Set up ssh-agent

Checking out or updating the PHENIX sources will establish many ssh(1) connections to cci.lbl.gov. While not necessary for a successful installation, having an ssh-agent(1) running on the machine used to access cci.lbl.gov will save much error-prone password-typing.

Creating a cryptographic key pair

The first step is to generate a pair of cryptographic keys. One of the keys is public and the other is private; together they define an identity. This step can be skipped if a key pair already exists. In the example below, the key pair is generated in the default location, /home/user/.ssh/id_rsa. The passphrase, yourverysecretpasswordhere below, is used to encrypt the private key, and this passphrase will be needed to use the key pair. The passphrase should be complex—when using ssh-agent(1) one will not need to type it very often. It is possible to generate a key pair without a passphrase, but then anybody who gains access to the key also acquires access to all systems the key unlocks.

$ ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/home/user/.ssh/id_rsa):
Enter passphrase (empty for no passphrase): yourverysecretpasswordhere
Enter same passphrase again: yourverysecretpasswordhere
Your identification has been saved in /home/user/.ssh/id_rsa.
Your public key has been saved in /home/user/.ssh/id_rsa.pub.
The key fingerprint is:
ba:f2:7a:78:aa:b0:33:1d:53:de:63:01:62:15:d6:c9 user@host

Then, start the ssh-agent, and add the newly generated identity.

$ eval `ssh-agent`
$ ssh-add
Enter passphrase for /home/user/.ssh/id_rsa: yourverysecretpasswordhere
Identity added: /home/user/.ssh/id_rsa (/home/user/.ssh/id_rsa)

ssh(1) will now find the running ssh-agent(1), and attempt to use the keys it holds to authenticate with the remote system.

Distribute the public key to the remote system

The remote system has to be aware of the key pair to grant access. This means that the public component of the key pair has to be present in ~/.ssh/authorized_keys on the remote system.

$ cat ~/.ssh/id_rsa.pub | ssh user@remote.host "cat - >> ~/.ssh/authorized_keys"

This will append the public key of the recently created key pair to ~/.ssh/authorized_keys on the remote system.

Using keychain

An ssh-agent started from command line only affects ssh-connections started from the same shell, and separate ssh-agents would be required for each new login shell. The private key would have to be unlocked from every login shell separately. Furthermore, ssh-agents are not terminated when the login shell is exited, this has the potential to clutter the system with "orphaned" ssh-agents. keychain is a shell script that was written to avoid this situation: it probes the system for any running ssh-agents, and attempts to use a present agent instead of starting a new process. If no ssh-agents are running, keychain starts a new process and uses ssh-add(1) to add keys from ~/.ssh.

At CCI and SLAC keychain is installed at ~hattne/gentoo/usr/bin/keychain (at NERSC it is at /global/project/projectdirs/lcls/gentoo/usr/bin/keychain). It is recommended to run keychain from the shell configuration files for interactive, login shells. To do so, bash(1) users can add the following lines to ~/.bash_profile:

 test -x /path/to/keychain && \
   eval `/path/to/keychain --agents ssh --eval id_rsa --inherit any-once --stop others`

csh(1) would modify ~/.login instead.

Using agent forwarding

It is not necessary to start ssh-agents on every system logged in to. If an ssh-agent is running on the host used to access the remote system from, the agent can be forwarded using the ssh(1)’s -A option to forward the authentication agent when logging in. Note that forwarding the agent connection has security implications.