More Trickiness With SSH



More Trickiness With SSH

I saw an article on reddit about SSH trickery. SSH is a very subversive protocol, able to work around many kinds of unwise security policies. Here’s a couple more useful things to know.

1. Better Lurking Through .ssh/config-ery.

Where you’ve got machines lurking behind other machines, inaccesible from the Internet, you can add a clause like this to your .ssh/config file:

Host: lurker
ProxyCommand: ssh /bin/nc %h %p
This causes ‘ssh lurker’ to open an ssh connection to, then use nc (may be called netcat on your system, or you may have to install it yourself) to connect on to lurker (the %h %p interpolates the target hostname and port into the proxy command)

2. Reverse Tunnelling

So you’ve noticed the -L option, right, and you understand that by running:

ssh -L 3128:localhost:3128 gateway.home
you are establishing a tunnel home to your proxy server, and you can now configure your web browser to use localhost:3128 as its proxy server to keep your web traffic private.

But did you know this one? Let’s say you’ve got a machine stuck out in DMZ land and you want to apt-get upgrade the poor thing, pronto. You can’t access the web from this box: security policy. You can’t access your internal proxy: ditto. All you can do is ssh into it. Try this:

ssh -R
From your shell on dmzbox, you can now configure the http proxy as localhost:3128 and start sucking down packages via the reverse tunnel.

3. Tunnel Tunnelling

Every now and then, you need to get control of a box which is sadly hidden away behind a broken hotel NAT network or some kind of Get Smart style VPN setup. You can’t even get an ssh in. It’s either read Unix commands over an international phone line at 3am your time, or train a pigeon to tap out the following:

ssh -L 2222:localhost:22
which, when run on the remote box, opens an ssh tunnel back home, through which you can ssh back into the remote box with ssh -p 2222 localhost

4. ssh tunnels with tap and -w

There’s also a (newish) “-w” option, which turns ssh into a full-on VPN solution rather than just a port-at-a-time port forwarder.

The useful piece of information which I haven’t seen elsewhere is this: you don’t need to allow root ssh logins to use it. Instead, you can use ‘tunctl’ to preconfigure tun or tap devices on each end with the -u option to set their permissions to a non-root user. The easiest place to do this, on Debian/Ubuntu systems, is in /etc/network/interfaces, for example, in host1:/etc/network/interfaces:

auto tap9
iface tap9 inet static
pre-up tunctl -u nick -t $IFACE
post-down tunctl -d $IFACE
and in host2:/etc/network/interfaces:

auto tap9
iface tap9 inet static
pre-up tunctl -u nick -t $IFACE
post-down tunctl -d $IFACE
Now you can ‘ifup’ those interfaces, and then start the VPN by running:

user@host2$  ssh -o Tunnel=Ethernet -w9:9 host1
And the tunnel will be up and running, without needing to create the tunnel as root. You could easily take this one further for an automatic tunnel, setting


© 2009-2011 Nick Moore. Published at:

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» How to Secure SSH with Google Authenticator’s Two-Factor Authentication


How to Secure SSH with Google Authenticator’s Two-Factor Authentication
Published on August 14th, 2012  |  Written by: Chris Hoffman

Want to secure your SSH server with easy-to-use two-factor authentication? Google provides the necessary software to integrate Google Authenticator’s time-based one-time password (TOTP) system with your SSH server. You’ll have to enter the code from your phone when you connect.

Google Authenticator doesn’t “phone home” to Google — all the work happens on your SSH server and your phone. In fact, Google Authenticator is completely open-source, so you can even examine its source code yourself.

Install Google Authenticator
To implement multifactor authentication with Google Authenticator, we’ll need the open-source Google Authenticator PAM module. PAM stands for “pluggable authentication module” – it’s a way to easily plug different forms of authentication into a Linux system.

Ubuntu’s software repositories contain an easy-to-install package for the Google Authenticator PAM module. If your Linux distribution doesn’t contain a package for this, you’ll have to download it from the Google Authenticator downloads page on Google Code and compile it yourself.

To install the package on Ubuntu, run the following command:

sudo apt-get install libpam-google-authenticator

(This will only install the PAM module on our system – we’ll have to activate it for SSH logins manually.)

Create an Authentication Key
Log in as the user you’ll be logging in with remotely and run the google-authenticator command to create a secret key for that user.

Allow the command to update your Google Authenticator file by typing y. You’ll then be prompted with several questions that will allow you to restrict uses of the same temporary security token, increase the time window that tokens can be used for, and limit allowed acces attempts to hinder brute-force cracking attempts. These choices all trade some security for some ease-of-use.

Google Authenticator will present you with a secret key and several “emergency scratch codes.” Write down the emergency scratch codes somewhere safe – they can only be used one time each, and they’re intended for use if you lose your phone.

Enter the secret key in the Google Authenticator app on your phone (official apps are available for Android, iOS, and Blackberry). You can also use the scan barcode feature – go to the URL located near the top of the command’s output and you can scan a QR code with your phone’s camera.

You’ll now have a constantly changing verification code on your phone.

If you want to log in remotely as multiple users, run this command for each user. Each user will have their own secret key and their own codes.

Activate Google Authenticator
Next you’ll have to require Google Authenticator for SSH logins. To do so, open the /etc/pam.d/sshd file on your system (for example, with the sudo nano /etc/pam.d/sshd command) and add the following line to the file:

auth required

Next, open the /etc/ssh/sshd_config file, locate the ChallengeResponseAuthentication line, and change it to read as follows:

ChallengeResponseAuthentication yes

(If the ChallengeResponseAuthentication line doesn’t already exist, add the above line to the file.)

Finally, restart the SSH server so your changes will take effect:

sudo service ssh restart

You’ll be prompted for both your password and Google Authenticator code whenever you attempt to log in via SSH.

15 Responses

August 14, 2012 at 8:00 am
Excellent article. I use this authenticator for my main GMail account for a while now, and I really like the fact that I can have multiple secret keys and number generators on my phone. The trick is now to see whether I can replicate the secret key across multiple servers to have a single generated code to access multiple servers.

August 14, 2012 at 8:38 am
I’m in love with Google’s 2-factor auth. I use it for Google account(obviously), LastPass, SSH, and once implemented…dropbox =D

August 14, 2012 at 11:07 am
Tried this under my ubuntu 10.10 server. Even when adding a repo manually that I found it still could not find the libpam-google-authenticator package. Maybe it just doesn’t work for 10.10?

August 14, 2012 at 12:53 pm
Someone needs to add this to Tomato/DD-WRT

August 14, 2012 at 3:42 pm
I had this on my SSH server for a little while… it worked really well… and then I went to connect with WinSCP… there isn’t a great implementation for connecting via SFTP.

August 14, 2012 at 6:32 pm
I’m with Tim, I can’t really use this without WinSCP support…. sigh

August 14, 2012 at 11:01 pm
It seems that you’re using a libvte based terminal (gnome-terminal) in your example screenshots, since you are concerned enough about security to implement two factor auth, you really should check out this document, which documents a major security flaw in that library:

August 16, 2012 at 3:19 am
Interesting article, the idea looks interesting.

Just a quick question, can you still have several way to connect to your ssh server at the same time?

Basically, I’m usually connecting to my ssh server from my laptop or tablet, so I created a public/private pair of key (DSA), because it’s faster, more secure etc…

I would like to use the google system but only if I’m not able to use my private key. Is there is a a way to do that?

August 16, 2012 at 2:16 pm
For those that are concerned about your smart phone not have battery charge on being in an area that has no coverage, there is a simpler alternative at:

It uses simple USB flash drives and provides Two-Factor Authentication for:
1. OpenVPN
2. SSH
3. Web Single Sign On

August 16, 2012 at 6:01 pm
To both Tim and cpradio, not sure what you’re on about here. I’ve had Google Authenticator set-up for some time now on my SSH server, WinSCP seems to work fine for me. Just make sure you’re running the latest version. Failing that, if you use SSH keys, two factor authentication is bypassed. WinSCP supports both Challenge Response and SSH keys.

August 17, 2012 at 12:21 am
A million thank-yous!

August 17, 2012 at 3:07 pm
I think that the yubikey or softyubikey for gnome, kde, cinnamon and mate is a better solution. Please check them out. I solves all previous problems. Cheers!

August 22, 2012 at 9:02 am
@Toto: if you don’t have your private key on a device you are trying to login from, and you have setup the two-factor authentication with google authenticator, sshd will do two factor authentication. You don’t have to do anything, it will work exactly like you want. I have the same need and it works correctly i.e. uses key based login where private keys are available and two factor when keys are not available.

August 23, 2012 at 5:41 am
@aselvan : Thanks for your input, I actually tried that and it worked like a charm.

For those of you who are interested to do so, I recommend you to set the shared key as preferred login method.

September 13, 2012 at 12:30 am

I have two types of authentication on my server.
One is used just for my account to administer the server (not root) that has public/private keys
The second has just user and pass, but has shell access restricted, are locked in to using a shared ftp folder and are restricted from doing pretty much anything but ftp and http forwarding. (via a group policy in sshd)

I can see that above toto had asked if it was possible to use either public keys or authenticator if not available. is it possible to have the authenticator AND the pub/private key for the standard account, while leaving the accounts in my restricted group untouched.

its probably overkill, with what feels like 3 step protection, but i am curious if it is possible.

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