Ssh

Quickstart

• For any new host, you need to copy private key to .ssh and cat public key into .ssh/authorized_keys.

ssh newhost
export GOODHOST=bitpost.com
# -- YOU CAN COPY AND PASTE THIS --
mkdir -p .ssh
cd .ssh
scp $GOODHOST:.ssh/id_rsa* .
cat id_rsa.pub >> authorized_keys
# -- COPY AND PASTE END --

• Control-D to disconnect and ssh back in to make sure it's working

Clean room hardened config

If you're dealing with a machine that has some unknown ssh config, reset it to hardened key auth as follows. This is a good way to start in general, for completeness.

Client

ssh-keygen -o -a 256 -t ed25519 -C "$(hostname)-$(date +'%d-%m-%Y')"
#Make the .ssh directory unreadable for other users and groups
chmod 700 ~/.ssh			
chmod 700 /home/$USER/.ssh	 		
#Make the private SSH key read only
chmod 400 /home/$USER/.ssh/id_ed25519 
chmod 400 ~/.ssh/id_ed25519 				 	    	
#Make the local $USER own the SSH key pair files
chown $USER:$USER ~/.ssh/id_ed25519*		 
chown $USER:$USER /home/$USER/.ssh/id_ed25519*

Server

sudo rm /etc/ssh/ssh_host_*                  #Delete old SSH keys
rm ~/.ssh/id_*                               #Delete old SSH keys
sudo dpkg-reconfigure openssh-server         #Reset SSH config to defaults and generate new key files
rm /home/$USER/.ssh/id_*                     #Delete old SSH keys
vi /home/$USER/.ssh/authorized_keys          #paste public key here
cd /home/$USER/ && chmod g-w,o-w .ssh/       #The directory containing your .ssh directory must not be writeable by group or others
chmod 600 /home/$USER/.ssh/authorized_keys   #change permissions to r+w only for user
sudo service sshd restart                    #restart and reload keys into the SSH deamon

Generate fresh keys

• If you need a new identity, use this to generate the keys:

cd ~/.ssh 
# use ed25519, the newest cipher, it's smaller, performs better AND more secure
ssh-keygen -t ed25519 -C moodboom@gmail.com
chmod 400 id_rsa
cat id_rsa.pub >> authorized_keys

Shortcut and Key organization

• Always set up shortcuts to get to hosts fast in .ssh/config; see common for example, it's used everywhere.
• You can use any file for key by specifying an [IdentityFile] in .ssh/config.

Enable X11 forwarding

• server: install xauth
• server: enable in /etc/ssh/sshd_config
• server: make sure you have an X environment installed (install of any X app will pull it in)
• client: set [ForwardX11Trusted yes] in /etc/ssh/ssh_config (no doesn't work)
• client: make sure you have an X server environment - on windows, use x2go
• if you need to become root after you have ssh'ed (for example to run wireshark), do this magic to patch in x authorization, oh yeah!

sudo su -
[root ~]# touch .Xauthority
[root ~]# xauth add $(xauth -f ~ec2-user/.Xauthority list|tail -1)
[root ~]# xclock& # good to go!

NOTE: This step gets it going automatically, but that has messed with the ability to ssh/git through non-X servers like bitpost. Just use [ssh -Y] as needed.

• client: set [ForwardX11 yes] in /etc/ssh/ssh_config (in place of ssh -Y)

ProxyJump

You can set up a hostname configuration to jump directly through another machine if needed:

• make sure you are running ssh>=7.3 everywhere
  • if you have to jump through Windows, Cygwin has a nice sshd that works out of the box for the most part, with ProxyJump capability.
  • Ubuntu must be 17.04 or newer.
• configure a host shortcut AwsInstance from dmz to internet
• configure a host shortcut DmzHostfrom lab to dmx
• configure a host shortcut AwsInstanceJumpfrom lab to internet, with ProxyJump DmzHost