Limiting SSH Access with TCP Wrappers, AllowUsers, and IP Restrictions

Securing SSH access is crucial for hardening Linux servers, and one way to do this is by restricting access to specific users and IP addresses. This can be achieved through three key techniques: TCP Wrappers, AllowUsers/AllowGroups directives, and IP restrictions within sshd_config. TCP Wrappers are a host-based access control system that can restrict network services based on IP addresses or hostnames. They operate using two configuration files: /etc/hosts.allow and /etc/hosts.deny. For example, you can allow SSH access only from a specific IP address by configuring these files. However, modern systems often rely on firewalls for similar functionality, which is generally preferred for flexibility and scalability. The AllowUsers and AllowGroups directives in sshd_config can also be used to specify which user accounts and groups are allowed to access the server via SSH. IP restrictions can be combined with user-based restrictions using the Match Address block in sshd_config. This allows for more granular control over SSH access, ensuring that only authorized users from designated sources can access the server. By using these techniques together, you can build a solid security posture for your SSH services and reduce the attack surface of your Linux server.