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SSH Usage on CEP
We use the Secure Shell (ssh) on CEP to connect to different systems. This page explains how this can be used without having to supply a password each time you want to connect to a system. The image below tries to explain the process:
With normal ssh you always have to give a password. If you use a private and public key, you can access systems where your public key is in $HOME/.ssh/authorized_keys from the system where you have the private key. With the ssh-agent explained below, you can ssh to any system without having to supply a password. (very useful to run things on nodes of a cluster, or other remote machines.
Generating keys
Linux or OS X
The first thing you need to do is generate an authorisation key using the DSA algorithm, which means you need to do the following once. You need to have a somewhat recent version of OpenSSL on your system for this to work:
ssh-keygen -tdsa cp .ssh/id_dsa.pub .ssh/authorized_keys
Use cat or some editor if authorized keys already exists and can't be simply copied. Copy your .ssh/authorized_keys
to the $HOME
of each system you want access to. Please make sure you use a passphrase to encrypt your private key, to prevent easy access. When using the instructions below on the ssh-agent, you'll only have to provide it once each time you use the systems.
Windows
Windows users can download the Putty
toolkit from http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html. Please select the Windows Installer as that contains the whole package.
After installing, you'll have the Putty program, and several help programs such as Pageant
.
Select from the Programs
menu the Putty
section and select PuttyGen
.
- Select in the
Paramaters
section the key type (e.g., SSH-2 DSA). - Press the
Generate
button. - Now you'll have to move your mouse over the grey area below the progress bar.
- Once done, you'll get a screen like the one shown below.
- Enter a passphrase (and confirm).
- Now you can save the public and private key files in a location of your choice.
Using an SSH-Agent
An ssh-agent is a small program that when you start work is used to unlock the passphrase protected private key you generated above. The ssh-agent will from that point on automatically supply the right answers to any ssh session, if you use ssh -A
each time you connect to another system.
Detailed information on how to setup ssh agent forwarding can be found here and here.
Linux
The ssh-agent runs in the user's local PC, laptop, or terminal. Authentication data need not be stored on any other machine, and authentication passphrases never go over the network. Also, the connection to the agent is forwarded over SSH remote logins, and the user can thus use the privileges given by the identities anywhere in the network in a secure way.
If you start the ssh-agent it creates a socket in /tmp
and then generates a few commands on stdout
which serve to set environmental variables and to tell you which PID the agent has. An example:
SSH_AUTH_SOCK=/tmp/ssh-jpIaV4861/agent.4861; export SSH_AUTH_SOCK; SSH_AGENT_PID=4862; export SSH_AGENT_PID; echo Agent pid 4862;
By 'eval'uating this code, the variables SSH_AUTH_SOCK and SSH_AGENT_PID will be set. You can use the SSH_AGENT_PID, for example, to kill the agent when you log off. The agent itself recognises the variable and commits suicide when you type:
ssh-agent -k
The SSH_AUTH_SOCK
variable is mainly used by the program ssh-add
, which uses it to determine with which agent to communicate. To get your agent running in (t)csh type:
eval `ssh-agent` ssh-add
Please note the back-quotes. If you have bash, you might use the more readable:
eval $(ssh-agent) ssh-add
OS X
Install SSH Agent 1.1 and set it to Open at Login or use the same commands as on Linux.
Windows
Windows users that have downloaded the Putty
package (see above) and have keys generated and saved on their system can use the Pageant
program in the Putty
distribution.
To use it, select Programs
→ Putty
→ Pageant
. Use the Add Key
button to select the Private key file on your system. You will be asked to enter the passphrase.
Once you have done this, Pageant
will be used to forward the keys to any system you have access to, and on which you have added the public key in the $HOME/.ssh/authorized_keys
file.
To enable forwarding of your key in Putty
, make sure that the session you use in Putty
has the option 'Agent Forwarding' enabled and the location of the key file filled in the Connection/SSH/Auth section (see screenshot below).