SSH is a protocol for securely exchanging data between two computers over an untrusted network. SSH protects the privacy and integrity of the transferred identities, data, and files. It runs in most computers and in practically every server. It ships standard on UNIX, Linux, and macOS machines and it is used in over 90% of all data centers in the world. The SSH protocol and SSH keys secure hundreds of millions of maintenance sessions, file transfers, and automated processes - every day. How Does SSH Server Work? Availability of SSH Servers Quality Equals Security Standardized Security The SSH protocol works on the client/server-model. The SSH client always initiates the setup of the secure connection, and the SSH server listens for incoming connection requests (usually on TCP port 22 on the host system) and responds to them. In the connection setup phase, the SSH server authenticates itself to the client by providing its public key. This allows the SSH client to verify that it is actually communicating with the correct SSH server (instead of an attacker that could be posing as the server). After a successful authentication the server provides the client access to the host system. This access is governed with the user account permissions at the target host system. The secure connection between the client and the server is used for remote system administration, remote command execution, file transfers, and securing the traffic of other applications. Automated SSH sessions are very often used as a part of many automated processes that perform tasks such as logfile collection, archiving, networked backups, and other critical system level tasks. Most server operating systems come with a native, preinstalled SSH server implementation. Those that are an exception to the rule are usually installed with an SSH server from a trusted security solution vendor, such as SSH Communications Security, Bitvise, or VanDyke Software. These companies sell SSH software and provide the technical support and maintenance services for it. The open source community maintains the OpenSSH project that provides a free to use, non-commercial SSH implementation. As security software, the SSH server has strict requirements for software quality. The SSH server process executes with wide system privileges, and acts as an access control "gatekeeper" to the host system. This makes the SSH server an attractive target for hackers and malware. The pivotal security role of the SSH server places stringent requirements for its code quality and reliability. Bugs and defects in the code can lead to serious security vulnerabilities. The SSH protocol has been standardized by the Internet Engineering Task Force (IETF). The standards are open and were authored as a joint effort by many security specialists and companies. As the original inventor of the protocol, SSH Communications Security was a key contributor in the standardization effort.Contents
How Does SSH Server Work?
Availability of SSH Servers
Quality Equals Security
Standardized Security
FAQs
How does SSH daemon work? ›
The sshd is the daemon that listens for connections from clients on port 22. It is normally started when z/OS® UNIX is initialized. It forks a new process for each incoming connection. The forked processes/connections handle key exchange, encryption, authentication, command execution, and data exchange.
What is a sshd daemon? ›sshd (OpenSSH Daemon) is the daemon program for ssh(1). Together these programs replace rlogin(1) and rsh(1), and provide secure encrypted communications between two untrusted hosts over an insecure network. sshd listens for connections from clients.
How do you check is the SSH daemon is running? ›If the SSH daemon is running, you should see an entry with “sshd” in the process name field. If you'd like to check for active SSH connections on a server, you can use the “netstat” command. This command is available on most operating systems, including Linux, macOS, and Windows.
Where is the Sshd config file? ›The SSH server has its own set of configuration files, including the SSH server system-wide configuration file named sshd_config. By default, these files reside in the /etc/ssh directory on the remote host.
What is the main configuration file for SSH server? ›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. The user-specific configuration file ~/.ssh/config is used next.
How to use SSH command? ›You can start an SSH session in your command prompt by executing ssh user@machine and you will be prompted to enter your password. You can create a Windows Terminal profile that does this on startup by adding the commandline setting to a profile in your settings.json file inside the list of profile objects.
What is the difference between sshd and SSH? ›sshd is the OpenSSH server process. It listens to incoming connections using the SSH protocol and acts as the server for the protocol. It handles user authentication, encryption, terminal connections, file transfers, and tunneling.
How do daemons work in Linux? ›In Unix-like systems, including Linux, daemons are background processes that start at system boot and continue running until the system is shut down. They are typically initiated by the init process, the first process that starts when a Unix-like system boots up, and the parent of all other processes.
How does SSH work in Linux step by step? ›SSH operates on TCP port 22 by default (though SSH port can be changed if needed). The host (server) listens on port 22 (or any other SSH assigned port) for incoming connections. It organizes the secure connection by authenticating the client and opening the correct shell environment if the verification is successful.
On which port can a SSH daemon run? ›By default, the SSH server still runs in port 22.
How does a SSH handshake work? ›
SSH handshake is a process in the SSH protocol responsible for negotiating initial trust factors for establishing a secure channel between an SSH client and SSH server for an SSH connection. The handshake process includes: SSH protocol version exchange. Key Exchange.