ssh.md

Basic SSH Commands

The following are fundamental SSH commands. Commit as many to memory as you can.

COMMAND	DESCRIPTION
ssh	Connect to a remote server
ssh pi@raspberry	Connect to the device raspberry on the default SSH port 22 as user pi
ssh pi@raspberry -p 3344	Connect to the device raspberry on a specific port 3344 as user pi
ssh -i /path/file.pem [email protected]	Connect to [email protected] via the key file /path/file.pem as user admin
ssh [email protected] 'ls -l'	Execute remote command ls -l on 192.168.2.2 as user root
$ ssh [email protected] bash < script.sh	Invoke the script script.sh in the current working directory spawning the SSH session to 192.168.3.3 as user user
ssh [email protected] "tar cvzf - ~/ffmpeg" > output.tgz	Compress the ~/ffmpeg directory and download it from a server Best.local as user friend
ssh-keygen	Generate SSH keys (follow the prompts)
ssh-keygen -F [ip/hostname]	Search for some IP address or hostname from ~/.ssh/known_hosts (logged-in host)
ssh-keygen -R [ip/hostname]	Remove some IP address or hostname from ~/.ssh/known_hosts (logged-in host)
ssh-keygen -f ~/.ssh/filename	Specify file name
ssh-keygen -y -f private.key > public.pub	Generate public key from private key
ssh-keygen -c -f ~/.ssh/id_rsa	Change the comment of the key file ~/.ssh/id_rsa
ssh-keygen -p -f ~/.ssh/id_rsa	Change passphrase of private key ~/.ssh/id_rsa
ssh-keygen -t rsa -b 4096 -C "[email protected]"	Generate an RSA 4096-bit key with "[email protected]" as a comment: -t: Type of key (rsa, ed25519, dsa, ecdsa) -b: The number of bits in the key -C: Provides a new comment
scp	Copy files securely between servers
scp user@server:/folder/file.ext dest/	Copy from remote to local destination dest/
scp dest/file.ext user@server:/folder	Copy from local to remote
scp user1@server1:/file.ext user2@server2:/folder	Copy between two different servers
scp user@server:/folder/* .	Copies from a server folder to the current folder on the local machine
scp -r	Recursively copy entire directories
scp -r user@server:/folder dest/	Copy the entire folder to the local destination dest/
scp user@server:/folder/* dest/	Copy all files from a folder to the local destination dest/
scp -C	Option to compress data
scp -v	Option to print verbose info
scp -p	Option to preserve the last modification timestamps of the transferred files
scp -P 8080	Option to connect to remote host port 8080
scp -B	Option for batch mode and prevent you from entering passwords or passphrases
sftp	Securely transfer files between servers
sftp -p	Option to preserve the last modification timestamps of the transferred files
sftp -P 8080	Option to connect to remote host port 8080
sftp -r	Recursively copy entire directories when uploading and downloading. SFTP doesn't follow symbolic links encountered in the tree traversal.

SSH Configurations and Options

Have you ever wondered how SSH remembers your login credentials for various machines? This section is a brief reference on how to do so.

COMMAND	DESCRIPTION
man ssh_config	Open OpenSSH SSH client configuration files. This manual lists all the OpenSSH parameters you can change.
cat /etc/ssh/ssh_config | less	View your OpenSSH client system-wide configuration file
cat /etc/ssh/sshd_config | less	View your OpenSSH server system-wide configuration file; the "d" stands for the server "daemon"
cat ~/.ssh/config | less	View your SSH client user-specific configuration file
cat ~/.ssh/id_{type} | less	View your SSH client private key; type is any of rsa, ed25519, dsa, ecdsa.
cat ~/.ssh/id_{type}.pub | less	View your SSH client public key; type is any of rsa, ed25519, dsa, ecdsa.
cat ~/.ssh/known_hosts | less	View your SSH client logged-in hosts
cat ~/.ssh/authorized_keys | less	View your SSH client authorized login keys
ssh-agent	Hold private SSH keys used for public key authentication (RSA, DSA, ECDSA, Ed25519)
ssh-agent -E fingerprint_hash	Specify the hash algorithm used when displaying key fingerprints. Valid fingerprint_hash options are sha256 (default) and md5.
ssh-agent -t lifetime	Set up a maximum lifetime for identities/private keys, overwritable by the same setting in ssh-add. Examples of lifetime: - 600 = 600 seconds (10 minutes) - 23m = 23 minutes - 1h45 = 1 hour 45 minutes
ssh-add	Add SSH keys to the ssh-agent
ssh-add -l	List your private keys cached by ssh-agent
ssh-add -t lifetime	Set up a maximum lifetime for identities/private keys. Examples of lifetime: - 600 = 600 seconds (10 minutes) - 23m = 23 minutes - 1h45 = 1 hour 45 minutes
ssh-add -L	List the public key parameters of all saved identities
ssh-add -D	Delete all cached private keys
ssh-copy-id	Copy, install, and configure SSH keys on a remote server
ssh-copy-id user@server	Copy SSH keys to a server as a user
ssh-copy-id server1	Copy to some alias server server1 with the default login
ssh-copy-id -i ~/.ssh/id_rsa.pub user@server	Copy a specific key to a server as a user

Remote Server Management

The operating systems of SSH servers are mostly Unix/Linux, so once you've logged in to a server via SSH, the following commands are largely the same as their counterparts in Unix/Linux.

COMMAND	DESCRIPTION
cd	Change the current working directory
kill	Stop a running process
ls	List files and directories
mkdir	Create a new directory
mv	Move files or directories
nano	Edit a file in the terminal using Nano
ps	List running processes
pwd	Display the current working directory
tail	View the last few (10, by default) lines of a file
top	Monitor system resources and processes
touch	Create a new file or update the timestamp of an existing file
vim	Edit a file in the terminal using Vim
exit	Close the SSH session

Advanced SSH Commands

This table lists some complex SSH utilities that can help with network administration tasks: SSH File System (SSHFS), data compression, and X11 forwarding.

To conduct X11 forwarding over SSH, do these three things:

1. Set up your client (~/.ssh/config) to forward X11 by setting these parameters:

   Host *`\
   ForwardAgent yes\
   ForwardX11 yes
   

2. Set up your server (/etc/ssh/sshd_config) to allow X11 by setting these parameters:\

   X11Forwarding yes
   X11DisplayOffset 10
   X11UseLocalhost no
   

3. Set up X11 authentication on your server by installing xauth.

   COMMAND	DESCRIPTION
   sshfs	Mount a remote server's file system on a local directory. Remember to install this program onto your machine before use. Example installation commands: - sudo apt install sshfs # Ubuntu/Debian - sudo yum install fuse-sshfs # CentOS
   ssh -C hostname	Compress SSH traffic to improve performance on slow connections. Alternatively, insert Compression yes into your SSH configuration files
   ssh -o "Compression yes" -v hostname	An alternative method to compress SSH traffic to improve performance on slow connections. This is the same as inserting Compression yes into your SSH config files
   ssh -X user@server	Enable X11 forwarding over SSH: forward graphical applications from a remote server as a user to a local machine.
   ssh -o ForwardX11=yes user@server	Enable X11 forwarding over SSH: forward graphical applications from a remote server as a user to a local machine.
   ssh -x	Disable X11 forwarding
   ssh -Y	Enable trusted X11 forwarding. This option is riskier than ssh -X as it forwards the entire display of the SSH server to the client.

Tunneling

These SSH command line options create secure tunnels.

OPTIONS	DESCRIPTION	SYNTAX / EXAMPLE
-L	Local port forwarding: forward a port on the local machine (SSH client) to a port on the remote machine (ssh_server as user), the traffic of which goes to a port on the destination machine. The parameters local_port and remote_port can match.	ssh user@ssh_server -L local_port:destination:remote_port # Example ssh [email protected] -L 2222:10.0.1.5:3333
-J	ProxyJump; ensure that traffic passing through the intermediate/bastion hosts is always encrypted end-to-end. ProxyJump is how you use bastion hosts to connect to a remote host with a single command.	ssh -J proxy_host1 remote_host2 ssh -J user@proxy_host1 user@remote_host2 Multiple bastion hosts/jumps ssh -J user@proxy_host1:port1,user@proxy_host2:port2 user@remote_host3
-R	Remote port forwarding: forward a port remote_port on the remote machine (ssh_server as user) to a port on the local machine (SSH client), the traffic of which goes to a port destination_port on the destination machine. An empty remote means the remote SSH server will bind on all interfaces. Additional SSH options in the example: -N: don't execute remote commands; useful for dedicated port forwarding -f: run SSH in the background.	ssh -R [remote:]remote_port:destination:destination_port [user@]ssh_server Example ssh -R 8080:192.168.3.8:3030 -N -f [email protected]
-D	Set up a SOCKS Proxy to tunnel traffic from a remote_host on which you're the user to a local_port_number. Additional SSH options in the example: -q: quiet mode; don't output anything locally -C: compress data in the tunnel, save bandwidth -N: don't execute remote commands; useful for dedicated port forwarding -f: run SSH in the background.	ssh -D local_port_number user@remote_host # Example ssh -D 6677 -q -C -N -f [email protected]