Techniques for Firewall Evasion
Understanding Firewalls
Firewalls are security devices that monitor and control incoming and outgoing network traffic based on predefined security rules. They act as a barrier between a trusted internal network and an untrusted external network, such as the internet. Firewalls can pose a challenge for network scanners like Nmap, as they may block or filter certain types of traffic.
Firewall Evasion Techniques
To bypass firewalls and conduct successful network scans, Nmap provides several techniques:
1. TCP SYN Scan
As mentioned earlier, the TCP SYN scan is a stealthy scan that does not complete the full TCP handshake. This can help bypass firewalls that are configured to block full TCP connections.
nmap -sS 192.168.1.100
2. Idle/Zombie Scan
The idle/zombie scan uses an "idle" or "zombie" host to perform the scan, making it harder for the firewall to detect the actual source of the scan.
nmap -sI zombie_host 192.168.1.100
3. Fragmented Packets
Firewalls may be configured to block large packets, so Nmap can split the packets into smaller fragments to bypass this restriction.
nmap -f 192.168.1.100
4. Decoy Scans
Nmap can launch the scan from multiple source IP addresses, including decoy IP addresses, to make it more difficult for the firewall to identify the actual source of the scan.
nmap -D decoy1,decoy2,ME 192.168.1.100
5. Timing and Delay Options
Firewalls may be configured to block scans that are too fast or too aggressive. Nmap provides various timing and delay options to slow down the scan and evade detection.
nmap -T4 -min-rate 100 192.168.1.100
By combining these techniques, you can increase the chances of successfully scanning a network protected by a firewall. However, it's important to use these methods responsibly and only for legitimate purposes, as they can also be used for malicious activities.