Overview
The OpenSSH client is a package that includes various utilities for secure communication with remote systems. It primarily performs functions such as remote login, file copying, and file system management through commands like SSH, SCP, and SFTP.
Key Included Commands
The main commands included in the openssh-client package are as follows:
- ssh: Securely log in to remote servers
- scp: Copy files between remote servers and local systems
- sftp: Interactive file transfer with remote servers
- ssh-keygen: Generate and manage SSH key pairs
- ssh-agent: Load SSH keys into memory to avoid repeated password entry
Installation
The OpenSSH client is included by default in most Linux distributions. If it is not installed, you can install it using the following commands.
Debian/Ubuntu
sudo apt update
sudo apt install openssh-clientInstall using the APT package manager.
CentOS/RHEL/Fedora
sudo yum install openssh-clientsInstall using the YUM or DNF package manager.
Usage Examples
Here are common usage examples for the main commands included in the openssh-client package.
Log in to a remote server using SSH
ssh username@your_server_ipConnect to a remote server using your username and IP address or hostname.
Connect to SSH on a specific port
ssh -p 2222 username@your_server_ipIf the default SSH port (22) is not used, use the -p option.
Copy a local file to a remote server using SCP
scp /path/to/local/file.txt username@your_server_ip:/path/to/remote/directory/Copy a local file to a specific path on the remote server.
Copy a remote file to the local system using SCP
scp username@your_server_ip:/path/to/remote/file.txt /path/to/local/directory/Copy a file from the remote server to your local system.
Start an interactive file transfer session using SFTP
sftp username@your_server_ipConnect to the SFTP client to interactively transfer or manage files.
Tips & Precautions
Useful tips and security precautions when using the OpenSSH client.
Using SSH Keys
Enhance security and automate the login process by using SSH keys instead of passwords. Generate keys with `ssh-keygen` and copy your public key to the server with `ssh-copy-id`.
- Set up passwordless login
- Improve security (prevent brute-force attacks)
- Useful for automated scripts
Utilizing ~/.ssh/config File
Simplify connection commands and pre-configure specific options by storing connection information for frequently accessed servers in the `~/.ssh/config` file.
- Set aliases: `Host myserver`
- Specify username, port, and key file path: `User`, `Port`, `IdentityFile`
- Configure automatic login and tunneling
Security Precautions
Maintain security by observing the following when using the SSH client:
- Never expose your private key (`id_rsa`) externally.
- Set a strong passphrase for your private key.
- Disable unnecessary port forwarding.
- Do not trust unknown host key fingerprints.