SSH: A quick guide

The secure shell or ssh is much more than a secure replacement for telnet. Using ssh is not only secure but also convenient. We will have a look at ssh in this article. The objective is not to explain all the features of ssh, for that you can consult the man page, but to examine some of the key features and their use. All the code in this poset should work if you cut and paste (without the $ prompt of course) it on to the terminal. Also by ssh I mean openssh throughout.

Your .ssh directory

All the files used by ssh are inside the .ssh directory in your home area. Here is a list of them and their use.

• known_hosts: This file contains the public keys of some of the hosts that you have logged in to.
• id_rsa: RSA private key.
• id_rsa.pub: RSA public key.
• id_dsa: DSA private key.
• id_dsa.pub: DSA public keys.
• authorized_keys: List of public keys of users who are authorised to access this account.

Managing known_hosts

The known_hosts file contains the public keys of all the hosts that you have logged in to. It is a good idea to get these known hosts from a trusted source. When your ssh client contacts a server, it receives public key of the server. If there is a mismatch, ssh warns you that the key has changed. This could be due to a man-in-the-middle attack or due to a system reinstallation. When you get such a message it is better to be sure that there is no tampering. Be especially careful if you in an unknown LAN or WiFi network like that of an airport or a hotel. Having a trusted known_hosts file is a very good security measure.

Key based login.

Usually one uses ssh with passwords to login. Although this is secure in the sense that the passwords sent are encrypted, it has all the problems of password based authentication. An alternative is to use public key/private key based authentication. The public key access is more secure and in fact more convenient than the password based access. Here is the step by step procedure.

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$ ssh-keygen # Generate the public key/private key pair.

You will find the generated keys inside the .ssh directory. The files with extension .pub are the public keys. Copy them into the .ssh/authorized_keys file of the remote machine.

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$ scp .ssh/id_rsa.pub @remote:
$ ssh remote
ppk@remote: mkdir .ssh
ppk@remote: cat id_rsa.pub >> .ssh/authorized_keys
               # copy the key to the authorized_keys file.
ppk@remote: chmod 644 .ssh/authorized_keys
               # Ensure that it is readable only to user.

The last step is particularly important. Ssh will refuse to login if it finds that the .ssh/authorized_keys is writeable to someone other than the user. Otherwise an intruder could leave his public key and will have unrestricted access. So do not forget to change permissions. Many have been stumped by this and ssh does not give any indication on where the problem is.

In case you connect to many hosts it is a good idea to install the same public key in all the different hosts you log into. Thus you need to remember only one passphrase for all these hosts.

Generating keys from a Windows machine

Of course the best option is to install yourself an operating system, one of the BSD’s or GNU/Linuxes for example. However if you don’t have that option, you will also be forced to use other ssh clients like putty. My experience with these clients are limited and that prevents me from giving a detailed procedure. Usually they have a click-click interface to generate keys. The keys generated are however not in the format expected by by openssh. Don’t you worry. The correct format is only a command line away.

As before you have to copy the public key to the remote machine. The command

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$ ssh-keygen -i -f pubkeyfile

will convert an SSH2 compatible key format, which is what many of the commercial ssh-client uses, to openssh compatible key format and print it on the standard output. So after copying the public key to the remote machine, you can type

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$ ssh-keygen -i -f pubkeyfile >> .ssh/authorized_keys

on the remote machine.

Passphrase, Empty Passphrase and SSH-agents.

While generating a public key/private key pair one is asked for a passphrase. The passphrase is used to keep you private key encrypted on the disk. It is never sent across the network or used in the protocol. Thus one can use an empty passphrase in which case the private key is kept unencrypted on the disk. In case your machine is a private laptop this is not such a bad idea. The advantage of an empty passphrase is that you will never have to type any passwords while ssh-ing or scp-ing. However there is always a risk of your private key getting compromised if the local machine from which you log on to the remote machine is a shared machine. You could, for example, forget to logout from the common terminal. So it is a good idea to have a passphrase.

A better alternative to an empty passphrase is to use an ssh-agent. The ssh-agent keeps you private key with it and does all authentication on your behalf. Here is a quick example.

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$ ssh-agent bash  # start a new shell session with an ssh-agent running
$ ssh-add         # add your public keys to the agent.
$ ssh remote      # No passphrase will be asked
ppk@remote: exit
$ scp foo ppk@remote:  # No passphrase will be asked.

I like to use ssh-agent in conjunction with screen (another cute program). This is what I do.

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$ ssh-agent screen # start a screen session with an ssh-agent
$ ssh-add          # in any of the windows of the screen.

Now no passwords are asked in any of the windows of the screen session. Usually I leave my screen session running in the office machine (which is physically secure as only I have the key to my office) and when I connect from home, I attach my self to the already running screen in my office machine.

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ppk@home: ssh office
ppk@office: screen -x  # connect to the already running screen

When I am done I detach the screen. Thus one can go on for months without typing any passphrase for any of the ssh/scp/sftp sessions.

SSH-tunneling or port forwarding.

One of the most powerful uses of ssh is its ability to port forward. You can build an ssh tunnel and connect a local port to a remote port. For all purpose this local port is the remote port. For example suppose there is an smtp server (mail server) running on remote which relays mails only from remote. You can set up a tunnel that connects your local port with that of the remote smtp port provided you have shell access to the remote host. Here is how you do it.

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$ ssh -N -L 2500:remote:25 ppk@remote &

Now you have a smtp server “running” at port 2500 of your machine. All the traffic to port 2500 is redirected via the ssh tunnel to the port 25 of the remote machine. If you want to actually forward the port 25 of you local machine, you need to be root on your local machine as this is a privileged port. However you don’t need root access on remote.

Using tunnel devices and ssh port forwarding one can also setup vpn like network. We wont go into the details in this article.