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

• 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

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/608976