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

From: AMOL (
Date: 08/22/05

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    To: "Niranjan S Patil" <>
    Date: Mon, 22 Aug 2005 14:53:26 +0530

    Hi Niranjan,
    Nice question!

    Any IDS in inline mode, or Firewall will block the packets as per the rules
    defined for blocking/allowing.
    Generally,Port 80-HTTP and 443-HTTPS are among the most common ports in the
    "allowed" ones.
    And yes; your Firewall doesn't know more than source (IP:PORT) destination
    (IP:PORT) and state (if you have an option of a stateful inspection of
    From your scenario it looks like you have a packet filter Firewall.A
    firewall implemented with the Packet Filters work at Network Layer of
    ISO/OSI stack.
    Hence it cant stop telnet connection to the server listening on "allowed"

    Similar is the case for Inline IDS.

    But as a security measure you can make sure that hosts on your network are
    NOT practicing things like: running telnet server on port 443. Strictly. And
    you can implement ALG (or simply enable it if its already present)option in
    your Firewall.
    A firewall implemented with the Application Layer Gateways(ALG) work at the
    Application Layer of ISO/OSI stack.

    Hope this may help a little.


    -----Original Message-----
    From: Niranjan S Patil []
    Sent: Monday, August 15, 2005 9:06 PM
    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

    Is this normal with all IDS?

    Any help is appreciated.

    Niranjan S Patil

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