[Bro] Problem with connections in S1 and SF state

Sven Dreyer sven at dreyer-net.de
Wed Apr 27 04:40:39 PDT 2016

Dear list,

we are still having problems with messed up src and dst ip/port of some 

What we did up to now:
- Disabled bridging and used a single interface (eth1) for packet 
capturing, as suggested by Seth Hall
- Disabled offloading by running "ethtool -K eth1 tso off ufo off gso 
off gro off lro off" in the interface's post-up script
- Changed hardware from a "PC Engines APU 1C4" embedded board to a 
"Thomas Krenn LESv2" system
- Check the history field in conn.log for "ShA" flags, as suggested by 
Justin S. Azoff

I found several entries like this in conn.log:
1457953693.259152       CaUcf02Z9xpSSHPz24       50993     41023   tcp     ssl     302.736749      987     7059 SF 
       T       T       0       ShADadFfR       23      2167    16 7899 

According to conn.log, this is a connection from (internal IP 
of our server) port 50993 (the IMAP-TLS port the servers uses) to (a notebook computer) port 41023, lasting for 302 seconds. 
My understanding is that bro saw the whole connection from establishment 
to termination (according to the history field).

In fact, the notebook established a connection to the IMAP service of 
our server. So src ip and port and dst ip and port are twisted in conn.log.

The problem is reproducible (at least in our environment). I captured a 
pcap file on the same machine running bro, using the same interface and 
stripped it down so that it contains only the above mentioned connection 
- 39 frames and about 15 kB (see attachment). If I inject this pcap file 
in the network using

tcpreplay -i eth1 twisted.pcapng

bro shows a connection from => (wrong!) in 
conn.log. If instead I read the pcap file using "bro -r", conn.log shows 
a connection from => (correct!).

Does anyboby have a further idea what we could do to track the problem down?

Thanks and best regards, Sven

Am 17.11.2015 um 21:38 schrieb Sven Dreyer:
> Dear list,
> I'm having trouble understanding some log entries from my conn.log. I
> already learned from this mailing list that bro cannot surely detect who
> initiated a connection if it does not see the initial connection setup,
> which seems logical to me.
> But if I look to my conn.log file, I find entries like these:
> 1446190221.687738 Cbu3fj3FYdODxvLF1h      87.152.221.xxx  50993
> 192.168.100.yyy 36709   tcp     ssl     122.745965      1238    5340
> S1      F       T       0       ShAD
> ad      20      2050    19      6112    (empty)
> 1446190138.746769 CykNrp4VEfzbrJ2vm6      87.152.221.xxx  50993
> 192.168.100.yyy 36679   tcp     ssl     223.406750      1384    18908
> S1      F       T       0       ShAD    ad      39      2956    36
> 20360   (empty)
> It looks like our IMAP server (87.152.221.xxx running on port 50993)
> initiated a connection to my notebook (192.168.100.yyy). That should not
> be possible due to lack of port forwarding for this connection.
> So my first guess is that bro didn't see the initial connection setup
> (midstream traffic, OTH state). But I took a look into the documentation
> on https://www.bro.org/sphinx/scripts/base/protocols/conn/main.bro.html
> regarding the reported states (S1), which says:
> S1  Connection established, not terminated.
> This looks to me like bro saw the connection setup. Or did I get
> something wrong here?
> Oh and by the way: the next paragraph reads:
> SF  Normal establishment and termination. Note that this is the same
> symbol as for state S1. You can tell the two apart because for S1 there
> will not be any byte counts in the summary, while for SF there will be.
> I don't understand this. Do S1 and SF really only differ in byte count
> zero or non-zero? It seems to me that they also differ in "connection
> still alive" and "connection was terminated".
> Looking further trough the logs, I also find entries with "SF" flag in
> whuch source and destination seem twisted:
> 1445338094.186121    C9uuKp4dE9nrHo46bd      87.152.220.xxx  50993
> 192.168.100.yyy 20108   tcp     -462.348551       401     754     SF
>     F       T       0       DdAfFa  13      921     12      1234    (empty)
> Does anybody have a hint? Did I misunderstand something?
> I'm running bro 2.4.1.
> Thanks a lot!
> Sven
> _______________________________________________
> Bro mailing list
> bro at bro-ids.org
> http://mailman.ICSI.Berkeley.EDU/mailman/listinfo/bro

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