Mozilla has an excellent [[https://wiki.mozilla.org/Security/Guidelines/OpenSSH|list of security guidelines for OpenSSH]].

== Copy an SSH public key to a remote machine ==

This:

{{{#!highlight sh
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.

{{{#!highlight sh
for pid in $(pidof sshd) ; do
	echo "disabling oom on pid $pid"
  echo -17 | sudo tee /proc/$pid/oom_adj > /dev/null
done
}}}

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.:

{{{
HostKey /etc/ssh/ssh_host_ed25519_key
}}}

Once it's there, create a new host key with:

{{{#!highlight sh numbers=off
sudo ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key
}}}

== Backups and transferring stuff over SSH ==

Using tar to transfer over SSH:

{{{#!highlight sh numbers=off
tar czhf - $SRC_DIR | \
  ssh $SSH_USER@$SSH_HOST \
    "tar xzhf - -C $DST_DIR"
}}}

or doing so in two commands:

{{{#!highlight sh numbers=off
# 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:

{{{#!highlight sh numbers=off
ssh -C $USER@$REMOTE_HOST "dd if=/dev/sda" | dd of=/dev/sda
}}}

== SSH tunnels ==

{{{#!highlight sh
# $LOCAL_IP: 'localhost' or machine from local network
# $LOCAL_PORT: open port on local machine
# $REMOTE_IP: remote localhost or IP from remote network
# $REMOTE_PORT: open port on remote site

# Forward Tunnel: map port from remote machine/network on local machine
ssh -L $LOCAL_PORT:$REMOTE_IP:$REMOTE_PORT $USER@$SERVER

# Reverse Tunnel: make local port accesible to remote machine
ssh -R $REMOTE_PORT:$LOCAL_IP:$LOCAL_PORT $USER@$SERVER
}}}

== Interesting reading ==

 * [[https://stribika.github.io/2015/01/04/secure-secure-shell.html|Secure Secure Shell]], with a corresponding [[https://gist.github.com/cmavr8/eb4a9e596bd0e3e85f97d907de288c54|ansible playbook securing ssh]]

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CategoryCheatSheet