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Re: Why does Debian allow all incoming traffic by default

On Fri, Sep 21, 2018 at 09:32:45PM +0300, Reco wrote:
> 	Hi.
> On Fri, Sep 21, 2018 at 07:14:03PM +0100, Brian wrote:
> > On Fri 21 Sep 2018 at 19:25:22 +0300, Reco wrote:
> > 
> > > 	Hi.
> > > 
> > > On Fri, Sep 21, 2018 at 08:55:21AM -0400, Henning Follmann wrote:
> > > > On Fri, Sep 21, 2018 at 08:34:50AM +0530, Subhadip Ghosh wrote:
> > > > > Hi,
> > > > > 
> TCP RST attack requires exactly that. That, and an absence of a
> firewall.
> > There is no point with a standard Debian installation (which is what the
> > OP inquired about). Debian is already a good netizen.
> Good person makes a TCP connection to unprotected (as in - no firewall
> interference) host. Since there's nothing on a host that does not listen
> appropriate TCP port - host's kernel sends back TCP RST packet.
> Good person's connection terminates, everyone's happy. That's how it
> goes in your typical LAN.
Sorry that is not how a RST attack works.
You send a TCP package two either or both ends where the RST flag is set by
faking your address. This way mostTCP implementation close the exsisting
connection. The china firewall works that way. It is a kind of denial of
service attack.

If you send a TCP package to a computer not listening it will send a ICMP
error back.

> Evil person makes a TCP connection to unprotected host, but forges
> source IP. Host sends TCP RST to this forged IP, host acting as a
> 'reflector' to an attack. And being a bad netizen at the same time.
> Evil person takes as many of such hosts as possible - and there goes
> your old-fashioned RST DDOS.


> I recall that you've stated that your servers do not run any kind of
> packet filter. So, just in case - one cannot harm the reflector that
> way.

On those machines where I run a firewall, I use by default REJECT and not
DROP. This also sends a ICMP back. In most cases this is desireable. If you
drop the package without error the TCP sender will just think the package
was lost and will resend the package. So in most cases REJECT might be
better than DROP anyway.

> So, in this regard Debian is imperfect, but at least they give you right
> tools to solve the problem (iptables suite), and do not force braindead
> firewall policies by default (like RHEL does).
> Reco


Henning Follmann           | hfollmann@itcfollmann.com

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