SSH

general overview

# add -v for verbose mode

# connect
ssh <user>@<ip or url>

# connect on non standard port
ssh -p 12345 <user>@<ip or url>

# connect use rsa private key saved under ~/.ssh/ instead of password
ssh -i ~/.ssh/id_rsa <user>@<ip or url>

ssh tunnel

nice explanation of ssh tunnels

# ssh tunnel which connects my local port 1025 with the remote port 4000 over the standard ssh port 22
ssh -L1025:192.168.x.x:4000 -v <user>@<ip or url>

# if ssh is forwarded it can be used via -p flag
ssh -p1025 <user>@192.168.x.x

handle identity with password

For .bashrc or .zshrc to cache password on startup if not cached already:

if [ -z "$SSH_AUTH_SOCK" ]; then
  eval "$(ssh-agent -s)" > /dev/null
fi

# Function to check if the key is already added
check_key_added() {
  ssh-add -l | grep -q "$(ssh-keygen -lf ~/.ssh/id_ed25519 | awk '{print $2}')"
}

# Add the key if it's not already added
if ! check_key_added; then
  echo "Add ssh key for 8 hours"
  ssh-add -t 28800 ~/.ssh/id_ed25519 # 8 hours timeout
fi

generate ssh key

The -i flag can be omitted if the standard file names id_rsa (private key) and id_rsa.pub (public key) are used.

# generate a private and public key pair in ~/.ssh/ folder
ssh-keygen

# permisions of pub files
chmod 700 ~/.ssh
chmod 644 ~/.ssh/id_rsa.pub
chmod 600 ~/.ssh/id_rsa

trust ssh key on remote server

# your user: copy your public key to the authorized_keys file in your home directory on a remote machine
# allows access with your private key instead of a password
ssh-copy-id -i ~/.ssh/id_rsa user@host

# other user:  copy your public key to the authorized_keys file in another home direcotry
cat ~/id_rsa.pub | ssh your_user@remote.server.com “sudo tee -a /USER/.ssh/authorized_keys”

location of known_hosts

Known hosts are saved in following locations - as entry in /etc/ssh/ssh_known_hosts - as entry in /etc/ssh/ssh_config - as *.pub file in /etc/ssh/ folder

add and remove entries from known_hosts

```shell

remove old entry from known_host

ssh-keygen -R SERVER_NAME -f ~/.ssh/known_hosts

add new key to known_hosts

ssh-keyscan SERVER_NAME >> ~/.ssh/known_hosts