The Secure Shell (SSH) provides protected, encrypted communications with other systems. Because SSH is an entry point into the system, disable SSH if it isn't required. Optionally, you can edit the /etc/ssh/sshd_config file to restrict its use.
Important:
After applying changes to the configuration file, you must restart the sshd service for the changes to take effect.
Restrict Root Access
Set PermitRootLogin to no to prohibit root from logging in with SSH. Then, elevate a user's privileges after logging in.
PermitRootLogin no
Restrict Specific Users
You can restrict remote access to certain users and groups by specifying the AllowUsers, AllowGroups, DenyUsers, and DenyGroups settings, for example:
DenyUsers carol danAllowUsers alice bob
For more information about configuring users and groups, see Oracle Linux 8: Setting Up System Users and Authentication or Oracle Linux 9: Setting Up System Users and Authentication.
Set a Timeout Period
The ClientAliveInterval and ClientAliveCountMax settings cause the SSH client to time out automatically after a period of inactivity, for example:
# Disconnect client after 300 seconds of inactivityClientAliveCountMax 0ClientAliveInterval 300
Disable Password Authentication
The PasswordAuthentication and PubkeyAuthentication settings define the method of authentication the SSH client implements for users: either with a password or with an SSH public key. By default, OpenSSH uses passwords for authentication. However, if you have configured key based authentication, which is more secure, you can optionally disable that functionality:
PasswordAuthentication noPubkeyAuthentication yes
For more information, see the sshd_config(5) manual page.
FAQs
Installing OpenSSH Server and Enabling sshd
- If the packages aren't installed, run the following command: sudo dnf install openssh openssh-server.
- Start the sshd service and configure it to start following a system reboot: sudo systemctl start sshd sudo systemctl enable sshd.
sshd_config is the configuration file for the OpenSSH server. ssh_config is the configuration file for the OpenSSH client. Make sure not to get them mixed up. Creating a read-only backup in /etc/ssh means you'll always be able to find a known-good configuration when you need it.
How to check SSH server configuration? ›
Check whether you have enabled SSH
- To check whether the SSH server has been installed, run the following command: systemctl -t service|grep sshd.
- If the SSH is not installed on your node, run the following commands to install and start the SSH server. yum install openssh-server. systemctl enable sshd. systemctl start sshd.
Harden your Linux server: Best practices for securing SSH,User Privileges, firewall configurations
- Update regularly: ...
- Strong Passwords and SSH Keys: ...
- Modify SSH Port: ...
- Using Firewalls to protect server. ...
- Disable Root login. ...
- Enabling Only One User for Login. ...
- Effective Monitoring strategies:
Install OpenSSH for Windows Server
- Open Settings, select System, then select Optional Features.
- Scan the list to see if the OpenSSH is already installed. ...
- Open the Services desktop app. ...
- In the details pane, double-click OpenSSH SSH Server.
OpenSSH is the open-source version of the Secure Shell (SSH) tools used by administrators of Linux and other non-Windows for cross-platform management of remote systems. Beginning with Windows 10 build 1809 and Windows Server 2019, OpenSSH is available as a feature on demand.
What is better than OpenSSH? ›
OpenSSH alternatives
- Cowrie (SSH/telnet honeypot)
- Dockpot (SSH honeypot based on Docker)
- Fail2ban (log parser and blocking utility)
SSH has three components: transport layer protocol (TLP), user authentication protocol, and connection protocol.
What are the three types of SSH? ›
SSH uses three encryption methods: symmetric encryption, asymmetric encryption, and hashing. Symmetric encryption involves a secret key that is used for both the encryption and decryption of an entire SSH connection. The symmetric key is created using an agreed key exchange algorithm.
How to setup SSH configuration? ›
Procedure
- Open the SSH configuration file /etc/ssh/sshd_config and set the appropriate SSH key type. The HostKey directive can be set to specify various SSH key types. ...
- Restart the SSH server to apply new settings. ...
- Restart the noded service to activate your changes.
The ssh program on a host receives its configuration from either the command line or from configuration files ~/. ssh/config and /etc/ssh/ssh_config . Command-line options take precedence over configuration files.
How do I know if my SSH server is active? ›
You can try ssh localhost to test if it is running; if it respons with something like Connection refused , then it is not running. These commands must be run as root. If the server does not start automatically, try using the service sshd start command, or just reboot the computer.
What is the security flaw in OpenSSH? ›
Is OpenSSH a vulnerability? A critical vulnerability affecting millions of OpenSSH servers allows remote code execution, bypassing authentication safeguards. Discovered by the security research team at Qualys, the bug, known as regreSSHion (CVE-2024-6387), predominantly impacts Glibc-based Linux systems.
Which version of OpenSSH is secure? ›
In a nutshell, it says that OpenSSH versions on OSes other than OpenBSD are vulnerable, up to version 9.7p1; version 9.8 is safe. The vulnerability is very slow: on a 32-bit Linux system with address space randomization (ASLR), the attack has actually be demonstrated, and takes 6-8 hours.
Does OpenSSH use SSL? ›
SSH doesn't use Transport Layer Security (TLS) protocols or Secure Socket Layer (SSL). To be clear, TLS is the successor to SSL, so they're considered synonyms. TLS/SSL is used for encryption in the HTTPS and FTPS protocols, not the SFTP protocol.
How do I start an OpenSSH server service? ›
Start, stop, and restart the OpenSSH server on Linux
- $ sudo systemctl start ssh.service ##<-- Linux start sshd.
- $ sudo systemctl stop sshd.service ##<-- stop the server.
- $ sudo systemctl restart sshd.service ##<-- restart the server.
- $ sudo systemctl status sshd.service ##<-- Get the current status of the server.
To generate an SSH key on your Linux server, run the command ssh-keygen . The command can take flags if you would like to customize the type of key that is generated and the signing algorithms that are used to generate the key.
How do I create OpenSSH in Windows? ›
Open the Windows 10 Start menu and search for “Apps & Features”. In the “Apps & Features” heading, click “Optional Features”. Scroll down the list to see if “OpenSSH Client” is listed. If not, click the plus sign next to “Add a feature”, select OpenSSH Client, and click “Install”.
How to setup SSH VPN server? ›
So the high level steps are:
- Open a putty session and configure it to act as a tunnel.
- From this session connect to your default SSH server at home.
- Open another putty session and configure it use the previous putty session as proxy.
- SSH connect to any machine at home using the local subnet IP address.
