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.

Suppose that you wish to limit the number of connections to port 22 to 3/minute from individual internet hosts to the firewall. Add an action named SSHLimit (see the Action documentation). Leave the action.SSHLimit file empty. Create /etc/shorewall/SSHLimit with the following contents: run_iptables -A $CHAIN -m recent --name SSHA --set if [ -n "$LEVEL" ]; then run_iptables -N $CHAIN% log_rule_limit $LEVEL $CHAIN% SSHLimit REJECT "" "" -A run_iptables -A $CHAIN% -j reject run_iptables -A $CHAIN -m recent --name SSHA --update --seconds 60 --hitcount 4 -j $CHAIN% else run_iptables -A $CHAIN -m recent --update --name SSHA --seconds 60 --hitcount 4 -j reject fi run_iptables -A $CHAIN -j ACCEPT Add this rule to /etc/shorewall/rules: #ACTION SOURCE DEST PROTO DEST PORT(S) SSHLimit net $FW tcp 22 If you wish to log the rejects at the 'info' level then use this rule instead: #ACTION SOURCE DEST PROTO DEST PORT(S) SSHLimit:info net $FW tcp 22 If you wish to use SSHLimit 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 SSHLimit net loc:192.168.1.5 tcp 22 - 206.124.146.178 You can use SSHLimit with DNAT on earlier releases provided that you omit the ORIGINAL DEST entry on the second SSHLimit rule. This rule will be quite secure provided that you specify 'norfc1918' on your external interface. The above can be generalized into a flexible 'Limit' target. 'Limit' as described here is included as a standard part of Shorewall beginning with version 3.0.4. The following is included to show how 'Limit' is implemented; if you are running Shorewall 3.0.4 or later, you can omit the following two steps. Add an action named Limit. Leave the action.Limit file empty. Create /etc/shorewall/Limit with the following contents: set -- $(separate_list $TAG) run_iptables -A $CHAIN -m recent --name $1 --set if [ -n "$LEVEL" ]; then run_iptables -N $CHAIN% log_rule_limit $LEVEL $CHAIN% $1 REJECT "" "" -A run_iptables -A $CHAIN% -j reject 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 reject fi run_iptables -A $CHAIN -j ACCEPT Now if you want to limit the number of connections to port 22 to 3/minute from individual internet hosts to the firewall, you can add this rule: #ACTION SOURCE DEST PROTO DEST PORT(S) Limit:none:SSHA,3,60 net $FW tcp 22 If you want rejected 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 As you can see, 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).