SSH - network services

A guide about how to configure and use SSH in Ubuntu. The following questions will be answered in this guide:

How to log into a remote machine?

How to configure the login properly with an SSH key and a config file?

How to transfer files from one machine to another?

Log into a remote machine

Different remote machines are usually used during the software development, like spring, blueberry or the cluster fabulous. The most direct way to log into one of these machines is to open a terminal and to enter the following command:

ssh username@machine

For instance,

ssh boemer@blueberry.ltas.ulg.ac.be

The terminal then asks for the password and the connection is established. Once logged in, it is possible to execute all kinds of commands on the remote machine. Finally, to leave the remote machine, the command exit has to be executed.

Always having to type the full username@machine specification as well as the password can become tiring in the long run. For this reason, it is explained in the following section how to log into a remote machine by a command as simple as the following one, without having to enter a password:

ssh blueberry

Login configuration

In the first part of this section, it is explained how to log into a remote machine without a password. In the second part, the creation of a config file is explained.

SSH keys

SSH keys are similar to passwords. They can be created by the command ssh-keygen. During the creation of the key files, the user is asked to enter a keyphrase. This keyphrase is an additional password, which can be used with the key files. If you do, however, not want to add any further security, you can simply press Enter. The ssh-keygen command then creates two files in the .ssh directory (in $HOME): a private identification key called id_rsa and a public verification key id_rsa.pub.

The public key has to be transfered to the remote machine by the command ssh-copy-id username@machine, e.g. ssh-copy-id boemer@blueberry.ltas.ulg.ac.be. This command adds the alpha-numeric key in id_rsa.pub to the file .ssh/authorized_keys on the remote machine.

When you try to connect for the next time to the remote machine, no password will be asked since the ssh command compares your private key in .ssh/id_rsa with the public key in .ssh/authorized_keys on the remote machine. If the comparison is successful, the connection is established. In reality, a more complex operation than a simple comparison is performed. See RSA.

You might want to connect from one remote machine immediately to another remote machine without entering the password, e.g.

ssh boemer@blueberry.ltas.ulg.ac.be
ssh boemer@fabulous.ltas.ulg.ac.be

The easiest way to do so is to copy your private key id_rsa to the first remote machine, provided that the public key id_rsa.pub has already been transfered to the second remote machine by the ssh-copy-id command. The private key is transfered to the first remote machine by the following command:

scp .ssh/id_rsa boemer@blueberry.ltas.ulg.ac.be:.ssh/

It might happen that your file permissions in the .ssh folder are not sufficiently restrictive. To change these permissions execute the following command in the home directory:

chmod -R 700 .ssh

This command sets the permission recursively for all files in the .ssh directory to “read/write/execute only allowed by the user”.

Previously, it was suggested to copy your private key id_rsa on all machines. Notice that whoever has this key can access your machines. It is as if he had your password. So pay attention to not leave this file on machines accessible by people, who you do not know.

Config file

To log into a remote machine by typing

ssh blueberry

instead of

ssh boemer@blueberry.ltas.ulg.ac.be

a configuration file named config has to be created in the .ssh folder. In the previous example, this file should contain the following content:

Host blueberry
    HostName blueberry.ltas.ulg.ac.be
    User boemer
    IdentityFile ~/.ssh/id_rsa

Additional defintions can simply be added to this file. If these abbreviations should also work on other machines, the config file has to be copied into the .ssh folder of these machines,

File transfer

Besides using FileZilla, files can also be directly transfered by SSH, or more precisely, by SCP, i.e. Secure Copy. For instance, the config file of the previous section can be copied into the .ssh folder of another machine, say blueberry, by the following command:

scp .ssh/config boemer@blueberry.ltas.ulg.ac.be:.ssh/

Or, in short,

scp .ssh/config blueberry:.ssh/

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