cheat-fork-echo/cheat/cheatsheets/iptables

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# Show hit for rules with auto refresh
watch --interval 0 'iptables -nvL | grep -v "0 0"'
# Show hit for rule with auto refresh and highlight any changes since the last refresh
watch -d -n 2 iptables -nvL
# Block the port 902 and we hide this port from nmap.
iptables -A INPUT -i eth0 -p tcp --dport 902 -j REJECT --reject-with icmp-port-unreachable
# Note, --reject-with accept:
# icmp-net-unreachable
# icmp-host-unreachable
# icmp-port-unreachable <- Hide a port to nmap
# icmp-proto-unreachable
# icmp-net-prohibited
# icmp-host-prohibited or
# icmp-admin-prohibited
# tcp-reset
# Add a comment to a rule:
iptables ... -m comment --comment "This rule is here for this reason"
# To remove or insert a rule:
# 1) Show all rules
iptables -L INPUT --line-numbers
# OR iptables -nL --line-numbers
# Chain INPUT (policy ACCEPT)
# num target prot opt source destination
# 1 ACCEPT udp -- anywhere anywhere udp dpt:domain
# 2 ACCEPT tcp -- anywhere anywhere tcp dpt:domain
# 3 ACCEPT udp -- anywhere anywhere udp dpt:bootps
# 4 ACCEPT tcp -- anywhere anywhere tcp dpt:bootps
# 2.a) REMOVE (-D) a rule. (here an INPUT rule)
iptables -D INPUT 2
# 2.b) OR INSERT a rule.
iptables -I INPUT {LINE_NUMBER} -i eth1 -p tcp --dport 21 -s 123.123.123.123 -j ACCEPT -m comment --comment "This rule is here for this reason"