[Bro] Conn log shows massive file transfer inbetween normal browsing

[Azoff, Justin S]

> On May 16, 2018, at 3:25 PM, Eric Hacecky <hacecky at jlab.org> wrote:
> 
> I'm having some anomalies in my conn.log.
> 
> Scenario:
> 
> Internal host on my network (10.10.10.10) is browsing autotrader (20.20.20.20)
> 
> Inbetween normal bro logs for the related traffic, I have things like this showing up:
> 
> // conn.log
> 1524177777.577777	Ccq8hi7x7jIegYyKE7	10.10.10.10	63971	20.20.20.20	443	tcp	-	0.015780	1284714853	0	S0	T	F	0	Sa	1	48	1	44	(empty)
> 
> 
> As I'm reading this, it shows my internal host sent ~1.2gigs of data in .015 seconds to this external host.
> 
> S0 for the conn_state "Connection attempt seen, no reply."
> 
> 
> So bro thinks my host tried to send 1.2 gigs off-site but failed? (there are many more similar log entries for the same host)
> 
> Any ideas what can cause this?
> 
> Thanks,
> Eric

that's probably a websocket connection or something that is idle for long periods of time.  Since it's idle for so long bro is assuming the connection ended and is then getting confused when they start talking again.  You can fix it by redeffing this value to be higher:

## If a TCP connection is inactive, time it out after this interval. If 0 secs,
## then don't time it out.
##
## .. bro:see:: udp_inactivity_timeout icmp_inactivity_timeout set_inactivity_timeout
const tcp_inactivity_timeout = 5 min &redef;

You can figure out how high it needs to be based on how frequently you are seeing that connection logged. 

— 
Justin Azoff