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SSH

Server​

# Is sshd running?
ps -aux | grep ssh

# Install if missing
sudo apt-get install openssh-server

# Configure (port, root login, key auth)
sudo vi /etc/ssh/sshd_config
#   Port 11422
#   PermitRootLogin yes
#   PubkeyAuthentication yes
sudo ufw allow 11422
sudo systemctl restart ssh

ssh-agent​

eval $(ssh-agent -s)
ssh-add path/to/key            # default key is ~/.ssh/id_rsa

Key permissions​

The private key must not be group- or world-readable, or the client refuses it (Permissions 0644 for '...id_rsa' are too open):

chmod 600 ~/.ssh/id_rsa

Authorizing a key on a server​

cd ~                                  # the target user's home
mkdir .ssh && chmod 700 .ssh
touch .ssh/authorized_keys && chmod 600 .ssh/authorized_keys
cat id_rsa.pub >> .ssh/authorized_keys
rm id_rsa.pub

The 700 on .ssh and 600 on authorized_keys matter: sshd ignores the keys if the permissions are looser, and the failure is silent from the client's side.

Trusting a host​

The first connection to a new host prompts to accept its fingerprint. To add it non-interactively:

ssh-keyscan -t rsa github.com >> ~/.ssh/known_hosts

Per-host configuration​

~/.ssh/config maps a short alias to a host and a specific key — the way to use different keys for different accounts on the same provider:

Host github-rojarsmith
  Hostname github.com
  IdentityFile ~/.ssh/id_rsa_account_name

A Git remote then uses the alias in place of the hostname:

# Real:  [email protected]:<account>/<repo>.git
# Alias: git@github-rojarsmith:<account>/<repo>.git

Converting PuTTY keys to OpenSSH​

sudo apt install putty-tools
puttygen private_key.ppk -O private-openssh -o id_rsa
puttygen public_key.ppk  -O public-openssh >> id_rsa.pub

# Straight into a server's authorized_keys
puttygen public.ppk -O public-openssh >> ~/.ssh/authorized_keys

Running a remote command from Windows​

plink.exe -ssh user@host -pw password -m c:\path\command.txt > output.txt