openssh-server

sshd [options]

-4
    Forces sshd to use IPv4 addresses only.

-6
    Forces sshd to use IPv6 addresses only.

-D
    Do not daemonize. Useful for debugging.

-e
    Log to standard error instead of syslog.

-g secs
    Specifies the grace time for login. If a user fails to authenticate in 'secs' seconds, the connection is closed.

-i
    Specifies that sshd should not listen on any interfaces.

-p port
    Specifies the port that sshd listens on. The default is 22.

-q
    Quiet mode. Nothing is logged to syslog.

-T
    Test configuration file validity and exit.

-t
    Test mode. Check the configuration file and key files.

-v
    Verbose mode. Causes sshd to print debugging messages about progress.

The openssh-server command starts the OpenSSH daemon (sshd), which listens for incoming connections from SSH clients. SSH (Secure Shell) provides encrypted communication between two computers over a network. This allows users to securely log in to the server, execute commands remotely, and transfer files using protocols like scp and sftp. It's a cornerstone of secure system administration for Linux servers.

The openssh-server package includes the necessary tools and configuration files to establish and manage SSH connections. The daemon is typically configured using the /etc/ssh/sshd_config file, which controls various aspects of security, authentication, and connection settings. Using strong passwords or, even better, key-based authentication is strongly recommended to protect servers from unauthorized access. sshd is often managed by systemd as a service.

Incorrect configuration of sshd can lead to security vulnerabilities. Always review the /etc/ssh/sshd_config file carefully and use strong authentication methods.

Firewall rules must allow traffic on the SSH port (default 22).

Key-based authentication is generally more secure than password authentication. It involves creating a public/private key pair. The public key is placed on the server, and the private key is kept secret on the client machine. The SSH client uses the private key to authenticate to the server without needing to transmit a password.

SSH supports port forwarding, also known as tunneling. This allows you to securely forward traffic through the SSH connection. This can be useful for accessing services on the server that are not directly exposed to the internet, or for bypassing firewalls.

OpenSSH was developed as a fork of the original SSH protocol to provide a free and open-source alternative. It has become the de facto standard for secure remote access on Unix-like systems. Development focused on security, code correctness, and adherence to open standards.

ssh(1), scp(1), sftp(1), ssh-keygen(1), sshd_config(5)