[SSHd] Limiting access from authorized IP's

Mel fbsd.questions at rachie.is-a-geek.net
Fri Apr 18 19:37:52 UTC 2008

On Friday 18 April 2008 20:53:37 Paul Schmehl wrote:
> --On Friday, April 18, 2008 20:30:53 +0200 Mel
> <fbsd.questions at rachie.is-a-geek.net> wrote:
> > On Friday 18 April 2008 16:53:49 Paul Schmehl wrote:
> >> Firewalls are for preventing access to running services.  By definition,
> >> if you are running a service, you want it to be accessed.
> >
> > That's your assumption.
> > First of all, firewalls are for preventing unwanted connections, this is
> > not necessarily the same as access to running services.
> > Prime examples: cable modem and windows hosts broadcast spam on an ISP's
> > network, ping floods. User scans [1], vulnerability scans, open relay
> > scanners, spammers fall into running services category.
> They don't fall into the category of services that you authorized or
> approved of.  Keep in mind, we're talking about *hosts*, individual
> workstations if you will, not enterprises.

Well, I don't particularly like someone using my bandwidth to find out if I 
changed my mailserver config to such that I would now be an open relay, every 
10-20 minutes for weeks on end, so I want it to be over with at the TCP 
level, not at the daemon level.

Individual hosts are exactly the target for these scans. Same with the 
webserver, there are a great number of requests that seperate a scan from a 
legitimate user.

> >> For an individual host it makes a great deal more sense to only run
> >> those services you intend to use ***and keep them up to date and
> >> properly configured***.
> >
> > It is an illusion to think that the patch always comes before the
> > exposure.
> It's a worse illusion to believe the firewall is going to help.  If the
> service is exposed and compromised, the firewall wouldn't be blocking it
> anyway.

In a targetted scenario, this is correct. However, scans precede the attack 
and one example I gave with grok, you can limit the chances that the attacker 
gets the information he needs to exploit the bug he's looking for.
> Furthermore, if the host is compromised, the firewall is one of the 
> first things that will be disabled.

That would require root. So there's something else wrong in the chain, or it 
is one of those unfortunate services that run as root.

> > Secondly, pending the ammount of services you offer, this can be a full
> > task and especially for the "hobby" category, it is more time-efficient
> > to shut off any unauthorized traffic to begin with.
> > Say, some webapp allows uploading a file and executing it. It is then
> > quite easy to add a daemon to your server, that you have not configured.
> > With a firewall in default block mode, this daemon does not receive
> > connections. Even when the patch is released before exposure, you could
> > be, say sleeping and it can be too late. For some this is paranoia, for
> > others common sense.
> Again, the firewall is providing a false sense of security in exactly the
> scenario you propose.  Where do you think hacker's daemons are running
> these days?  **On the ports that you can't close on the firewall**.

I'm curious which those are.

> >> [4] # grep sshd /etc/defaults/rc.conf
> >> sshd_enable="NO"                # Enable sshd
> >
> > No? Surely you're not using inetd?
> I haven't used inetd in years.  I'm not sure why you think I would be.

Well, since sshd_enable is set to no, I assumed inetd would be where you've 
started it.


Problem with today's modular software: they start with the modules
    and never get to the software part.

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