Iptables

iptables command line utility allows to modify Linux kernel firewall rules.

Basic commands

• iptables -L
• NAT: iptables -t nat -L

• iptables-save and iptables-restore
• apt-get install iptables-persistent

• Add: iptables -A
• Delete: iptables -D

Examples

KVM VNC remote viewer
iptables -t nat -A PREROUTING -i eno1 -p tcp --dport 5900 -j DNAT --to 127.0.0.1:5900
sysctl -w net.ipv4.ip_forward=1
sysctl -p /etc/sysctl.conf

• Port forwarding:

iptables -t nat -I PREROUTING --src 0/0 --dst 10.10.10.x -p tcp --dport 80 -j REDIRECT --to-ports 8123
/proc/sys/net/ipv4/ip_forward

• Block all output traffic: iptables -A OUTPUT -o ethXXX -j DROP
• Open a port: iptables -I INPUT -p tcp --dport XXX -j ACCEPT
• Block all but a range

iptables -I OUTPUT -m iprange --dst-range <remote_ip>  -j ACCEPT
iptables -I INPUT -m iprange --src-range <remote_ip> -j ACCEPT
iptables -P INPUT DROP
iptables -P OUTPUT DROP
netfilter-persistent save

• Block all but one IP

iptables -I OUTPUT -d <remote_ip>  -j ACCEPT
iptables -I INPUT -s <remote_ip> -j ACCEPT
iptables -I OUTPUT -d <remote_ip> -j ACCEPT
iptables -I INPUT -s <remote_ip> -j ACCEPT
iptables -P INPUT DROP
iptables -P OUTPUT DROP

• Allow ssh connections only from specific IPs:

iptables -A INPUT -p tcp --dport 22 -s YourIP -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j DROP
netfilter-persistent save

• Clear iptables rules

iptables -P INPUT ACCEPT
iptables -P FORWARD ACCEPT
iptables -P OUTPUT ACCEPT
iptables -t nat -F
iptables -t mangle -F
iptables -F
iptables -X

• Flush

iptables -F
iptables -t nat -F

Activities

1. Read iptables Ubuntu howto: https://help.ubuntu.com/community/IptablesHowTo
2. Read archlinux documentation: https://wiki.archlinux.org/index.php/iptables
3. Read Stackoverflow iptables questions: https://stackoverflow.com/questions/tagged/iptables?tab=Votes
4. Review your current iptables configuration
5. iptables-save

Related terms

• fail2ban
• Shorewall

See also

• iptables ufw firewalld nftables firewall-cmd ipfw (FreeBSD) PF (OpenBSD), netsh advfirewall
• nftables
• Palo Alto firewalls: PAN-OS
• Port knocking, fail2ban fwknop, DenyHosts

https://serverfault.com/a/200658

https://serverfault.com/a/608976