Date: Thu, 30 Jan 2014 19:31:44 +0100 From: Fabian Wenk <fabian@wenks.ch> To: freebsd-security@freebsd.org Subject: Re: portscans and blackhole Message-ID: <52EA9A90.4040608@wenks.ch> In-Reply-To: <52E93941.7080002@hfbk-hamburg.de> References: <52DD08F7.1000306@hfbk-hamburg.de> <52E910B0.4030606@wenks.ch> <52E93941.7080002@hfbk-hamburg.de>
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Hello
On 29.01.14 18:24, sa9k063 wrote:
> On 01/29/2014 03:31 PM, Fabian Wenk wrote:
>> system will see this as a “Connection refused”. By setting the TCP
>> blackhole MIB to a numeric value of one, the incoming SYN segment is
>> merely dropped, and no RST is sent, making the system appear as a
>> blackhole. By setting the MIB value to two, any segment arriving on
>> a closed port is dropped without returning a RST. This provides
>> some degree of protection against stealth port scans.
>
> This added to the confusion and thus made me ask. The manpage says
> for both values of net.inet.tcp.blackhole={1,2} that no RSTs are
> sent out.
> Both seem to drop SYNs and suppress sending a RST.
>
> Reading it again, the only conclusion i could get to regarding the
> difference between 1 and 2 would be that for a value of 2, all other
> tcp packets with flags other than SYN are additionally ignored. Is
> this a better way to understand it ?
Yes. I read it this way:
If set to 1, it does drop and not send RST only for SYN packets,
if set to 2, it does drop and not send RST for all packets.
>> So it is possible, that you are hit with something else then SYN
>> packets and should probably set net.inet.tcp.blackhole=2, or even
>> with UDP packets, then also set net.inet.udp.blackhole=1.
>
> this remains as a likely explanation, ie FIN scans etc.
>
>> What output does 'sysctl -a | grep blackhole' show?
>
> it used to be
>
> net.inet.tcp.blackhole: 1
> net.inet.udp.blackhole: 1
>
> since setting the tcp value to 2 no more messages like these popped
> up supporting your line of thought.
Then the behavior does match the man page and how I did
understand it.
bye
Fabian
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