event DNS::log_dns (rec: DNS::Info)
{
  if(condition)
        rec$foo = T;
}

-------------------------

without any results.
For example if i want to modify conn.log, i can use

event connection_state_remove(c: connection)

For dns.log, which event should be called?
Thanks
Fab
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