Advanced IP Network Scanning Methods - Network Scanning Countermeasures

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Here is a checklist of countermeasures to use when considering technical modifications to networks and filtering devices to reduce the effectiveness of network scanning and probing undertaken by attackers:

• Filter inbound ICMP message types at border routers and firewalls. This forces attackers to use full-blown TCP port scans against all of your IP addresses to map your network correctly.
  
• Filter all outbound ICMP type 3 “unreachable” messages at border routers and firewalls to prevent UDP port scanning and firewalking from being effective.
  
• Consider configuring Internet firewalls so that they can identify port scans and throttle the connections accordingly. You can configure commercial firewall appliances (such as those from Check Point, NetScreen, and WatchGuard) to prevent fast port scans and SYN floods from being launched against your networks (however, this functionality can be abused by attackers using spoofed source addresses, resulting in DoS). On the open source side, there are many tools such as portsentry that can identify port scans and drop all packets from the source IP address for a given period of time.
  
• Assess the way that your network firewall and IDS devices handle fragmented IP packets by using fragtest and fragroute when performing scanning and probing exercises. Some devices crash or fail under conditions in which high volumes of fragmented packets are being processed.
  
• Ensure that your routing and filtering mechanisms (both firewalls and routers) can’t be bypassed using specific source ports or source routing techniques.
  
• If you house publicly accessible FTP services, ensure that your firewalls aren’t vulnerable to stateful circumvention attacks relating to malformedPORTandPASVcommands.
  

• If a commercial firewall is in use, ensure the following:
  
  -- The latest service pack is installed.
  
  -- Antispoofing rules have been correctly defined so that the device doesn’t accept packets with private spoofed source addresses on its external interfaces.
  
  -- Fastmode services aren’t used in Check Point Firewall-1 environments.

• Investigate using reverse proxy servers in your environment if you require a high-level of security. A reverse proxy will not forward fragmented or malformed packets to the target systems, so a number of low-level attacks are thwarted.

• Be aware of your own network configuration and its publicly accessible ports by launching TCP and UDP port scans along with ICMP probes against your own IP address space. It is surprising how many large companies still don’t properly undertake even simple port scanning exercises.  

DISCLAIMER: The content provided in this article is not warranted or guaranteed by Developer Shed, Inc. The content provided is intended for entertainment and/or educational purposes in order to introduce to the reader key ideas, concepts, and/or product reviews. As such it is incumbent upon the reader to employ real-world tactics for security and implementation of best practices. We are not liable for any negative consequences that may result from implementing any information covered in our articles or tutorials. If this is a hardware review, it is not recommended to open and/or modify your hardware.

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This article is excerpted from chapter four of Network Security Assessment, Second Edition, written by Chris McNab (O'Reilly, 2007; ISBN: 0596510306). Check it out today at your favorite bookstore. Buy this book now.