Iptables Tutorial 1.1.19

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# bad_tcp_packets chain

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$IPTABLES -A bad_tcp_packets -p tcp –tcp-flags SYN,ACK SYN,ACK

–m state –state NEW -j REJECT –reject-with tcp-reset

$IPTABLES -A bad_tcp_packets -p tcp ! –syn -m state –state NEW -j LOG

–log-prefix «New not syn:»

$IPTABLES -A bad_tcp_packets -p tcp ! –syn -m state –state NEW -j DROP

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# allowed chain

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$IPTABLES -A allowed -p TCP –syn -j ACCEPT

$IPTABLES -A allowed -p TCP -m state –state ESTABLISHED,RELATED -j ACCEPT

$IPTABLES -A allowed -p TCP -j DROP

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# TCP rules

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$IPTABLES -A tcp_packets -p TCP -s 0/0 –dport 21 -j allowed

$IPTABLES -A tcp_packets -p TCP -s 0/0 –dport 22 -j allowed

$IPTABLES -A tcp_packets -p TCP -s 0/0 –dport 80 -j allowed

$IPTABLES -A tcp_packets -p TCP -s 0/0 –dport 113 -j allowed

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# UDP ports

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#$IPTABLES -A udp_packets -p UDP -s 0/0 –source-port 53 -j ACCEPT

#$IPTABLES -A udp_packets -p UDP -s 0/0 –source-port 123 -j ACCEPT

$IPTABLES -A udp_packets -p UDP -s 0/0 –source-port 2074 -j ACCEPT

$IPTABLES -A udp_packets -p UDP -s 0/0 –source-port 4000 -j ACCEPT

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# In Microsoft Networks you will be swamped by broadcasts. These lines

# will prevent them from showing up in the logs.

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#$IPTABLES -A udp_packets -p UDP -i $INET_IFACE -d $INET_BROADCAST

#–destination-port 135:139 -j DROP

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# If we get DHCP requests from the Outside of our network, our logs will

# be swamped as well. This rule will block them from getting logged.

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#$IPTABLES -A udp_packets -p UDP -i $INET_IFACE -d 255.255.255.255

#–destination-port 67:68 -j DROP

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# ICMP rules

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$IPTABLES -A icmp_packets -p ICMP -s 0/0 –icmp-type 8 -j ACCEPT

$IPTABLES -A icmp_packets -p ICMP -s 0/0 –icmp-type 11 -j ACCEPT

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# 4.1.4 INPUT chain

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# Bad TCP packets we don't want.

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$IPTABLES -A INPUT -p tcp -j bad_tcp_packets

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# Rules for special networks not part of the Internet

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$IPTABLES -A INPUT -p ALL -i $LO_IFACE -s $LO_IP -j ACCEPT

$IPTABLES -A INPUT -p ALL -i $LO_IFACE -s $LAN_IP -j ACCEPT

$IPTABLES -A INPUT -p ALL -i $LO_IFACE -s $INET_IP -j ACCEPT

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# Rules for incoming packets from anywhere.

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$IPTABLES -A INPUT -p ALL -d $INET_IP -m state –state ESTABLISHED,RELATED

–j ACCEPT

$IPTABLES -A INPUT -p TCP -j tcp_packets

$IPTABLES -A INPUT -p UDP -j udp_packets

$IPTABLES -A INPUT -p ICMP -j icmp_packets

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# If you have a Microsoft Network on the outside of your firewall, you may

# also get flooded by Multicasts. We drop them so we do not get flooded by

# logs

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#$IPTABLES -A INPUT -i $INET_IFACE -d 224.0.0.0/8 -j DROP

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# Log weird packets that don't match the above.

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$IPTABLES -A INPUT -m limit –limit 3/minute –limit-burst 3 -j LOG

–log-level DEBUG –log-prefix "IPT INPUT packet died: "

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# 4.1.5 FORWARD chain

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# Bad TCP packets we don't want

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$IPTABLES -A FORWARD -p tcp -j bad_tcp_packets

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# Accept the packets we actually want to forward

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$IPTABLES -A FORWARD -p tcp –dport 21 -i $LAN_IFACE -j ACCEPT

$IPTABLES -A FORWARD -p tcp –dport 80 -i $LAN_IFACE -j ACCEPT

$IPTABLES -A FORWARD -p tcp –dport 110 -i $LAN_IFACE -j ACCEPT

$IPTABLES -A FORWARD -m state –state ESTABLISHED,RELATED -j ACCEPT

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# Log weird packets that don't match the above.

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$IPTABLES -A FORWARD -m limit –limit 3/minute –limit-burst 3 -j LOG

–log-level DEBUG –log-prefix "IPT FORWARD packet died: "

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# 4.1.6 OUTPUT chain

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# Bad TCP packets we don't want.

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$IPTABLES -A OUTPUT -p tcp -j bad_tcp_packets

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# Special OUTPUT rules to decide which IP's to allow.

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$IPTABLES -A OUTPUT -p ALL -s $LO_IP -j ACCEPT

$IPTABLES -A OUTPUT -p ALL -s $LAN_IP -j ACCEPT

$IPTABLES -A OUTPUT -p ALL -s $INET_IP -j ACCEPT

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# Log weird packets that don't match the above.

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$IPTABLES -A OUTPUT -m limit –limit 3/minute –limit-burst 3 -j LOG

–log-level DEBUG –log-prefix "IPT OUTPUT packet died: "

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# 4.2 nat table

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# 4.2.1 Set policies

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# 4.2.2 Create user specified chains

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# 4.2.3 Create content in user specified chains

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# 4.2.4 PREROUTING chain

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# 4.2.5 POSTROUTING chain

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# Enable simple IP Forwarding and Network Address Translation

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$IPTABLES -t nat -A POSTROUTING -o $INET_IFACE -j SNAT –to-source $INET_IP

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# 4.2.6 OUTPUT chain

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# 4.3 mangle table

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# 4.3.1 Set policies

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# 4.3.2 Create user specified chains

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# 4.3.3 Create content in user specified chains

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# 4.3.4 PREROUTING chain

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# 4.3.5 INPUT chain

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# 4.3.6 FORWARD chain

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# 4.3.7 OUTPUT chain

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# 4.3.8 POSTROUTING chain

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