Because OpenSSH allows you to run commands on remote systems, showing you the results directly, as well as just logging in to systems it’s ideal for automating common tasks with shellscripts and cronjobs. One thing that you probably won’t want is to do though is store the remote system’s password in the script. Instead you’ll want to setup SSH so that you can login securely without having to give a password.
Thankfully this is very straightforward, with the use of public keys.
To enable the remote login you create a pair of keys, one of which you simply append to a file upon the remote system. When this is done you’ll then be able to login without being prompted for a password — and this also includes any cronjobs you have setup to run.
If you don’t already have a keypair generated you’ll first of all need to create one.
If you do have a keypair handy already you can keep using that, by default the keys will be stored in one of the following pair of files:
- ~/.ssh/identity and ~/.ssh/identity.pub
- (This is an older DSA key).
- ~/.ssh/id_rsa and ~/.ssh/id_rsa.pub
- (This is a newer RSA key).
If you have neither of the two files then you should generate one. The DSA-style keys are older ones, and should probably be ignored in favour of the newer RSA keytypes (unless you’re looking at connecting to an outdated installation of OpenSSH). We’ll use the RSA keytype in the following example.
To generate a new keypair you run the following command:
skx@lappy:~$ ssh-keygen -t rsa
This will prompt you for a location to save the keys, and a pass-phrase:
Generating public/private rsa key pair. Enter file in which to save the key (/home/skx/.ssh/id_rsa): Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/skx/.ssh/id_rsa. Your public key has been saved in /home/skx/.ssh/id_rsa.pub.
If you accept the defaults you’ll have a pair of files created, as shown above, with no passphrase. This means that the key files can be used as they are, without being "unlocked" with a password first. If you’re wishing to automate things this is what you want.
Now that you have a pair of keyfiles generated, or pre-existing, you need to append the contents of the .pub file to the correct location on the remote server.
Assuming that you wish to login to the machine called mystery from your current host with the id_rsa and id_rsa.pub files you’ve just generated you should run the following command:
ssh-copy-id -i ~/.ssh/id_rsa.pub username@mystery
This will prompt you for the login password for the host, then copy the keyfile for you, creating the correct directory and fixing the permissions as necessary.
The contents of the keyfile will be appended to the file ~/.ssh/authorized_keys2 for RSA keys, and ~/.ssh/authorised_keys for the older DSA key types.
Once this has been done you should be able to login remotely, and run commands, without being prompted for a password:
skx@lappy:~$ ssh mystery uptime 09:52:50 up 96 days, 13:45, 0 users, load average: 0.00, 0.00, 0.00