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sshkey

Ssh without password

You can connect to the servers using the SSH protocol without the need to insert the password every time you connect. The trick is to generate a encripted key that can be used by the systems to recognize yourself and allow your access to local resources. This behaviour can be used when you are using the Servers from the Epfl Network, but also when you connect to the server from outside the Epfl.

Setup

Generate the public/private ssh key pair

Note that this doc applies only to Unix (Mac/Linux). There are similar things on window but they might not work exactly as described here.

  • Login in your workstation and open a Terminal Windows (if you are using the Graphical interface)
  • Execute this command
$ ssh-keygen -t dsa
  • When the system ask a name for the file id_dsa just press enter
  • The system will ask your for a password. This is the password that is used to encrypt your private key file (id_dsa) so that, even if the file gets stolen, it cannot be used. Try to generate a strong password.
  • Now you have to append the content of your public key file ~/.ssh/id_dsa.pub into the ~/.ssh/authorized_keys on the machines where you want to be able to ssh into (e.g. if you are generating the key pair on your laptop, you might want to add the public key on the cluster lth.epfl.ch):
$ cat ~/.ssh/id_dsa.pub | ssh USERNAME@lth.epfl.ch 'cat - >> ~/.ssh/authorized_keys'
  • In our cluster, all the machines have access to the same home directory. Therefore you can use standard local copy:
$ cat ~/.ssh/id_dsa.pub >> ~/.ssh/authorized_keys

From now, every time you connet to a server, the server will recognize your workstation automatically (it read the ~/.ssh/authorized_keys file everytime you connect) and you can login without bothering with the password…. well almost! Curious? Continue reading.

In fact, since your private key is encrypted, you need to type the password to decrypt it each time you want to use it. Seems like we have just replaced the “enter remote machine password” with “enter local key encryption password”.

Setup ssh-agent

Ssh-agent is a program that keeps in memory your decrypted private ssh key and serves it to ssh when it is needed. In this way you only have to type the decryption password only once. Keeping the decrypted key (or the password) in memory is considered safer than letting the key file un-encrypted.

On desktop sessions

All modern desktop environment (os X, Gnome, KDE), do take care automatically the ssh key decryption business and you don't have to do anything. Yahoo!

On console

If you are not on a full desktop environment (e.g. if you ssh to a machine from where you want to connect to [several] another machines), then you have to setup the ssh-agent business by hand.

The two (actually three) steps are

  • launch ssh-agent and setup the environment variables accordingly:
$ eval `ssh-agent -s`
  • ask ssh-agent to manage your id_dsa key (eventually with a lifetime -t time_in_seconds)
$ ssh-add [-t time_in_seconds] ~/.ssh/id_dsa
  • before logging out you have to remember to kill the ssh-agent:
$ ssh-agent -k

Since this is quite cumbersome, I suggest to add the following to your ~/.bash_profile:

# only for interactive shell
if [ "$PS1" ]; then
  # run ssh agent if not already running
  SSHAGENT=/usr/bin/ssh-agent
  if [ -z "$SSH_AUTH_SOCK" -a -x "$SSHAGENT" ]; then
    eval `$SSHAGENT -s`
    alias ssh='ssh-add; unalias ssh; ssh'
    trap "$SSHAGENT -k" 0
  fi
fi

This will start ssh-agent and add ssh-add only the first time you try to use ssh. This way you will have to type the password only if you really need to ssh.

sshkey.txt · Last modified: 2019/11/06 10:00 by admin