ssh (OpenSSH client)

ssh is the OpenSSH ssh client for logging or executing commands into remote devices/machines

• Man page: https://linux.die.net/man/1/ssh
• ssh --help

     ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port]
        [-E log_file] [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file] [-J destination] [-L address]
        [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port] [-Q query_option] [-R address] [-S ctl_path]
        [-W host:port] [-w local_tun[:remote_tun]] destination [command]

• -X Enables X11 forwarding
• -N Do not execute a remote command. This is usefull for just forwarding ports

• ssh -vvv
• ssh -V
• ssh -t
• ssh -O
• ssh -o ConnectTimeout=100
• ssh -i
• ssh -L
• ssh -R

Windows: Settings > Apps and click “Manage optional features” under Apps & features.

Configuration

• Configuration file: ~/.ssh/config

Usage

• ssh Remote_Username@Remoter_Server_Name_Or_IP
• ssh -vvv REMOTE_USERNAME@REMOTE_SERVER_NAME_OR_IP (-v increase verbosity, maximum 3)
  • ssh -vvv REMOTE_USERNAME@REMOTE_SERVER_NAME_OR_IP 2>&1 | grep "STRING_TO_SEARCH"

Execute a remote command:

• ssh USERNAME@SERVER_NAME '/path/to/remote/scripts/backup.sh'

• ssh -Q kex SERVER

Example commands

• ssh -o StrictHostKeyChecking=accept-new username@hostname.com (since OpenSSH 7.5, March 2017)
• ssh -o StrictHostKeyChecking=no username@hostname.com
• ssh -Q kex SERVER
• ssh -o MACs=hmac-sha2-256 HOSTNAME

Multiplexing options

ssh -O check server.example.org or ssh -O check username@server.example.org 
Master running (pid=26289)

ssh -O check username@server.example.org 
Control socket connect(/path/): No such file or directory

To stop an specific multiplexed session:

ssh -O stop username@server.example.org

Avoid using Multiplexing for a connections:

ssh -o "ControlMaster=no" server.example.org

See also: Configure OpenSSH to reuse ssh connections

Logs

ssh -i id_ed25519_xxxxx.pub SERVER_NAME
Load key "id_ed25519_xxxxx.pub": invalid format
Solution:
Use private key instead of public key 
ssh -i id_ed25519_xxxxx SERVER_NAME

• Unable to negotiate with (KexAlgorithms)

Activities

• Understand differences between stop and exit: ssh -O exit username@server.example.org
• Configure OpenSSH to reuse ssh connections
• Execute a command in parallel by ssh
• ssh -o PreferredAuthentications=keyboard-interactive -o PubkeyAuthentication=no host1.example.org
• Reverse SSH tunneling

Configuration

• CheckHostIP

See also

• OpenSSH: ssh, ssh -O, ssh -t, ssh -T, ssh -V, /etc/ssh/ssh config, ssh --help
• SSH clients: ssh (OpenSSH), putty, plink, mosh, paramiko, conch pssh, sshpass and autossh

http://man7.org/linux/man-pages/man1/ssh.1.html

https://serverfault.com/a/132973

https://stackoverflow.com/a/48330113