[Bro] check rx and tx hosts for files

Kellogg, Brian (GS IT PG-DR) bkellogg at dresser-rand.com
Mon Oct 10 12:34:37 PDT 2016

Thanks, I did something similar.  Always concerned I’m doing it the hard way.

From: fatema bannatwala [mailto:fatema.bannatwala at gmail.com]
Sent: Monday, October 10, 2016 3:32 PM
To: Kellogg, Brian (GS IT PG-DR)
Cc: bro at bro.org
Subject: Re: check rx and tx hosts for files

Hi Brian,

I had the kind of same use-case where I had to exclude file extraction for certain subnets.
Hence this is what I have done in my script:

# White list of subnets to exclude file extraction for.
global subnet_map: table[subnet] of string = {
      [x.x.x.x/25] = "VIP subnet1",
      [y.y.y.y/26] = "VIP subnet2",
      [z.z.z.z/24] = "VIP subnet3",
} &default ="";

event file_sniff(f: fa_file, meta: fa_metadata)

    # check for right source to extract.
    if(f$source != "HTTP")

    #check the right mime-type to extract.
    if ( ! meta?$mime_type || meta$mime_type !in ext_map )

    # get the recieving hosts from the record.
    local rx_addr: set[addr];
    rx_addr = f$info$rx_hosts;

    # check if the rx host is in VIP subnets
    for (i in rx_addr)
     if ( i in subnet_map )

    if ( meta?$mime_type )
         local fname = fmt("%s-%s.%s", f$source, f$id, ext_map[meta$mime_type]);
         Files::add_analyzer(f, Files::ANALYZER_EXTRACT, [$extract_filename=fname]);

You can define the rx or tx which you want to exclude/include and modify accordingly.
I am sure there might be some more efficient ways to do this, I will let other more experience people to answer that :)

Hope this helps.

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