Bering-uClibc 6.x - User Guide - Advanced Topics - Setting Up SSH password-less login and Port Knocking

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Setting Up SSH password-less login and Port Knocking
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SSH password-less login

SSH (dropbear) allows password-less login by using RSA security key pairs. To set it up you will have to generate the RSA key pairs on your client machine (PuTTYgen on Windows, ssh-keygen OSX & Linux) and copy the public key in your router file /root/.ssh/authorized_keys. If you have more than one client machine, repeat the same process on each of them, and append their respective public keys in your router file /root/.ssh/authorized_keys. On Linux or OSX you can use the ssh-copy-id utility script that might help you do that.

Make sure you have this format:

firewall# pwd
firewall# cat authorized_keys
l7bUlGYoouP3tpqfXxIBbQEeMr/2X99 me@ProBook-6560b

You can now login in your firewall without a password from those client machines obviously. You could disable password login but be careful not to lock yourself out... I prefer to use a very very very strong password anyways, you never know when you might have to login from none of your prepared client machines. But you can always keep, at your own risks, a copy of one of the private keys on a portable secure medium..., phone, earrings ...

If you intend to login from the internet, you have to add the following rule to /etc/shorewall/rules

SSH(ACCEPT) net fw

You can always choose another port like 65022 for SSH to listen to. The port scanners wont usually reach there. There are ongoing debates about the questionable security of this obfuscation... Anyway, if you do, add this rule then:

ACCEPT  net fw tcp 65022

and tell dropbear to listen on port 65022...

lrcfg -> 3) -> 13) -> 1)

Port Knocking

If you don't like or trust a simple ssh -p 65022 root@your.ipaddrs.router everytime you login, you could always use the default SSH port 22 but keep it hidden with a technique called "port knocking". The port 22 is kept normally closed until you send a proper sequence of random port connection attempts on normally closed ports, using TCP or UDP packets, hence the name Port Knocking. When the port knocking software detects the right combination sequence, it will then open up port 22 for 60 seconds for instance, thus giving you time enough to make your SSH login connection attempt.

The major gain of this technique is that all your ports are closed. Secondly you won't fill your logs with port 22 scanners attempts. Port 22 will stay open for 60 seconds only. If you want to open another session and missed the previous time window, you will have to knock again. Remember that you are still protected by a very secure RSA key exchange on you ssh login, but if you have to, you should use a very very very strong password, remember it's just a matter of time to get cracked.

Near the end of this article: " " you will find in Examples a simple Port Knocking setup that works quite well.

If you were using this last example you would have to type:

ssh -p 1600 root@your.ipaddrs.router   # knock on port 1600... opens port 22 for 60 seconds
ssh root@your.ipaddrs.router           # to connect...

The port knocking feels more secure, because most scanners attempts would be looking for open ports on a typical clueless server, and unless you are a person/site of interest, they might not try to dig deeper if they find no opened port. But, if they still persist anyway, they would be slowed down, having to try knocking on every ports, while looking if it opens up an ssh port. Knocking on one port only is not too hard to find though, but you can knock on more than one and even with UDP or TCP packets. If you look again in at example: " Stateful Port Knocking (knock with a sequence of ports) ", in the PERL module, you will find examples of 8 ports knocks... there is no limit...

Whatever you do, herrrr... don't forget to save Doctor Freeman !!!

lrcfg -> s)

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