sshd configuration guide — linux services explained

sshd configuration guide — linux services explained

Ever been intrigued by the workings of the SSHD process? Ever wondered how it enables successful server connections and secure data transfers? Well, you’re in for a treat! We’re about to unravel the mysteries of the SSHD process, from installation to troubleshooting.

Think of the SSHD process, or Secure Shell Daemon, as the gatekeeper to your data castle. It’s responsible for ensuring secure and encrypted transmission between your computer and the server. But understanding and troubleshooting this gatekeeper can sometimes feel like trying to crack a cryptic code.

Fret not! We’ve got you covered with this comprehensive guide. We’re about to embark on a journey through the realm of SSHD, from its installation on diverse Linux distributions to its roles and responsibilities in server connections. We’ll also delve into troubleshooting techniques that can help you solve common SSHD puzzles.

So, are you ready to unlock the secrets of the SSHD process? Let’s dive in!

TL;DR: What is the SSHD process?

The SSHD process, or Secure Shell Daemon, is like the gatekeeper to your data castle. It ensures secure and encrypted transmission between your computer and the server. It’s responsible for user authentication, terminal connections, file transfers, and tunneling. For more advanced methods, background, tips and tricks, continue reading the article.

SSHD Overview

So, let’s delve into the crux of the matter: what exactly is the SSHD process? SSHD, or Secure Shell Daemon, is a network protocol that acts as a secure channel in a computer network. Picture it as the gatekeeper of a castle, controlling the entry and exit of all. The SSHD process shoulders the responsibility of handling user authentication, encryption, terminal connections, file transfers, and tunneling.

Imagine a user trying to connect to a server. This is where the SSHD process comes into play. It scrutinizes the user’s credentials and, if authenticated, establishes an encrypted connection. This encryption ensures that the data transferred between the user and the server is secure and doesn’t fall into the wrong hands.

Example of SSHD process role in user authentication and encryption:

Password: ********

This command attempts to establish an SSH connection to the server using the user’s credentials.

But the SSHD process doesn’t stop at authentication and encryption. It also manages terminal connections, enabling users to execute commands on the server remotely.

Example of SSHD process role in terminal connections:


This command attempts to list the files in the home directory of the server. It oversees file transfers, ensuring that data is securely moved from one location to another.

Example of SSHD process role in file transfers:

scp /path/to/local/file

This command attempts to copy a local file to a remote directory on the server. It even takes care of tunneling, a technique used to transport network data securely.

User AuthenticationScrutinizes the user’s credentials
EncryptionEstablishes an encrypted connection
Terminal ConnectionsEnables users to execute commands on the server remotely
File TransfersOversees secure data transfer from one location to another
TunnelingUsed to transport network data securely

In essence, the SSHD process is the backbone of secure server connections. Without it, data security and integrity could be at risk.

But where does the OpenSSH server process fit into all of this? The OpenSSH server process is a suite of security-related network-level utilities based on the Secure Shell protocol. The SSHD process we’ve been discussing is actually a part of this suite. It’s the daemon that runs in the background, vigilantly waiting for SSH connections from clients.

Installing the SSH Server on Linux

Before we jump into the installation process, it’s vital to determine if the SSH Server is already operational on your Linux system. Wondering how to do this? Just execute the following command in your terminal:

ps -A | grep sshd

If the SSHD process is up and running, this command will return a line that includes ‘sshd’. If it draws a blank, that means the SSH Server is not currently active on your system.

Setting up SSH Server on Debian-derived and Red Hat derived distributions

The installation process differs slightly based on your Linux distribution. Here are detailed instructions for both Debian-derived distributions like Ubuntu and Red Hat derived distributions like CentOS.

Debian-derived Distributions

  1. Open a terminal window.
  2. Refresh your package list by typing:
sudo apt-get update
  1. Install the OpenSSH server by typing:
sudo apt-get install openssh-server

Red Hat derived Distributions

  1. Open a terminal window.
  2. Install the OpenSSH server by typing:
sudo yum install openssh-server

In both scenarios, the SSHD process should kickstart automatically post-installation. If it doesn’t, you can manually start it by typing:

sudo service sshd start

or simply reboot your system.

SSH Server for Windows and IBM z/OS Mainframes

While this guide is primarily Linux-focused, it’s worth mentioning that SSH servers are also accessible for other operating systems. For Windows, you can utilize OpenSSH for Windows or freeSSHd. For IBM z/OS mainframes, IBM Ported Tools for z/OS provides an SSH server.

Lastly, remember to run installation commands as root. This ensures that the commands have the necessary permissions to install software on your system.

Understanding SSHD Processes

With the SSH server now installed, it’s time to dive deeper into the startup process and the roles of different SSHD processes.

Booting Up with the System

The SSHD process, like a diligent worker, is designed to start automatically when your system boots. It does this by registering itself with the system’s init process, which is the first process that the Linux kernel starts. This ensures the SSHD process is up and running, ready to handle incoming SSH connections as soon as your system is operational.

The Master Server and Its Minions: Child Processes

The SSHD process operates using a master server and child processes. The master server, like a watchful hawk, listens for incoming connections. Once a connection is established, it forks a child process to handle that connection. This allows the master server to continue its vigil for new connections while the child process takes care of the existing one.

Ensuring Seamless Service during Upgrades or Restarts

The SSHD process is designed to minimize disruption to users when it’s upgraded or restarted. When the master server is restarted, it doesn’t terminate existing SSH connections. Instead, it forks a new master server to listen for new connections, while the old master server stays alive to handle the existing connections until they’re closed. This ensures that users don’t experience any disruption in their SSH sessions.

Terminating Individual Processes and Running Multiple Master SSHD Processes

At times, it may be necessary to terminate individual SSHD processes, for example, if a particular SSH session is causing problems. You can do this using the ‘kill’ command followed by the process ID (PID) of the SSHD process.

Moreover, it’s also possible to run multiple master SSHD processes on different ports. This can be useful in situations where you want to segregate different types of SSH traffic, for example, separating administrative SSH traffic from regular user SSH traffic.

The Parent-Child Structure of OpenSSH’s SSHD Server Process

OpenSSH’s SSHD server process operates using a parent-child structure. The parent process listens for connections and creates a new child process for each new connection. This structure allows for minimal interruption to users during restarts or upgrades, as we discussed earlier. It’s one of the many ways that the SSHD process ensures a smooth and secure SSH experience.

Mastering SSHD Configuration

Think of the configuration file as the command center of the SSHD process. It’s where you can tailor various aspects of its behavior to your needs. The configuration file for the OpenSSH server typically resides at /etc/ssh/sshd_config. Here, you can tweak settings such as the port the SSHD process listens on, the authentication methods it accepts, and more.

To open and edit the SSHD config file run this command:

nano /etc/ssh/sshd_config

The SSHD configuration file is a feature-rich utility that allows you to manage a lot of what the server can do. Here are some important configuration options:

  1. Port: This option allows you to set the port SSHD listens on.
Port 22
  1. PermitRootLogin: This controls whether or not the root user is allowed to login. It’s generally considered good security practice to disable root login over SSH. Choices range from “yes”, “prohibit-password”, “forced-commands-only”, to “no”.
PermitRootLogin no
  1. PasswordAuthentication: Determines if password authentication is allowed. Disabling password authentication and using keys is more secure.
PasswordAuthentication no
  1. AllowUsers: Allows only specific users to log in through SSH. This setting can boost your security by limiting the users who can access the server.
AllowUsers user1 user2
  1. PubkeyAuthentication: Allows or disallows authentication using SSH keys. It is considered more secure than password authentication.
PubkeyAuthentication yes
  1. ChallengeResponseAuthentication: This option controls whether challenge response authentication is allowed (e.g., one time passwords).
ChallengeResponseAuthentication no
  1. MaxAuthTries: Specifies the maximum number of authentication attempts permitted per connection. When the number of failures reaches half this value, additional failures are logged.
MaxAuthTries 3

Lastly, ensure that you restart the SSHD process by executing the command service sshd restart or systemctl restart sshd for the changes to take effect. Remember to always be cautious while editing this file; faulty configuration might lock you out of your own server.

Using the Syslog Subsystem for Logging

The SSH server relies on the syslog subsystem for logging. Syslog is a standard for message logging, facilitating the separation of the software that generates messages, the system that stores them, and the software that reports and analyzes them. When an event occurs, the SSH server dispatches a message to the syslog subsystem, which then dutifully logs the message to a file.

The Magic of VERBOSE Logging Level

Setting the logging level to VERBOSE is a game-changer for effective troubleshooting. When the logging level is VERBOSE, the SSH server chronicles more detailed information about each connection attempt. This can be a lifesaver when diagnosing connection issues. You can set the logging level to VERBOSE by adding the following line to your SSHD configuration file:


This line sets the logging level to VERBOSE.


Don’t forget to restart the SSHD process after modifying the configuration file.

The Home of Log Files

The location of the SSH log files is subject to your Linux distribution. On Debian-derived systems, SSH logs usually call ‘/var/log/auth.log’ their home. On Red Hat derived systems, you can find them chilling in ‘/var/log/secure’.

The Significance of Centralized SIEM Systems

In enterprise environments, employing a centralized Security Information and Event Management (SIEM) system for log management is often a smart move. SIEM systems collate and aggregate log data generated across the organization’s IT infrastructure. This enables centralized log analysis and reporting, making it a breeze to spot suspicious activities and potential security threats.

Troubleshooting SSH Connection Problems

Even the best-configured SSHD process can occasionally stumble upon connection problems. But don’t worry, we have an arsenal of tools and techniques at our disposal to diagnose and resolve these issues.

Your Toolkit for Diagnosing Connection and Authentication Problems

There are several tools available to help diagnose connection and authentication problems. These include the SSH client itself, log files, and the debug mode of the SSHD process.

The Power of the SSH Client -v Option

One of the most straightforward ways to diagnose connection problems is by leveraging the verbose mode of the SSH client. You can do this by adding the -v option when connecting to the server. For example:

ssh -v

This command attempts to establish an SSH connection to the server and spits out verbose debug information. This information can be a goldmine when trying to identify the cause of the connection problem.

Harnessing Log Files and Debug Mode to Diagnose Problems

Log files and the debug mode of the SSHD process are two other potent tools for diagnosing connection problems. As we discussed earlier, the SSHD process logs information about each connection attempt to the syslog subsystem. You can view these logs to gain insights into what might be causing the connection problem.

In addition to log files, you can also run the SSHD process in debug mode. This can be done by halting the SSHD service and then manually running the SSHD process with the -d option. For example:

sudo service sshd stop
sudo /usr/sbin/sshd -d

When run in debug mode, the SSHD process doesn’t detach from the console and prints verbose debug information to the console. This can provide detailed insights into user authentication and other issues, helping you diagnose and resolve problems more effectively.

Testing New Configurations Before A Remote Restart

When making changes to the SSHD configuration file, it’s crucial to test the new configuration before restarting the server remotely. If there’s an error in the configuration file, the SSHD process may fail to start, potentially locking you out of the server.

You can test the configuration file using the -t option. For example:

sudo /usr/sbin/sshd -t

If there’s an error in the configuration file, this command will output an error message and exit. If the configuration file is correct, the command won’t output anything and exit with a status of 0.

Example of testing new configurations before restarting the server remotely:

sudo /usr/sbin/sshd -t

This command checks the SSHD configuration file for any errors.

SSH Server Summary

In this guide, we’ve embarked on an enlightening journey through the world of the SSHD process. We’ve peeled back the layers to understand what the SSHD process is, its pivotal role in server connections, and how it juggles user authentication, encryption, terminal connections, file transfers, and tunneling. We’ve also unraveled the relationship between the OpenSSH server process and SSHD.

Finally, we’ve examined various tools and techniques for diagnosing and resolving SSH connection problems, including the SSH client -v option, log files, running the server in debug mode, and the importance of testing new configurations before restarting the server remotely.

SSH client -v optionProvides verbose debug information during connection attempt
Log filesLogs information about each connection attempt
Running the server in debug modeProvides verbose debug information to the console
Testing new configurations before restarting the server remotelyPrevents potential lockout from the server due to configuration errors

Armed with this knowledge, you’re now ready to conquer the world of SSHD with confidence. Whether you’re installing the SSH server, configuring the SSHD process, or troubleshooting connection issues, you have the tools and knowledge you need to ensure a secure and smooth SSH experience. Remember our castle gatekeeper analogy? You’re now the master of your own SSHD castle!