Secure Tips and Tricks for "iptables" Rules

"iptables" is an interface tool used to manage the Linux firewall rules (such as packet-filtering) implemented by the netfilter framework. "iptables" usually have multiple tables, and tables may contain multiple chains. Chains typically contain multiple rules that determine how to manage packets. To summarize the structure of iptables, there are Tables > Chains > Rules.

This article provides various tips and tricks for "iptables" rules that can be added to a desktop system to make a stronger firewall. The commands must be executed with Root privileges.

NOTE: Install "iptables-persistent" to make newly added rules permanent (not be lost during reboot).

Actions

• ACCEPT - Allow the packet to be delivered to specified destination
• DROP - Drop the packet without informing the packet's status to the source/destination
• QUEUE - Queue the packets to user-space. In other words, forward all the packets to some other utility that will manage packet filtering
• REJECT - Reject the packets and send information to the source/destination about the rejection
• RETURN - Stop executing the next set of rules of the current chain for this packet, then return to the calling chain

Secure Rules

# Drop Various Attacks
iptables -A INPUT -p tcp --tcp-flags FIN,SYN FIN,SYN -j DROP
iptables -A INPUT -p tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG FIN -j DROP
iptables -A INPUT -p tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG FIN,PSH,URG -j DROP
iptables -A INPUT -p tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG FIN,SYN,RST,PSH,ACK,URG -j DROP
iptables -A INPUT -p tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG NONE -j DROP
iptables -A INPUT -p tcp --tcp-flags SYN,RST SYN,RST -j DROP

# Prevent source address 127.0.0.1 from sending data through various interfaces
iptables -A INPUT -p all -s localhost -i wlan0 -j DROP
iptables -A INPUT -p all -s localhost -i eth0 -j DROP

# Drop Fragments
iptables -A INPUT -f -j DROP

# Drop ICMP (Ping) Packets
iptables -A INPUT -p icmp -m icmp --icmp-type 8 -j DROP

# Do not respond to pings
iptables -A OUTPUT -p icmp --icmp-type echo-reply -j DROP

# Drop Invalid Packets
iptables -A INPUT -m state --state INVALID -j DROP
iptables -A FORWARD -m state --state INVALID -j DROP
iptables -A OUTPUT -m state --state INVALID -j DROP

# Drop LAND (Local Area Network Denial) Packets
# In this attack, a packet is spoofed to make the source address appear as the IP-address of the target.  In other words, the source and destination IP-addresses are the same.
iptables -A INPUT -s 127.0.0.0/8 -j DROP

# Drop Null Packets
iptables -A INPUT -p tcp --tcp-flags ALL NONE -j DROP

# Drop excessive RST Packets to avoid Smurf-Attacks
iptables -A INPUT -p tcp -m tcp --tcp-flags RST RST -m limit --limit 2/second --limit-burst 2 -j ACCEPT

## Drop Smurf-Attacks
# Smurf-Attacks send a large number of ICMP "echo broadcast" packets with a spoofed source IP-address being the target's IP-address. The machines on the network recieve this broadcast message and reply to the target with "echo reply" packets. One way to block this attack is to block all the ICMP packets. However, if that cannot be done, then a limit may be applied to the ICMP packets allowed.
iptables -A INPUT -p icmp -m icmp --icmp-type address-mask-request -j DROP
iptables -A INPUT -p icmp -m icmp --icmp-type timestamp-request -j DROP
iptables -A INPUT -p icmp -m limit --limit 2/second --limit-burst 2 -j ACCEPT
iptables -A INPUT -p icmp -j DROP

# Drop Spank DoS Attacks
# Computers answer TCP packets that are coming from a multicast-address. This can be used for the Spank DoS Attack or stealth-scans.
iptables -A INPUT -s 224.0.0.0/4 -j DROP
iptables -A INPUT -d 224.0.0.0/4 -j DROP
iptables -A INPUT -s 240.0.0.0/5 -j DROP
iptables -A INPUT -d 240.0.0.0/5 -j DROP
iptables -A INPUT -s 0.0.0.0/8 -j DROP
iptables -A INPUT -d 0.0.0.0/8 -j DROP
iptables -A INPUT -d 239.255.255.0/24 -j DROP
iptables -A INPUT -d 255.255.255.255 -j DROP

# Drop SYN Flood Packets
# This is a type of DOS (Denial Of Service) attack.
iptables -A INPUT -p tcp -m state --state NEW -m limit --limit 2/second --limit-burst 2 -j ACCEPT
iptables -A INPUT -p tcp -m state --state NEW -j DROP

# Drop XMAS Packets
# A Christmas-Tree Packet is a packet that has all flags of any protocol set. The FIN, URG, and PSH bits in the TCP header are set. This packet is called an "Xmas Tree" packet because all the fields of header are "lightened up".
iptables -A INPUT -p tcp --tcp-flags ALL FIN,PSH,URG -j DROP
iptables -A INPUT -p tcp --tcp-flags ALL ALL -j DROP

# Prevent Port-scans

# Use only one of the two given port-scan lock-out systems
# Lock-out systems that attempted a port-scan (lock lasts a day)
iptables -A INPUT -m recent --name portscan --rcheck --seconds 86400 -j DROP
iptables -A FORWARD -m recent --name portscan --rcheck --seconds 86400 -j DROP
iptables -A INPUT -m recent --name portscan --remove
iptables -A FORWARD -m recent --name portscan --remove
# Lock-out systems that attempted a port-scan (lock lasts a week)
iptables -A INPUT -m recent --name portscan --rcheck --seconds 604800 -j DROP
iptables -A FORWARD -m recent --name portscan --rcheck --seconds 604800 -j DROP
iptables -A INPUT -m recent --name portscan --remove
iptables -A FORWARD -m recent --name portscan --remove

# Log Port-Scan Attempts
iptables -A INPUT -m recent --name portscan --set -j LOG --log-prefix "Portscan:"
iptables -A INPUT -m recent --name portscan --set -j DROP
iptables -A FORWARD -m recent --name portscan --set -j LOG --log-prefix "Portscan:"
iptables -A FORWARD -m recent --name portscan --set -j DROP

# Block Packets used by Port-Scans
iptables -A INPUT -p tcp --tcp-flags ACK,FIN FIN -j DROP
iptables -A INPUT -p tcp --tcp-flags ACK,PSH PSH -j DROP
iptables -A INPUT -p tcp --tcp-flags ACK,URG URG -j DROP
iptables -A INPUT -p tcp --tcp-flags ALL ALL -j DROP
iptables -A INPUT -p tcp --tcp-flags ALL FIN -j DROP
iptables -A INPUT -p tcp --tcp-flags ALL FIN,PSH,URG -j DROP
iptables -A INPUT -p tcp --tcp-flags ALL NONE -j DROP
iptables -A INPUT -p tcp --tcp-flags ALL SYN,FIN -j DROP
iptables -A INPUT -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP
iptables -A INPUT -p tcp --tcp-flags ALL URG,PSH,FIN -j DROP
iptables -A INPUT -p tcp --tcp-flags ALL URG,PSH,SYN,FIN -j DROP
iptables -A INPUT -p tcp --tcp-flags FIN,RST FIN,RST -j DROP
iptables -A INPUT -p tcp --tcp-flags SYN,ACK,FIN,RST RST -m limit --limit 1/s -j ACCEPT
iptables -A INPUT -p tcp --tcp-flags SYN,ACK SYN,ACK -m state --state NEW -j DROP
iptables -A INPUT -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP
iptables -A INPUT -p tcp --tcp-flags SYN,RST SYN,RST -j DROP

Miscellaneous Commands

Here are some miscellaneous "iptables" commands.

# Save Rules to a File
iptables-save > ~/firewall.txt

# Restore/Load Rules from File
iptables-restore < $HOME/firewall.txt

# Reload/Restart UFW
ufw reload

# List Rules
iptables -L
iptables -L -n
iptables -L -n -v
iptables -L -n -v --line-numbers

# List Rules by Table
iptables -L -t filter
iptables -L -t mangle
iptables -L -t nat
iptables -L -t raw

# List ICMP Types
iptables -p icmp -h

Further Reading

• Intro to IPTables - https://dcjtech.info/topic/intro-to-iptables/
• Uncomplicated Firewall (ufw) - https://dcjtech.info/topic/uncomplicated-firewall-ufw/
• DCJTech Security Article Index - https://dcjtech.info/topic/security/