Mozilla has an excellent list of security guidelines for OpenSSH.
Copy an SSH public key to a remote machine
1 cat ~/.ssh/id_rsa.pub | ssh remoteuser@remotehost 'mkdir .ssh ; shat >> .ssh/authorized_keys'
will copy a public key to a remote machine, but most likely you want to use ssh-copy-id included with recent versions of OpenSSH.
Protect sshd from kernel OOM events
The kernel out-of-memory killer kills processes when a system runs out of RAM. Killing SSH typically does not help fix anything, and makes remotely-accessible systems inaccessible.
This is done by a few distributions, but when working on an arbitrary remote machine may be useful to run just in case.
Generate new SSH host key
Make sure /etc/ssh/sshd_config contains a path to the host key, e.g.:
Once it's there, create a new host key with:
sudo ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key
Backups and transferring stuff over SSH
Using tar to transfer over SSH:
tar czhf - $SRC_DIR | \ ssh $SSH_USER@$SSH_HOST \ "tar xzhf - -C $DST_DIR"
or doing so in two commands:
# Backup ssh root@$SSH_HOST "tar cpf - / --exclude=/sys --exclude=/proc --exclude=/dev" | pv | gzip | cat > backup.tar.gz # Restore cat backup.tar.gz | ssh root@$SSH_HOST "pv | tar zxvf - -C /"
Cloning a disk with dd over SSH:
ssh -C $USER@$REMOTE_HOST "dd if=/dev/sda" | dd of=/dev/sda
1 # $LOCAL_IP: 'localhost' or machine from local network 2 # $LOCAL_PORT: open port on local machine 3 # $REMOTE_IP: remote localhost or IP from remote network 4 # $REMOTE_PORT: open port on remote site 5 6 # Forward Tunnel: map port from remote machine/network on local machine 7 ssh -L $LOCAL_PORT:$REMOTE_IP:$REMOTE_PORT $USER@$SERVER 8 9 # Reverse Tunnel: make local port accesible to remote machine 10 ssh -R $REMOTE_PORT:$LOCAL_IP:$LOCAL_PORT $USER@$SERVER
Secure Secure Shell, with a corresponding ansible playbook securing ssh