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ArticlesPort Knocking with iptables
More info to come soon, but this example should work out of the box. To use it save the script to a file and then, as root, `iptables-restore < script`. This script assumes that your SSH server is running on the default port of 22 and that you are not using iptables for anything else. # START SCRIPT *filter # Allows all loopback (lo0) traffic and drop all traffic to 127/8 that doesn't use lo0 -A INPUT -i lo -j ACCEPT -A INPUT -i ! lo -d 127.0.0.0/8 -j REJECT # Accepts all established inbound connections -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT # Allows all outbound traffic # You can modify this to only allow certain traffic -A OUTPUT -j ACCEPT # Allows HTTP and HTTPS connections from anywhere (the normal ports for websites) #-A INPUT -p tcp --dport 80 -j ACCEPT #-A INPUT -p tcp --dport 443 -j ACCEPT # Allows SSH connections # Below is a three port knock which will allow a new ssh session. Client must 'knock' on ports 3820, 5446, and 3015 first. Each knock has a 15 second window before the user has to start over. # After the knocking is complete the established connection rule comes into place and this won't be tested. -A INPUT -p tcp --dport 3820 -m recent --set --rsource --name SSH_AUTH_KNOCK1 -m limit --limit 15/min -j LOG --log-prefix "ssh knock 1 " --log-level 7 -A INPUT -p tcp --dport 5446 -m recent --rcheck --rsource --seconds 15 --name SSH_AUTH_KNOCK1 -m recent --set --rsource --name SSH_AUTH_KNOCK2 -m limit --limit 15/min -j LOG --log-prefix "ssh knock 2 " --log-level 6 -A INPUT -p tcp --dport 3015 -m recent --rcheck --rsource --seconds 15 --name SSH_AUTH_KNOCK2 -m recent --set --rsource --name SSH_AUTH -m limit --limit 15/min -j LOG --log-prefix "ssh knock 3 " --log-level 6 -A INPUT -p tcp --dport 22 -m state --state NEW -m recent --rcheck --rsource --seconds 15 --name SSH_AUTH -j ACCEPT # Allow ping #-A INPUT -p icmp -m icmp --icmp-type 8 -j ACCEPT # Deny Ping -A INPUT -p icmp -j DROP # log iptables denied calls -A INPUT -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7 # Reject all other inbound - default deny unless explicitly allowed policy -A INPUT -j DROP -A FORWARD -j DROP COMMIT #END SCRIPT See also: http://www.netfilter.org, iptables-save(8), iptables-restore(8), ip6tables(8)  This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. Rsync Mini Howto
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