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We are going to make assumptions which are very probable for many firewalls, but won't be accurate, because they can vary from firewall to firewall. We can list as anti-scanning technologies:

  • Firewall
  • NAT (Network Address Translation)
  • IDS (Intrusion Detection System)

We are going to discuss each and how they will affect the result.


A firewall is a network security system that controls network traffic passing between two networks, and is based on a configured rule set. A firewall establish a barrier between a trusted internal network, and an external network (such as internet), a firewall inspects the packets passing between these networks; and either allows or denies the forwarding of the packet, based on the content of the packet header or sometimes on the content of the packet payload.

Decisions are primarily made based on the socket which is the combination "SourceIP,SourcePort and DestIP,DestPort". There are generally three types of firewall:

  • Stateless Packet Filter Firewall
  • Stateful Packet Filter Firewall
  • Application (aka Proxy) Firewall

We are not interested in the normal scans done with -sS or with -sT because in these cases we simply receive a response (or the traffic is normally forwarded) if the port is open and a drop of the packet (aka no response) if the port is closed

Stateless Firewall

In a stateless firewall, the only thing checked is the socket, so the rules are based upon the socket and the allowed ports. This firewall has no record of prior transactions. A stateless firewall don't inspect the TCP flags, since it only looks at the rule, so if we send a:

  • NFX or ACK packet scan, we receive a RST packet directly from the target for both the cases of open and closed ports, bypassing the firewall, while if the firewall has a rule about filtering that specific port, we won't receive any RST packet (In this case we are evading firewall)

Stateful Firewall

Stateful firewall, look at inbound packets and make their pass/drop decision based upon two objects:

  • state table
  • rule set (describing the allowed TCP flags)

In the state table we have records of approved connections recording the socket; once the connection has been approved, the following packets are inspected by the firewall only by looking the state table, the advantage of a stateful firewall is that looking for sockets after the approved connection is faster than looking everytime for the rule set. When a stateful firewall receives:

  • NFX or ACK packet it drops the packet, since the rule set, just states to drop the packets with these flags, so we receive no response, even in the case of ACK, the firewall will look the state table and notice of not having any history of conversation with the socket so again it will drop the packet

Application or Proxy Firewall

In this case we have a complete application who inspects the packets, and decides if it has to forward the packet or not. An application firewall will have its own TCP/IP stack protocols, so he will reply directly to the scanning machine, so we won't get a response from our target directly but from this firewall (this was not happening in the previous case, since we always had the case of either no response or response from the target directly). Application firewall must have an appropriate application for each protocol they want to support, if they don't have an appropriate module to interprete the application data, they typically behave as a stateful firewall. An application firewall will reply:

  • to an NFX or ACK request, the firewall will drop the packets, even if with an ACK request we could even receive a RST packet, but this will come from the firewall itself and not from the target machine

Firewall Commonalities are:

  • Default deny policy
  • Rate limiting
  • ICMP on UDP Scan