Port Knocking and Other Uses of 'Recent Match'

Tom 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”.

2023/02/18

Table of Contents

What is Port Knocking?
Implementing Port Knocking in Shorewall
Limiting Per-IP Connection Rate

Note

The techniques described in this article were superseded in Shorewall 4.5.19 with the introduction of Shorewall Events.

Note

The feature described in this article require 'Recent Match' in your iptables and kernel. See the output of shorewall show capabilities to see if you have that match.

What is Port Knocking?

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.

Implementing Port Knocking in Shorewall

In order to implement this solution, your iptables and kernel must support the 'recent match' extension (see FAQ 42).

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.

use Shorewall::Chains;

if ( $level ) {
    log_rule_limit( $level, 
                    $chainref, 
                    'SSHKnock',
                    'ACCEPT',
                    '',
                    $tag,
                    'add',
                    '-p tcp --dport 22   -m recent --rcheck --name SSH ' );

    log_rule_limit( $level,
                    $chainref,
                    'SSHKnock',
                    'DROP',
                    '',
                    $tag,
                    'add',
                    '-p tcp ! --dport 22 ' );
}

add_rule( $chainref, '-p tcp --dport 22   -m recent --rcheck --seconds 60 --name SSH          -j ACCEPT' );
add_rule( $chainref, '-p tcp --dport 1599 -m recent                       --name SSH --remove -j DROP' );
add_rule( $chainref, '-p tcp --dport 1600 -m recent                       --name SSH --set    -j DROP' );
add_rule( $chainref, '-p tcp --dport 1601 -m recent                       --name SSH --remove -j DROP' );

1;

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       DPORT
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       DPORT
SSHKnock:info    net               $FW            tcp         22,1599,1600,1601

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       DPORT         SPORT       ORIGDEST
DNAT-            net               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
```
Note
You can use SSHKnock with DNAT on earlier releases provided that you omit the ORIGDEST entry on the second SSHKnock rule. This rule will be quite secure provided that you specify 'routefilter' on your external interface and have NULL_ROUTE_RFC1918=Yes in shorewall.conf.

For another way to implement Port Knocking, see the Manual Chain documentation.

Limiting Per-IP Connection Rate

This information has been moved to the Actions article.