Tom Eastep 2005-05-14 2005 Thomas M. Eastep Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published SHby the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled GNU Free Documentation License.

Port knocking is a technique whereby attempting to connect to port A enables access to port B from that same host. For the example on which this article is based, see http://www.soloport.com/iptables.html which should be considered to be part of this documentation.

In order to implement this solution, your iptables and kernel must support the 'recent match' extension (see FAQ 42). These instructions also assume Shorewall version 2.2.0 or later. In this example: Attempting to connect to port 1600 enables SSH access. Attempting to connect to port 1601 disables SSH access (note that in the article linked above, attempting to connect to port 1599 also disables access. This is an port scan defence as explained in the article). To implement that approach: Add an action named SSHKnock (see the Action documentation). Leave the action.SSHKnock file empty. Create /etc/shorewall/SSHKnock with the following contents: if [ -n "$LEVEL" ]; then log_rule_limit $LEVEL $CHAIN SSHKnock ACCEPT "" "$TAG" -A -p tcp --dport 22 -m recent --rcheck --name SSH log_rule_limit $LEVEL $CHAIN SSHKnock DROP "" "$TAG" -A -p tcp --dport ! 22 fi run_iptables -A $CHAIN -p tcp --dport 22 -m recent --rcheck --name SSH -j ACCEPT run_iptables -A $CHAIN -p tcp --dport 1599 -m recent --name SSH --remove -j DROP run_iptables -A $CHAIN -p tcp --dport 1600 -m recent --name SSH --set -j DROP run_iptables -A $CHAIN -p tcp --dport 1601 -m recent --name SSH --remove -j DROP Now if you want to protect SSH access to the firewall from the Internet, add this rule in /etc/shorewall/rules: #ACTION SOURCE DEST PROTO DEST PORT(S) SSHKnock net fw tcp 22,1599,1600,1601 If you want to log the DROPs and ACCEPTs done by SSHKnock, you can just add a log level as in: #ACTION SOURCE DEST PROTO DEST PORT(S) SSHKnock:info net fw tcp 22,1599,1600,1601 If you wish to use SSHKnock with a forwarded connection, you must be using Shorewall 2.3.1 or later for fullest protection. Assume that you forward port 22 from external IP address 206.124.146.178 to internal system 192.168.1.5. In /etc/shorewall/rules: #ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL # PORT(S) DEST DNAT- net loc:192.168.1.5 tcp 22 - 206.124.146.178 SSHKnock net fw tcp 1599,1600,1601 SSHKnock net loc:192.168.1.5 tcp 22 - 206.124.146.178 You can use SSHKnock with DNAT on earlier releases provided that you omit the ORIGINAL DEST entry on the second SSHKnock rule. This rule will be quite secure provided that you specify 'norfc1918' on your external interface.