Archive for December, 2012

SSH private/public keys with passphrase and agent manager

2012-12-04 Leave a comment

Managing multiple systems that you only have access to by ssh can be annoying when you have to keep typing in ssh <HOST/IP>; then typing your password on each system.

This article explains how to use ssh pub/priv key pairs with a passphrase and ssh-agent.
The goal is to make logging in remotely more secure by using key pairs along with a passphrase but only having to use the passphrase once in a given time period.

First you’ll need to generate your own private/public key pair on the system you’ll be sshing FROM using the following command.

$ ssh-keygen -t rsa -b 4096 -f ~/.ssh/<ROLE>_rsa -C "Comment goes here"
Enter file in which to save the key (/home//.ssh/id_rsa):
Enter passphrase (empty for no passphrase):

You should never give your private key file or it’s contents to anyone. Think of it like a key to your house anyone that has it can access your “house”(server).

  • The comment just helps to let you know what the key is used for. I normally put the server(s) I’m going to push the public key too in the comments. This could be considered a minor security risk labeling where the key is used.
  • Some systems default to DSA, I recommend RSA 2048 bits or higher for the keys hence the keygen options -t rsa -b 4096. Two short write ups about DSA vs RSA can be found and .
  • Do not leave the passphrase empty as the worst case someone gets hold of your private key or gains access to your account they at least have to know the passphrase (back to the previous way of using just a password for security) to use it. An empty passphrase means anyone with the key can enter the server if they know which server the key is used on (hence the comment security risk).
  • If you generate more than one key pair and use the same passphrase, entering the passphrase will make all the key pairs that use that passphrase active at the same time.

Now that we have the keys generated the next step is to copy the .pub key text to the remote servers .ssh/authorized_keys file. I wrote this little bash script as a wrapper for the command used to publish the key. It prompts for what user to ssh with and the server host/ip then the ssh-copy-id command will prompt for the users password. It will automatically create the .ssh directory and append the key to the authorized_keys file. If your system doesn’t have ssh-copy-id I have included a bash 1 liner below the script that will check if the .ssh directory exists and create it then append the public key to the authorized_keys file.

if [[ -z "$sshusername" ]] && [[ -z "$serverIPaddress" ]] ; then
## Prompt for the user name to use
echo -en "\nIs $USER the account you want to use? \n" ;
select yn in "Yes" "No"; do
case "$yn" in
Yes ) sshusername="$USER" ; break ;;
No ) read -p "Type the username you want to use: " sshusername ; break;;
done ;
echo -en "\n" ;

## Prompt for the server name or IP
read -p "Type the Hostname or IP address of the server: " serverIPaddress ;
echo -en "\n" ;

## The actual command to copy the key over
ssh-copy-id "$sshusername"@"$serverIPaddress" ;

## Clean up of the vars we used
unset sshusername ;
unset serverIPaddress ;
## The system uses the variables?!?
echo -en "Your system currently has the following variables set.\n sshusername AS $sshusername\n serverIPaddress AS $serverIPaddress\n\n";
fi ;

One way to push the new pub key to the server is by using the ssh-copy-id binary command. If you are using some other port besides 22 you’ll need to include the username/host and port option all in quotes.

#ssh-copy-id -i <path to pub key> "username@host/ip -p <port>"
ssh-copy-id -i .ssh/
ssh-copy-id -i .ssh/ " -p 9999"

Bash one liner for pushing public key to remote system if youre unable to use ssh-copy-id. Be sure to put the correct and <HOST/IP> settings for the ssh command.

cat ~/.ssh/ | ssh @<HOST/IP> 'if [[ -z "$HOME/.ssh" ]] ; then mkdir $HOME/.ssh ; fi ; cat - >> ~/.ssh/authorized_keys'

Now we should have a priv/pub key par in our .ssh folder and on the remote system our public key should be in the authorized_keys file. To test if it’s working ssh to the system. You should be prompted with the following instead of the @<HOST/IP>’s password:

$ssh server
Enter passphrase for key '/home//.ssh/id_rsa':

Upon entering the correct passphrase you will now be logged in to the remote system. Exit out of the system and try sshing again you’ll notice you get prompted again for the passphrase. Great all this seemed to do was add an extra layer of security but didn’t stop the annoying issue of each time having to enter a password/passphrase.

Now it’s time to use a key manager such as ssh-agent which allows us to enter the passphrase once and after that anytime we try to ssh to any server that has our public key we’ll get direct access (for a given amount of time before we have to reenter the passphrase).

On my systems I create two alias commands some people want to have their system prompt for the password when they open a session the first time I prefer to start start it when I need too that way if a day I don’t need to ssh to a server (it happens some times…) my shell doesn’t have access.

The two aliases I use are below.

## type agent once on the machine your are using unless the time elapses or the system was rebooted
## Removes the old hostname agent file
## starts the agent with 28800 seconds (8hrs) to be active in memory
##   and story the environment variables to access the keys in memory in the .agent file
## run the .agent file as a Tlc script and then add the private key identities to the authentication agent
alias agent='rm -f "$HOME"/.ssh/`hostname`.agent ; ssh-agent -t 28800 | grep -v echo > "$HOME"/.ssh/`hostname`.agent ; source "$HOME"/.ssh/`hostname`.agent ; ssh-add'

## Any new shells you just need to run this alias to have them use the agent in memory
alias sshagent='if [ -e "$HOME"/.ssh/`hostname`.agent ]; then source "$HOME"/.ssh/`hostname`.agent ; fi'

Now you have the generated keys with the remote system(s) having the public key in the auth file and you are able to use an agent so you only have to enter your passphrase once allowing you to ssh to any systems with the public key with out any prompts.

A system is only as secure as it’s user.

Categories: Bash, Computers, Linux, Security