[Edit: I'm leaving this post up for historical reasons, but I've since modified the way I build my iptables firewalls—I typically add the rules I need from the command line one by one, then use CentOS's
service iptables save command (available in CentOS > 6.2) to save the rules so they'll persist after a restart.]
I've seen a ton of iptables configurations on the Internet, and none of them really got to the heart of what I need to do for the majority of my LAMP-based web servers (hosted on Linode, HostGator, Hot Drupal, and elsewhere). For these servers, I just need a really simple set of rules that restricts all incoming traffic except for web (port 80/443 for http/https traffic), ssh (usually port 22), smtp (port 25), and icmp ping requests.
The script below (save it as 'firewall.bash',
chmod u+x it to make it executable, and run it with
$ sudo /path/to/firewall.bash, then test your server (access websites, log on to it from another Terminal session, ping it, etc., and make sure that's all working)):
# Script to set up iptables firewall on a Linux machine.
# Drop spoofed packets.
if [ -e /proc/sys/net/ipv4/conf/all/rp_filter ]
for filter in /proc/sys/net/ipv4/conf/*/rp_filter
echo 1 > $filter
# Remove all rules and chains
# Accept traffic from loopback interface (localhost)
/sbin/iptables -A INPUT -i lo -j ACCEPT
# Accept SSH traffic (for administration)
/sbin/iptables -A INPUT -p tcp --dport ssh -j ACCEPT
# Accept port traffic on ports 22, 25, 80, 443 (SSH, SMTP, Apache http/https)
/sbin/iptables -A INPUT -p tcp -m multiport --destination-ports 22,25,80,443 -j ACCEPT
# Accept icmp ping requests
/sbin/iptables -A INPUT -p icmp -j ACCEPT
# Allow established connections:
/sbin/iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
# Log EVERYTHING (ONLY for Debug)
# /sbin/iptables -A INPUT -j LOG
# Uncomment to log the rest of the incoming messages (all of which are dropped)
# with a maximum of 15 log entries per minute
# /sbin/iptables -A INPUT -m limit --limit 15/minute -j LOG --log-level 7 --log-prefix "Dropped by firewall: "
# Drop all other traffic
/sbin/iptables -A INPUT -j DROP
/sbin/iptables -P FORWARD DROP
# End message
echo " [End iptables rules setting]"
- This script assumes iptables is located at /sbin/iptables. If it's not there, adjust the path accordingly.
- Show your current iptables configuration by running
$ sudo iptables -L
- Flush your iptables configuration back to allowing any traffic, anywhere, by running the attached bash script (rename to flush_iptables.bash)
The comments should hopefully help explain everything that's going on here, but for a fuller explanation of iptables, read here. Note that I take no responsibility for the security of your server! Use at your own risk.