Implement Port-Knocking Security with knockd

If You Are behind a Router

If you aren't directly connected to the Internet, but go through a router instead, you need to make some configuration changes. How you make these changes depends on your specific router and the firewall software you use, but in general terms you should do the following:

1) Forward the knock ports to your machine, so knockd will be able to recognize them.

2) Forward port 22 to your machine. Although in fact, you could forward any other port (say, 22960) to port 22 on your machine, and remote users would have to ssh -p 22960 your.site.url in order to connect to your machine. This could be seen as “security through obscurity”—a defense against script kiddies, at least.

3) Configure your machine's firewall to reject connections to port 22 and to the knock ports:

$ /usr/sbin/iptables -I INPUT 1 -p tcp --dport ssh -j REJECT
$ /usr/sbin/iptables -I INPUT 1 -p tcp --sport 7005:7007 -j REJECT

The command to allow SSH connections would then be:

$ /usr/sbin/iptables -I INPUT 1 -p tcp --dport ssh -j ACCEPT

And, the command for closing it again would be:

$ /usr/sbin/iptables -D INPUT -p tcp --dport ssh -j ACCEPT

Resources

The knockd page is at www.zeroflux.org/projects/knock, and you can find the knockd man page documentation at linux.die.net/man/1/knockd, or simply do man knockd at a console.

For more on port knocking, check www.portknocking.org/view, and in particular, see www.portknocking.org/view/implementations for several more implementations. Also, you might check the critique at www.linux.com/archive/articles/37888 and the answer at www.portknocking.org/view/about/critique for a point/counterpoint argument on port knocking.

Read en.wikipedia.org/wiki/Transmission_Control_Protocol for TCP flags, especially SYN. At www.faqs.org/docs/iptables/tcpconnections.html, you can find a good diagram showing how flags are used.

Port numbers are assigned by IANA (Internet Assigned Numbers Authority); see www.iana.org/assignments/port-numbers for a list.

To test your site, get nmap at nmap.org, and also go to GRC's (Gibson's Research Corporation) site at https://www.grc.com, and try the ShieldsUp test.

Check www.netfilter.org if you need to refresh your iptables skills.