RE: how to block connections running on non-default ports

["Burton Strauss" <Burton () FelisCatus org>]

No.


The short answer is that if you open a port, and allow internal users to
setup servers using that port, you can't control what goes through.  You
opened the port, remember??

A packet is just a collection of bits, which the receiver can interpret any
way they want.  The meaning of a packet is in that interpretation and that
is determined SOLELY by the receiver.

In your case, you setup a server on a port, and sent a packet the server
understood (both ends speaking the same language, as it were), which the
firewall was told to let through and so you got a telnet session.  Great -
things worked as designed...

Your firewall is open on 443 because that's normally used for https:// web
connections.  Many firewalls don't inspect more than the addresses and port
numbers, i.e. they don't look any deeper into the packet.  Even if they did,
they may not be able to interpret what's in a packet.

You are confused about how IDSes work.  At best they work by looking at
packet contents as well as port #s.  Sometimes they can recognize what looks
like a connection (say a web server request) on an unusual port and report
it.  But sometimes they can't.  Most simple commands could be aimed at many
different servers and so you can't say what they mean just from the packet
contents.  Sometimes, such as an HTTP get request, it's more 'obvious' and
so the IDS will guess and report it, but that's really all it is - a guess.


-----Burton

-----Original Message-----
From: Niranjan S Patil [mailto:niranjan.patil () gmail com] 
Sent: Monday, August 15, 2005 10:36 AM
To: security-basics () securityfocus com
Subject: how to block connections running on non-default ports

Hi list,

I recently noticed that our corporate IDS could not block some of
connections that are seemingly unauthorised.

I launched a telnet connection to a remote server on Internet on port
23 and it was successfully blocked by our firewall. I change the listening
port of the telnet server to 443 and launched another telnet connection on
port 443. Neither our firewall or IDS was able to block this connection.

Aren't IDS supposed to block such masqueraded connections, i.e., protocols
with non-default ports.

I have less knowledge on IDS, but isn't it simple for them to check packet
headers and block/filter if they are not on right protocol/port?

Is this normal with all IDS? 

Any help is appreciated.

--
Regards,
Niranjan S Patil