Tom Eastep 2005 2006 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 by 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. Access is enabled for 60 seconds. 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 --seconds 60 --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.

Debian users. This feature is broken in the Debian version 3.0.7 of Shorewall (and possibly in other versions). The file /usr/share/shorewall/Limit was inadvertently dropped from the .deb. That file may be obtained from Shorewall SVN and installed manually. Beginning with Shorewall 3.0.4, Shorewall has a 'Limit' action. Limit is invoked with a comma-separated list in place of a logging tag. The list has three elements: The name of a 'recent' set; you select the set name which must conform to the rules for a valid chain name. Different rules that specify the same set name will use the same set of counters. The number of connections permitted in a specified time period. The time period, expressed in seconds. Connections that exceed the specified rate are dropped. For example,to use a recent set name of SSHA, and to limiting SSH to 3 per minute: #ACTION SOURCE DEST PROTO DEST PORT(S) Limit:none:SSHA,3,60 net $FW tcp 22 If you want dropped connections to be logged at the info level, use this rule instead: #ACTION SOURCE DEST PROTO DEST PORT(S) Limit:info:SSHA,3,60 net $FW tcp 22 To summarize, you pass four pieces of information to the Limit action: The log level. If you don't want to log, specify "none". The name of the recent set that you want to use ("SSHA" in this example). The maximum number of connections to accept (3 in this example). The number of seconds over which you are willing to accept that many connections (60 in this example).

For those who are curious, the Limit action is implemented in Shorewall 3.0 and Shorewall 3.2 as follows: The file /usr/share/shorewall/action.Limit is empty. The file /usr/share/shorewall/Limit is as follows: set -- $(separate_list $TAG) [ $# -eq 3 ] || fatal_error "Rule must include <set name>,<max connections>,<interval> as the log tag" run_iptables -A $CHAIN -m recent --name $1 --set if [ -n "$LEVEL" ]; then run_iptables -N $CHAIN% log_rule_limit $LEVEL $CHAIN% $1 DROP "" "" -A run_iptables -A $CHAIN% -j DROP run_iptables -A $CHAIN -m recent --name $1 --update --seconds $3 --hitcount $(( $2 + 1 )) -j $CHAIN% else run_iptables -A $CHAIN -m recent --update --name $1 --seconds $3 --hitcount $(( $2 + 1 )) -j DROP fi run_iptables -A $CHAIN -j ACCEPT In Shorewall 3.3, Limit is made into a built-in action; basically that means that the above code now lives inside of Shorewall rather than in a separate file.