SSH Command Cheatsheet to become Linux Advance Pro User

Being in a Programming career, SSH is a widely known term and some like to call it as “Secure Shell” or “Secure Socket Shell”. SSH is a network protocol and it lets one to connect to different computer/server existing over an unsecured network (Internet or LAN) in a totally secured way. ssh is also a terminal program that connects to a server using an encrypted connection. This is useful when you do not have access to the required system directly and then you can connect to the required server system over internet if the SSH Port 22 is open on the Server System.

SSH can also be known as a cryptographic network protocol that transfer encrypted data over unsecured network. You can connect to the Remote Server Machine with user/password credential or through a public/private key authentication mechanism. It uses Client-Server Model where your local machine will be a Secure Shell Client application(like bash terminals or other GUI application) and Remote Server Machine will be a SSH-Server where session would run. SSH Server on your Remote Server Machine normally uses or listens on standard Transmission Control Protocol (TCP) port 22. It is an application layer protocol which resides at the 7th layer of the OSI Model. SSH was designed as a replacement for Telnet, which is also an application layer protocol that transferred data without encryption and in plain text which was sufficient for any middle-man/hacker to capture confidential data from live traffic.

Practical Use Cases of SSH includes:

1. Accessing resources in the remote server
2. Executing bash commands in the remote server, modifying files or doing any command-line operations.
3. Transferring files between 2 different machine.
4. Managing running application on the remote server.
5. Patching/Upgrading the remote server machine.
6. Accessing resources of Server Machine on local installed application. e.g. Database, File System, Remote Application
7. Using as a Jump Box to access other server inside the private network.
8. SSH can also be used to create secure tunnels for other application protocols, for example, to securely run X Window System graphical sessions remotely.
9. Live piping output from remote machine to other machine.

Secure Shell has 2 ways of authentication which includes strong password authentication and public key authentication.

Key based authentication comes in 2 pair:

Public key – Used with encryption function of Secure Shell. No need to protect it. This will be used by your SSH Remote Server for encryption connection to client.
Private key – Used for decryption function of Secure Shell. This must be protected as its a private and would be used by your SSH Client for connecting to remote SSH Server.

If you deal with aws cloud/azure/gcp or virtual machine or docker container, then you might be familiar with using Putty graphical tool or SSH on linux or windows to perform some task on remote server using command-line console.  There are quite many tricks to use ssh in most clever way. SSH can move contents securely, provides different authentication mechanism, mount a filesystem using sshfs, port forwarding and reverse port forwarding, or use it as a proxy to access another private resource. There are many and we many not be able to cover all of them. But we will try to compile a list of all useful ssh usecases.

SSH Basics

1. scp : Transfer files over ssh
2. sftp : Active Terminal FTP session to download or upload files over network.
3. ssh-keygen : Generate pair of public private key for your host.
4. ssh-copy-id : Transfer local public key to other remote system.
5. You can configure SSH presets related to different host in the ~/.ssh/config file. For example, suppose you want to connect to one of your server, you can set alias name for it and specify other details like port, username, identityFile, etc.

   Host ftpsever
     HostName mih0me.dyndns.org
     Port 1234
     User TheAl
     IdentityFile ~/.ssh/home_id
     ForwardX11 yes
     Compression yes
     TCPKeepAlive yes

6. You can control if you want to use single TCP socket or multiple socket for multiple session to same server host. This way you can connect to remote host efficiently. Specify below lines in ~/.ssh/config file.

   ControlMaster auto 
   ControlPath ~/.ssh/master-%r@%h:%p

   To set it for all hosts, use the Host * line in the config file.

   You can add -S none  within ssh command to override the global setting and avoid usage of ControlMaster incase you have a lot of data over multiple connections
7.  If you have multiple SSH Connection open and try to close the first ssh session, then it may hang if you try to exit it before all the other connections are close
   1. • To cause the original session to go to the background indefinitely, set below entry in ~/.ssh/config file.

      ControlPersist yes

      • If you want a little grace period you can set ControlPersist to a number like 180. That would cause the background session to end if there are no connections for three minutes. Set below entry in  ~/.ssh/config file.

      1. ControlPersist 180

8. set TCPKeepAlive to yes if you need the server and client to test their connection & remain active during idle periods. And so iff the connection becomes idle, you won’t get disconnected.

1. TCPKeepAlive yes

    

You can run below SSH commands on terminal or use it inside a bash script.

List of Popular Useful SSH Commands

ssh username@IPAddress

ssh -i pathtoPrivateKey.pem <username>@<ssh_host>

ssh-copy-id <username>@<ssh_host>

cat ~/.ssh/id_rsa.pub | <username>@<ssh_host> "mkdir ~/.ssh; cat >> ~/.ssh/authorized_keys"

ssh-keygen; ssh-copy-id <username>@<ssh_host>; ssh <username>@<ssh_host>

ssh <username>@<ssh_host> <commandHere>

service sshd restart,  systemctl reload sshd.service

ssh -v <username>@<ssh_host>

ssh-keygen

ssh -R 9000:localhost:9000 <username>@<ssh_host>

ssh -N -L 2001:localhost:80 <username>@<ssh_host>

ssh <username>@<ssh_host> "cat /path/to/remotefile" | diff /path/to/localfile -

sshfs <username>@<ssh_host>:/path/to/folder /path/to/mount/point

ssh -t <username>@reachable_host ssh <username>@unreachable_host

ssh <username>@host1 "cd /somedir/tocopy/ && tar -cf – ." | ssh <username>@host2 "cd /samedir/tocopyto/ && tar -xf -"

ssh -fX <username>@<ssh_host> <program>

ssh -MNf <username>@<ssh_host>

ssh -t <username>@<ssh_host> "screen -r"

ssh-keygen -R <the_offending_host>

sed -i 8d ~/.ssh/known_hosts

ssh host -l user $(<cmd.txt)
ssh host -l user “`cat cmd.txt`”

mysqldump –add-drop-table –extended-insert –force –log-error=error.log -uUSER -pPASS OLD_DB_NAME | ssh -C user@newhost "mysql -uUSER -pPASS NEW_DB_NAME"

yes | pv | ssh $host "cat > /dev/null"

ssh -t <username>@<ssh_host> /usr/bin/screen -xRR

rsync –partial –progress –rsh=ssh <source_file_path> <username>@<ssh_host>:<target_file_pathdirectory>

rsync –partial –progress –rsh=ssh  <username>@<ssh_host>:<remote_file_path> <local_file_path>

ssh <username>@<ssh_host> 'tshark -f "port !22" -w -' | wireshark -k -i –

ssh <username>@<ssh_host> tcpdump -w – 'port !22' | wireshark -k -i –

autossh -M50000 -t <username>@<ssh_host> 'screen -raAd mysession'

tar -cj /backup | cstream -t 777k | ssh host ‘tar -xj -C /backup’

ssh user@host cat /path/to/some/file | xclip

For more options related to SSH command, Refer official MAN Page Documentation : https://man7.org/linux/man-pages/man5/ssh_config.5.html and you can also refer https://explainshell.com/explain to get detailed usage idea of the command.

I hope above list would be helpful to you. I will try to keep the above list updated. Thanks for reading and share if you liked this post.