Secure Shell (SSH) is the standard program for connecting to remote servers and transferring data. It is very secure and well-supported, so it’s worth learning to use it properly. This page both gives a bit of a crash course (top) and more details (bottom) for all common connection methods.
Check the tabs below for your operating system and methods to see which method you want to use.
PowerShell is built in to Windows 10 and includes OpenSSH (the same as on Linux). Start the “Windows PowerShell” program. Then, follow the “Command line” instructions on most of this page if there isn’t a separate PowerShell tab. If you want to set up SSH keys there are a few differences but overall it is the same procedure.
This should work by default on recent Windows 10.
The Windows Subsystem for Linux lets you install a Linux operating system inside of Windows. This is what we recommend with Windows, if it works.
Install the Windows Subsystem for Linux and then use the “Command line” instructions. This will give you a top-level interface to scientific work on your computer.
This may not work if you do not have proper admin rights on your computer (e.g. if it is university managed). Ask your IT support.
This should only be used if the other methods don’t work.
PuTTY is a separate application that includes a terminal and SSH together. This used to be recommended before Windows 10. There aren’t detailed instructions below, but most of the ideas can be done with PuTTY somehow (except that SSH keys take more work).
MobaXterm is a separate application that allow SSH and also graphical applications. It’s liked by some people, but is freeware/commercial so isn’t discussed much more here. TODO: someone could describe it more if they wanted.
SSH is built-in to almost any distribution. If it’s not there,
try installing the
Start the Terminal application to follow the rest of the instructions. Then, follow the “Command Line” instructions on most of this page.
SSH should be built-in. Start the Terminal application. Then, follow the “Command Line” instructions on most of this page.
This guide uses Aalto University’s HPC cluster as an example, but should be applicable to other remote servers at Aalto as well and many other outsiders as well.
Basic use: connect to a server
The standard login command with the command line is:
$ ssh USER@triton.aalto.fi
USER is your username (Aalto: standard Aalto login, not
email address) and
triton.aalto.fi is the address of the server
you with to connect - replace this for your situation.
First time login: check host key
When connecting to a new computer, you will be prompted to affirm that you wish to connect to this server for the first time. This lets you make sure you are connecting to the right computer (which is important if you type a password!). You’ll get a message such as:
The authenticity of host 'triton.aalto.fi (22.214.171.124)' can't be established. ECDSA key fingerprint is SHA256:04Wt813WFsYjZ7KiAyo3u6RiGBelq1R19oJd2GXIAho. Are you sure you want to continue connecting (yes/no)?
If possible, compare the key fingerprint you get to the one for the
machine which you can find online (Triton cluster:
Triton ssh key fingerprints, Aalto servers),
and if they do not match, please contact the server administrator
immediately. If they do match, type
yes and press enter. You
will receive a notice:
Warning: Permanently added 'triton.aalto.fi,126.96.36.199' (ECDSA) to the list of known hosts.
The public key that identifies Triton will be stored in the file
~/.ssh/known_hosts and you shouldn’t get this prompt again. You
will be also asked to input your Aalto password before you are fully
logged in. You want to say “yes, save the key for the future” - it’s
more secure and you can always change it later if needed.
Checking known servers
You will not receive an authenticity prompt upon first login if the
server’s public key can be found in a list of known hosts. To check
whether a server, for example
kosh.aalto.fi, is known:
$ ssh-keygen -F kosh.aalto.fi
Your computer might come with some keys pre-loaded for your university’s computers, for example:
$ ssh-keygen -f /etc/ssh/ssh_known_hosts -F kosh.aalto.fi
SSH keys: better than just passwords
By default, you will need to type your password each time you wish to ssh into Triton, which can be tiresome, particularly if you regularly have multiple sessions open simultaneously. A more secure (and faster) way to authenticate yourself is to use a SSH key pair (this is public-key cryptography. The private key should be encrypted with a strong password xkcd has good and amusing recommendations on the subject of passwords. This authentication method will allow you to log into multiple ssh sessions while only needing to enter your password once, saving you time and keystrokes.
Generate an SSH key
While there are many options for the key generation program
ssh-keygen, here are the four main ones.
-t-> the cryptosystem used to make the unique key-pair and encrypt it.
-f-> filename of key
-C-> comment on what the key is for
Here are our recommended input options for key generation:
$ ssh-keygen -t ed25519
This works on Linux, MacOS, Windows
The PuTTYgen program can generate keys. We don’t go into more details right now. This provides a graphical application to generate keys and from here you would extract the OpenSSH format keys to copy to the servers.
Accept the default name of the key file by pushing enter with no extra
text(it will be automatically used later). Then, you will be prompted
to enter a password. PLEASE use a strong unique password. Upon
confirming the password, you will be presented with the key
fingerprint as both a SHA256 hex string as well as randomart
image. Your new key pair should be found in the hidden
directory (A directory called
.ssh in your user’s home directory).
ed25519 makes a private key named
and public key named
~/.ssh/id_ed25519.pub. The private key only
stays on your computer. The public key goes to other comuters.
Other key types were common in the past, and you may need to change
your filenames in some of the future commands (for exmaple
Copy public key to server
In order to use your key-pair to login to a server (for example: the
Triton cluster), you first need to securely copy the desired public
key to the machine with
ssh-copy-id. The script will also add the
key to the
~/.ssh/authorized_keys file on the server. You will be
prompted to enter your Aalto password to initiate the secure copy of
the file to Triton.
$ ssh-copy-id -i ~/.ssh/id_ed25519.pub USER@triton.aalto.fi
With this, we have to also make the directory and make sure the file has the right permissions.
$ ssh USER@triton.aalto.fi "mkdir -p ~/.ssh ; chmod go-rwx ~/.ssh" $ cat ~/.ssh/id_ed25519.pub | ssh USER@triton.aalto.fi "cat >> ~/.ssh/authorized_keys" $ ssh USER@triton.aalto.fi "chmod go-rwx ~/.ssh/authorized_keys"
Connect to the system via some method and get a shell. Copy the
OpenSSH public key (it should be one line, though a quite long
line). You’ll want to past the key as a new line in the file
~/.ssh/authorized_keys file on the other server. This is a
file in your home directory (
~), in the
From a terminal on the remote computer, you can run these
commands to make a
.ssh directory, edit the file, and set the
nano is a common editor, if it’s not
available you need to use a different one:
$ mkdir -p ~/.ssh $ nano ~/.ssh/authorized_keys ## Paste the key into that file and save. $ chmod -R go-rwx ~/.ssh/
You can also edit
.ssh/authorized_keys to manage your keys
You’ll need to grab the key from PUTTYgen and copy it to the remote server. Copy the key from PuTTYgen and then us ethe “Manual” instructions.
Connecting from outside of the Aalto network
Sometimes, you can’t connect directly to the computer you need to, since there is a jump host as some sort of a firewall. You need to connect to that computer first. This is described below in the section ProxyJump, but we give first workaround here. but roughly.
All this is easier if you set up a config file with ProxyJump
-J) first, and copy keys one at a time. (see as
described below). Once this is done, you
can copy your key to
kosh first, then
Aalto University: If you can connect by VPN, or to Eduroam, then you can directly access the Triton cluster and copy your key like above.
First copy the key to the jump host (like
copy to your final destination (like
$ ssh-copy-id -i ~/.ssh/id_ed25519.pub USER@kosh.aalto.fi $ ssh-copy-id -i ~/.ssh/id_ed25519.pub -J USER@kosh.aalto.fi USER@triton.aalto.fi
Like before, since
ssh-copy-id isn’t available, we have
to do extra steps to make sure the key is has the right
permissions - twice! You may need to enter your password
many times here.
## Copy stuff to our jump host $ ssh USER@kosh.aalto.fi "mkdir -p ~/.ssh ; chmod go-rwx ~/.ssh" $ cat ~/.ssh/id_ed25519.pub | ssh USER@kosh.aalto.fi "cat >> ~/.ssh/authorized_keys" $ ssh USER@kosh.aalto.fi "chmod go-rwx ~/.ssh/authorized_keys" ## Copy stuff to the real destination $ ssh -J USER@kosh.aalto.fi USER@triton.aalto.fi "mkdir -p ~/.ssh ; chmod go-rwx ~/.ssh" $ cat ~/.ssh/id_ed25519.pub | ssh -J USER@kosh.aalto.fi USER@triton.aalto.fi "cat >> .ssh/authorized_keys" $ ssh -J USER@kosh.aalto.fi USER@triton.aalto.fi "chmod go-rwx ~/.ssh/authorized_keys"
Login with SSH key
If the key is in one of the standard filenames, it should work directly.
SSH key agent
To avoid having to type the decryption password, the private key
needs to be added to the
ssh-agent with the command
You will need administrative permissions to be able to start a ssh-agent on your machine that can store and handle passwords.
Open Services from the start menu
Scroll down to OpenSSH Authentication Agent > double click
Change the Startup type to Automatic (Delayed Start), or anything that is not Disabled, then Apply, and also start the service manually if it is not yet running.
ssh-addto add the default key (to add a certain key, use
ssh-add ~/.ssh/id_ed25519, for example)
The program Pagent (“PuTTY Agent”) can unlock your keys once and give them to PuTTY each time they are needed. You can add keys and manage it from the small icon in the system tray. TODO: more instructions on using Pagent.
SSH is likely to automatically save the key the first time you use it, so that you don’t have enter your key’s password multiple times. If not, this will probably add it:
(You’ll get a message if
ssh-agent is not running. In this
case, to start a new agent, use
eval $(ssh-agent). It’ll
only work for this one shell, check the rest of the Internet for
how to do more.) TODO: is any more needed?
$ ssh-add --apple-use-keychain ~/.ssh/id_ed25519
Once the password is added, you can ssh as normal but will immediately be connected without any further prompts for passwords.
Often, you can’t connect directly to your target computer: you need to
go through some other firewall host. This is often done with two
ssh commands, but can be done with only one with the
-J (ProxyJump) option:
$ ssh -J FIREWALL.aalto.fi triton.aalto.fi
Both of these can take more options, for example if you need to specify your username you might need to do it twice:
$ ssh -J USER@FIREWALL.aalto.fi USER@triton.aalto.fi
Read more details at https://www.redhat.com/sysadmin/ssh-proxy-bastion-proxyjump, including putting this in your configuration file (or see below).
(Windows with PuTTY: Connection > Proxy > Proxy type=”SSH to proxy and use port forward.”, then enter the firewall host as “Proxy hostname” and port 22.
Connections can be even faster: you can re-use existing connections to
start new connections, so that future
ssh commands to the same
host are almost instant. It multiplexes across the same
connection, and is controlled by
ControlPersist. With a proper SSH key setup, the gain is
minimal, but it can be useful sometimes. It is not recommend to use
this unless you really want this, since there are some gotchas::
Connections hanging (e.g. unstable network, changing network) will cause all multiplexed connections to hang.
All multiplexed connections need to stop before the master process (first SSH connection) will stop. So if you try to exit the first SSH but child processes are using it, it will appear to hang - this may not be obvious.
If you are using with ProxyJump, there are two possible SSH processes which can hang and cause things to go wrong.
Only use this on your own computers that you control, for security reasons.
This works with OpenSSH. If you want to use this, to you ssh config
file (see below) add
ControlMaster auto and
/tmp/.ssh-USER-mux-ssh-%r@%h:%p (replacing USER with your username)
and test well. You might want
ServerAliveInterval 30 to kill
stuff soon if network goes down. We don’t give a full example to
prevent unintended problems. If you notice weird things happening
with your ssh, point your helpers to this section.
Config file: don’t type so many options
Remembering the full settings list for the server you are working on
each time you log in can be tedious. A ssh
config file allows you
to store your preferred settings and map them to much simpler login
commands. To create a new user-restricted
$ touch ~/.ssh/config && chmod 600 ~/.ssh/config
$ New-Item ~/.ssh/config
Open the created file to edit it as indicated below.
For a new configuration, you need specify in
config at minimum the
Host: the name of the settings list
User: your login name when connecting to the server (if different from the username on your computer)
Hostname: the address of the server
So for the simple Triton example, it would be:
# Configuration file for simplifying SSH logins # # HPC slurm cluster Host triton User LOGIN_NAME Hostname triton.aalto.fi
and you can use only this command to log in from now on:
$ ssh triton
Any additional server configs can follow the first one and must start
with declaring the configuration
# general login server Host kosh User LOGIN_NAME Hostname kosh.aalto.fi # light-computing server Host brute User LOGIN_NAME Hostname brute.aalto.fi
There are optional ssh settings that may be useful for your work, such as:
# Turn on X11 forwarding for Xterm graphics access ForwardX11 yes # Connect through another server (eg Kosh) if not connected directly to Aalto network ProxyJump USER@kosh.aalto.fi
Full sample config file
The following code is placed in the config file created above
~/.ssh/config on Mac/Linux or
# general login server Host kosh User LOGIN_NAME Hostname kosh.aalto.fi # Triton, via kosh Host triton_via_kosh User LOGIN_NAME Hostname triton.aalto.fi ProxyJump kosh
Now, you can just do command such as:
$ ssh triton $ rsync triton:/m/cs/scratch/some_file . ## And this works in any other tool that uses ssh.
directly, by using the
triton alias. Note that the Triton rule
uses the name
kosh which is defined in the first part of the
man ssh gives a detail of the SSH command line options
man ssh_config gives a detail of all of the config file options
https://blog.0xbadc0de.be/archives/300 - long-form guide
https://www.ssh.com/ssh/ - commercial site