Essential SSH Commands and Configuration Guide for Linux and Windows

Common SSH Commands

SSH Installation (Ubuntu)

sudo apt update # update repository
sudo apt install openssh-server # install OpenSSH server
vi /etc/ssh/sshd_config # edit configuration, uncomment port=22
sudo service ssh restart # restart service

1. SSH Connection Commands

# Basic connection
ssh username@hostname

# Connect to a non‑standard port
ssh -p 2222 username@hostname

# Use a private key for authentication
ssh -i /path/to/private_key username@hostname

2. SSH Key Management

# Generate an RSA key pair (4096‑bit)
ssh-keygen -t rsa -b 4096

# Copy the public key to the remote server
ssh-copy-id username@hostname

# View known hosts
cat ~/.ssh/known_hosts

3. SSH File Transfer (scp)

# Download a file from the server
scp username@servername:/path/filename /var/www/local_dir

# Upload a local file to the server
scp /path/filename username@servername:/path

# Recursively download a directory
scp -r username@servername:/var/www/remote_dir/ /var/www/local_dir

# Recursively upload a directory
scp -r local_dir username@servername:remote_dir

4. SSH Port Forwarding

# Local port forwarding
ssh -L local_port:target_host:target_port username@hostname

# Remote port forwarding
ssh -R remote_port:target_host:target_port username@hostname

# Dynamic (SOCKS) forwarding
ssh -D local_port username@hostname

# Common options
-L [local_ip:]local_port:target_ip:target_port   # local forwarding
-R [remote_ip:]remote_port:target_ip:target_port # remote forwarding
-D [local_ip:]local_port                       # dynamic forwarding
-N   # do not execute remote command (used for forwarding only)
-f   # run in background
-C   # enable compression
-q   # quiet mode
-v   # verbose (debug)
-4   # force IPv4
-6   # force IPv6
-p <port>   # specify server port
-i <key>    # specify private key file

5. SSH Configuration Management

# Edit user‑level configuration
vim ~/.ssh/config

# Example entry
Host myserver
    HostName hostname
    User username
    Port 2222
    IdentityFile ~/.ssh/id_rsa

Comparison of Common Uses

Security

Password login: convenient but less secure

Key‑based login: more secure and recommended

Transfer Method scp: simple, suitable for occasional transfers rsync: supports incremental sync, better for large or frequent transfers

Port Forwarding

Local forwarding – access remote internal services

Remote forwarding – allow remote access to local services

Dynamic forwarding – create a SOCKS proxy

Connection Management

One‑off connections – use directly

Frequent connections – configure

~/.ssh/config

Usage Recommendations

Prefer key‑based authentication.

Change the default port 22 for critical servers.

Configure frequent connections in ~/.ssh/config.

Use rsync instead of scp for large file transfers.

Protect private key files carefully.

Managing SSH Service on Ubuntu

Installation

sudo apt update
sudo apt install openssh-server

Service Control Methods

Traditional service command

# start
sudo service ssh start
# stop
sudo service ssh stop
# restart
sudo service ssh restart
# status
sudo service ssh status

/etc/init.d

scripts

# start
sudo /etc/init.d/ssh start
# stop
sudo /etc/init.d/ssh stop
# restart
sudo /etc/init.d/ssh restart
# status
sudo /etc/init.d/ssh status

Systemd systemctl (modern)

# start
sudo systemctl start sshd
# stop
sudo systemctl stop sshd
# restart
sudo systemctl restart sshd
# status
sudo systemctl status sshd

Directly start the daemon

# start daemon
sudo /usr/sbin/sshd
# start with a specific config file
sudo /usr/sbin/sshd -f /path/to/sshd_config

Service Names by Distribution

Ubuntu/Debian – service name ssh CentOS/RHEL – service name

sshd

Common Administrative Tasks

Use systemctl on modern systems (it wraps service).

Traditional /etc/init.d scripts are still supported.

Directly launching sshd is mainly for debugging.

Configuration File

# Main configuration file
sudo vim /etc/ssh/sshd_config

# Typical settings
Port 22                # SSH port
PermitRootLogin no     # Disallow root login
PasswordAuthentication yes  # Allow password auth (set to no for key‑only)

Checking Service Status

# Verify SSH process is running
ps aux | grep ssh

# Check if SSH port is listening
sudo netstat -tulpn | grep ssh

# Verify firewall allows SSH
sudo ufw status

Firewall Configuration

# Allow default SSH port
sudo ufw allow ssh

# Allow a custom port (e.g., 2222)
sudo ufw allow 2222/tcp

Troubleshooting

Service fails to start

# View detailed logs
sudo journalctl -u ssh
# Test configuration syntax
sudo sshd -t

Permission issues

# Fix directory permissions
sudo chmod 755 /etc/ssh
# Fix private key permissions
sudo chmod 600 /etc/ssh/ssh_host_*_key
# Fix public key permissions
sudo chmod 644 /etc/ssh/ssh_host_*_key.pub

Security Recommendations

Basic hardening:

PermitRootLogin no
PasswordAuthentication no
MaxAuthTries 3
Protocol 2

Use key‑based authentication:

# Generate key pair on client
ssh-keygen -t rsa -b 4096
# Copy public key to server
ssh-copy-id username@server_ip

Maintenance Commands

View current SSH connections

who
w
# View SSH login logs
sudo cat /var/log/auth.log | grep ssh

Backup configuration file

sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.backup

After modifying the SSH configuration, restart the service to apply changes:

sudo systemctl restart ssh

SSH Connection Timeout Issue

To keep idle SSH connections alive, add the following to ~/.ssh/config (or use the command‑line options):

Host *
    ServerAliveInterval 60   # send a keep‑alive every 60 seconds
    ServerAliveCountMax 3    # abort after 3 unanswered keep‑alives

Alternatively, specify the options directly when connecting:

ssh -o ServerAliveInterval=60 -o ServerAliveCountMax=3 user@remote-server

Closing Background SSH Connections on Windows (‑Nf)

Task Manager – locate ssh.exe and end the process.

PowerShell – Get-Process ssh | Stop-Process or taskkill /F /IM ssh.exe.

Git Bash – ps aux | grep ssh then kill <PID>.

When using -Nf, you can store the process ID for later termination:

ssh -Nf ... &
 echo $! > ssh_pid.txt

Or create batch files to start and stop the tunnel:

@echo off
rem start_ssh.bat
ssh -Nf ... & echo %ERRORLEVEL% > ssh_pid.txt

rem stop_ssh.bat
for /f %%i in (ssh_pid.txt) do taskkill /PID %%i /F
 del ssh_pid.txt