Sunday, December 02, 2012

SSH use specific private key


When connecting remotely to a Linux host most of us will (I hope) use SSH to establish a secure connection. Some people will use a single key pair for all the hosts they need to connect to when there is a need for pasword-less authentication to a Linux machine. However, when you are working in different environments and have different keys (and usernames) you can end up with a list of key pairs for all kind of hosts. Meaning you have to instruct the SSH client to use a specific key when connecting to a specific machine you can use the -i options.

For example if you have you private key stored in /user/jolouwer/backup_key for the server somehost.example.com you can use the following way to connect to this host. In our case we connect as the user dummy;

ssh -i  /user/jolouwer/backup_key dummy@somehost.example.com

In this way it will read the specific key located in /user/jolouwer/backup_key and use this to connect tot he server. Now you can use this way to connect to every server you like however you will have to find a way to remember which key to use for which server. More easy is to list them in your ssh config.

You can do so by adding some config to ~/.ssh/config
For our example we should add the following 2 lines to the file;

Host somehost.example.com
    IdentityFile /user/jolouwer/backup_key

this will ensure that the mentiond key is used every time you will connect from your workstation tot the somehost.example.com host. 
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