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SSH - Cheat Sheet

Last updated on July 14, 2022 -

A cheat sheet for popular SSH commands, key generation, SSH agents that I'm using a lot.

SSH-Keygen

Nowadays, most platforms recommend you to generate keys with the ed25519 algorithm.

ssh-keygen -t ed25519 -C "your@email.com"

If you prefer to go with RSA for compatibility reasons, use the following:

ssh-keygen -t rsa -b 4096 -C "your@email.com"

The -C file simply puts a comment on your public key, like below, so you can e.g. easily make out which public key belongs to which email address, in a busy Authorized_Keys file.

ssh-ed25519 KLAJSDLKSAJKLSJD90182980p1+++ your@email.com

Note: When generating SSH keys, make sure to protect your private key with a passphrase.

SSH with Keys

To use a specific private key to connect to a server, use:

ssh -i mykeyfile user@remotehost.com

Instead of specifying your key files manually with -i, use an SSH-Agent.

Authorized_Keys

Any remote host or service, like GitHub, that you want to use your SSH keys with, needs the public key of your SSH keypair.

For servers, you simply need to append your public key to the file ~/.ssh/authorized_keys.

Use one of the following commands to do that:

With services like GitHub, AWS etc. you would either use the UI to upload your public key, or, if available, command-line tools.

SCP

To upload a file to a remote server:

scp myfile.txt user@dest:/path

To recursively upload a local folder to a remote server:

scp -rp sourcedirectory user@dest:/path

To download a file from a remote server:

scp user@dest:/path/myfile.txt localpath

To recursively download a local folder to a remote server:

scp -rp user@dest:/remotedir localpath

Hint: When doing things recursively via SCP, you might want to consider rsync, which also runs over SSH and has a couple of advantages over SCP.

Hint 2: SCP has been deprecated and you should consider switching to (the less user-friendly) SFTP. The scp command uses the SFTP protocol since OpenSSH 9.

SSH-Agent

With a running OpenSSH agent (automatically available out of the box on most Linux distributions and macOS) simply use:

ssh-add privatekeyfile

To enable the OpenSSH agent on Windows, you’ll need to execute the following commands:

# By default the ssh-agent service is disabled. Allow it to be manually started for the next step to work.
# Make sure you're running as an Administrator.
Get-Service ssh-agent | Set-Service -StartupType Automatic

# Start the service
Start-Service ssh-agent

Note: On Windows/Linux adding a key to your ssh-agent once, even with a password, will make sure that the key gets associated with your 'login'. Meaning: When you restart your PC and log in again, you’ll have your identity automatically available again.

To get the same behavior on macOS, you’ll need to follow these instructions on StackExchange.

SSH Config

Create a file ~/.ssh/config to manage your SSH hosts. Example:

Host dev-meta*
    User ec2-user
    IdentityFile ~/.ssh/johnsnow.pem

Host dev-meta-facebook
    Hostname 192.168.178.1

Host dev-meta-whatsapp
    Hostname 192.168.178.2

Host api.google.com
    User googleUser
    IdentityFile ~/.ssh/targaryen.key

Note:

The Host directive can either

  • be a pattern (matching multiple follow-up Hosts)

  • refer to a made-up hostname (dev-facebook)

  • be a real hostname.

If it’s a made-up hostname, you’ll need to specify an additional Hostname directive, otherwise, you can leave it out. And to add to the overall confusion, a Host line can actually contain multiple patterns.

With the config file above, you could do a:

ssh dev-meta-facebook

Which would effectively do a ssh -i ~/.ssh/johnsnow.pem ec2-user@192.168.178.1 for you.

For a full overview of all available options, look at this article.

Git & Windows OpenSSH

To make Git use Window’s OpenSSH (and not the one it bundles), execute the following command:

git config --global core.sshcommand "C:/Windows/System32/OpenSSH/ssh.exe"

Exit Dead SSH Sessions

To kill an unresponsive SSH session, hit, subsequently.

Enter, ~, .

Multiple GitHub Keypairs

Trying to clone different private GitHub repositories, which have different SSH keypairs associated with them, doesn’t work out of the box.

Add this to your .ssh/config (this example assumes you have two GitHub keypairs, one for your work account and one for your personal account)

Host github-work.com
    Hostname github.com
    IdentityFile ~/.ssh/id_work

Host github-personal.com
    Hostname github.com
    IdentityFile ~/.ssh/id_personal

Then instead of cloning from github.com.

git clone git@github.com:marcobehlerjetbrains/buildpipelines.git

Clone from either github-work.com or github-personal.com.

git clone git@github-work.com:marcobehlerjetbrains/buildpipelines.git

SSH Agent Forwarding

Ever wanted to use your local SSH keys on a remote server, without copying your keys to that server? For example to git clone a private repository via SSH on a remote server?

Agent forwarding to the rescue. Edit your local .ssh/config file like so:

Host yourremoteserver.com
    ForwardAgent yes

Then simply ssh to your server and execute an _ssh-add -L. The server’s SSH agent should have all local SSH identities available and you can start cloning away!

SSH Agent Forwarding: Windows to WSL

If you want to use the Windows OpenSSH agent with all its identities from WSL, do the following:

  1. Install socat, e.g. on your WSL Distribution: e.g. apt install socat for Ubuntu/Debian.

  2. Download a build of npiperelay and put it somewhere on your (Windows) PATH.

  3. Put the following into your WSL ~/.bash_profile or ~/.bashrc.

# Configure ssh forwarding
export SSH_AUTH_SOCK=$HOME/.ssh/agent.sock
# need `ps -ww` to get non-truncated command for matching
# use square brackets to generate a regex match for the process we want but that doesn't match the grep command running it!
ALREADY_RUNNING=$(ps -auxww | grep -q "[n]piperelay.exe -ei -s //./pipe/openssh-ssh-agent"; echo $?)
if [[ $ALREADY_RUNNING != "0" ]]; then
    if [[ -S $SSH_AUTH_SOCK ]]; then
        # not expecting the socket to exist as the forwarding command isn't running (http://www.tldp.org/LDP/abs/html/fto.html)
        echo "removing previous socket..."
        rm $SSH_AUTH_SOCK
    fi
    echo "Starting SSH-Agent relay..."
    # setsid to force new session to keep running
    # set socat to listen on $SSH_AUTH_SOCK and forward to npiperelay which then forwards to openssh-ssh-agent on windows
    (setsid socat UNIX-LISTEN:$SSH_AUTH_SOCK,fork EXEC:"npiperelay.exe -ei -s //./pipe/openssh-ssh-agent",nofork &) >/dev/null 2>&1
fi

Enjoy!

Major thanks to Stuart Leeks, who I blatantly stole this code from - he did all the work @ https://stuartleeks.com/posts/wsl-ssh-key-forward-to-windows/.

Check out his WSL Book for more such tricks!

SSH Tunnels

Want to connect to a server that is hidden from the outside world, but accessible from a box you have SSH access to? Like an Amazon RDS database, which is only reachable from inside an AWS network?

Use SSH forwarding

ssh username@jumphost -N -f -L localport:targethost:targetport

The following command establishes an SSH tunnel between my local machine (@port 3307) and an RDS database (@port 3306), via an EC2 jump host (18.11.11.11).

ssh ec2-user@18.11.11.11 -N -f -L 3307:marcotestme.12345.eu-central-1.rds.amazonaws.com:3306

You could now, for example, use the mysql client to connect to localhost:3307, which will be transparently tunneled to RDS for you.

mysql -h localhost -P 3307

Note: A lot of tools/IDEs like IntelliJ IDEA, support opening up SSH tunnels by just clicking a checkbox in the UI.

Password Managers & SSH Agents

Password Managers like 1Password or Keepass can not only store your SSH keys, but they also come with their own ssh-agent, replacing your system’s ssh-agent.

This means, whenever you unlock your password manager on any machine that you have it installed on, you’ll have all your SSH identities instantly available.

Super useful!

Video

If you prefer video, you can see the guide in action on YouTube.

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I'm @MarcoBehler and I share everything I know about making awesome software through my guides, screencasts, talks and courses.

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