How to Check SSH Version in Linux: A Complete Guide

Secure Shell (SSH) is essential for anyone managing servers, cloud infrastructures, or local development machines on a Linux system. It provides encrypted, remote access, secure file transfers, and other functionalities critical for modern IT environments. However, not all SSH environments are created equal. SSH versions have unique features, performance enhancements, and security patches. Understanding which version you’re running and how to update or maintain it ensures that your connections stay reliable, secure, and compliant with industry standards.

This in-depth guide outlines the most effective methods for verifying your SSH version in Linux. It also covers why version tracking matters, a brief history of SSH, key security considerations, and advanced tips for optimal SSH maintenance. By the end, you’ll be well-equipped to verify both SSH client and server versions, understand the nuances of version numbering, and stay ahead of potential vulnerabilities.

What is SSH

SSH, short for Secure Shell, is the backbone for encrypted communication across untrusted networks. It replaced older protocols like telnet and rsh, transmitting data (including passwords) in plain Text. SSH uses cryptographic techniques to authenticate, verify users, and encrypt data, making it highly challenging for unauthorized parties to parties to intercept or read sensitive information.

On Linux systems, SSH typically comprises two components: a client (ssh) and a server (sshd). You use the SSH client when you initiate a connection to a remote host, while the SSH server (daemon) accepts incoming connections on that host. Both components are crucial in various scenarios, from simple file transfers to advanced port forwarding and secure tunneling.

As Linux administrators often manage multiple servers, each with its own configurations and security needs, checking the installed SSH version becomes a routine yet critical task. Different versions offer varying levels of security, support different cryptographic ciphers, and might include critical patches that protect against newly discovered vulnerabilities. Ensuring you know precisely which SSH versions you’re running—and upgrading them as necessary—is central to maintaining a robust, secure environment.

Why Is Knowing Your SSH Version Important?

Ensuring you know your current SSH version might seem like a minor detail. However, this knowledge ties directly into broader security and operational stability. Here are several reasons why it’s essential to keep track of SSH versions on Linux:

Security
Older SSH releases may contain known vulnerabilities or use outdated encryption algorithms. Attackers often exploit these weaknesses to gain unauthorized access to a system. Regularly checking and updating your SSH version minimizes the risk of an attack or data breach.

Feature Compatibility
SSH evolves with new features, bug fixes, and performance improvements. If you’re connecting to remote servers that use certain newer encryption ciphers or key exchange methods, running an older SSH client might cause errors or connectivity issues. Staying current with the latest version helps avoid compatibility headaches.

Troubleshooting
Many SSH-related errors, such as connection failures or cryptic warning messages, often originate from client-server mismatches or deprecated features. Identifying your SSH version quickly helps you zero in on potential version mismatches or known bugs that have already been documented and fixed in recent releases.

Compliance
Specific industries mandate strict standards for data encryption and authentication protocols. You may have to meet specific regulatory guidelines requiring regular cryptographic tool validation if you work in healthcare, finance, or government sectors. Verifying SSH versions is an integral part of meeting these guidelines.

Performance and Stability
Updates to SSH often fix memory leaks, optimize CPU usage, and enhance overall performance. If you notice slowness or suspect SSH connections are consuming too many system resources, your current SSH version could be part of the issue.

SSH Versions: A Brief History

SSH has undergone significant evolution since its inception in the mid-1990s. The first widely adopted version, SSH-1, had critical security flaws. Over time, SSH-2 was introduced, and the protocol was completely reworked to address these vulnerabilities. As a result, modern distributions almost exclusively rely on SSH-2. Here’s a brief timeline:

SSH-1
Offered early secure remote login but had vulnerabilities related to key exchanges and cryptographic functions.

SSH-2
Comprehensive overhaul that introduced better encryption algorithms, improved key exchanges, and stronger overall security. This is the version upon which modern implementations are based.

OpenSSH
The most prevalent open-source implementation of SSH. The OpenBSD project maintains OpenSSH and has become the default choice for Linux systems. It regularly integrates new security patches, performance tweaks, and additional features.

While other SSH implementations exist (like Dropbear, often used on embedded devices), OpenSSH remains the gold standard for most Linux distributions, its frequent updates underscore why you should regularly check and update your SSH components.

Common SSH Implementations in Linux

While “SSH” often refers broadly to any secure shell solution, multiple implementations are available. The most common ones on Linux include:

OpenSSH
By far the most widely used, OpenSSH provides both client (ssh) and server (sshd) programs, along with auxiliary tools like scp (secure copy) and sftp (secure FTP). OpenSSH is known for its robustness, reliability, and broad adoption across virtually every major Linux distribution.

Dropbear
Dropbear is a lightweight alternative for resource-constrained environments, including embedded systems or network appliances. While it’s smaller in footprint, It still provides essential SSH functionality but may lack some advanced features found in OpenSSH.

Commercial SSH
Organizations with specific compliance or support needs sometimes opt for commercial SSH solutions. These offerings can come with enterprise-grade backing, extended features, or proprietary encryption modules. Nonetheless, the fundamental steps to check SSH versions remain broadly consistent.

Most Linux administrators and users focus on OpenSSH, the default package in popular distributions like Ubuntu, Debian, Fedora, CentOS, and SUSE. This guide’s methods reflect how to check SSH versions, specifically in OpenSSH, but the underlying principles also apply to other implementations.

How to Check SSH Client Version

The SSH client initiates an outbound connection. When you type ssh user@hostname, you’re engaging the SSH client. Knowing the exact version of this client can be critical for troubleshooting or ensuring you meet specific security requirements. Below are standard techniques to determine which SSH client version is installed on your Linux system.

ssh -V

The most straightforward method to check your SSH client version is to run:

ssh -v

This command outputs a line similar to:

OpenSSH_8.4p1, OpenSSL 1.1.1f  31 Mar 2020

Here, OpenSSH_8.4p1 indicates that the client uses OpenSSH version 8.4p1. Additional information about the underlying cryptographic library (OpenSSL in this example) might also be displayed.

Using Package Managers

Your distribution package manager can show detailed information about installed SSH packages. This method can reveal the current version and whether updates are available. For instance:

Debian/Ubuntu (APT-based systems)

apt show openssh-client

Alternatively, you can list installed packages:

dpkg -l | grep openssh-client

CentOS/Red Hat/Fedora (YUM or DNF)

yum info openssh-clients

or

dnf info openssh-clients

SUSE Linux (Zypper)

zypper info openssh

These commands display the current version, installation source, and other pertinent details.

Checking Binary Properties

If the direct ssh -V command isn’t yielding results or you need deeper insights, examining the binary file properties can be helpful. For example:

file /usr/bin/ssh

or

strings /usr/bin/ssh | grep OpenSSH

These commands inspect the binary and may reveal embedded version strings. While this approach is more technical, it can be helpful in scenarios where traditional checks aren’t functioning (for instance, if the binary is customized or the ssh -V output is suppressed).

Reviewing System Logs

While not the most direct route, certain client-side logs could contain SSH version information. In many distributions, user-side SSH logs are minimal. System logs for the SSH client might be found in /var/log/syslog, /var/log/auth.log, or other authentication-related log files, depending on how logging is configured. You can search for references to OpenSSH or SSH connections:

grep ssh /var/log/auth.log

This might show version strings when connections are initiated, though it’s more common for server logs to store detailed version data.

How to Check SSH Server Version

If you’re an administrator managing a remote server, you also need to know the version of the SSH daemon that accepts incoming connections. This software, commonly called sshd, dictates which encryption algorithms, protocols, and authentication methods are supported.

sshd -V

A direct method, similar to checking the SSH client, is:

sudo /usr/sbin/sshd -V

On some systems, sshd may reside in /usr/sbin/, /usr/bin/, or even /usr/local/sbin/. The -V parameter provides version details:

OpenSSH_8.4p1, OpenSSL 1.1.1f  31 Mar 2020

Many distributions require root privileges to run sshd -V, so using sudo or logging in as root might be necessary.

Using Systemd or init Scripts

Another approach is to check the status of the SSH daemon via systems:

systemctl status ssh

or

systemctl status sshd

While this tells you if the SSH service is running, some distributions display the version in the service information. If it doesn’t appear, you can still confirm that the service is active, then proceed with other methods to determine the exact version.

Inspecting the sshd Binary

Similar to the SSH client, you can inspect the SSH server binary:

file /usr/sbin/sshd

or

strings /usr/sbin/sshd | grep "OpenSSH"

If the binary is stripped of symbols, this may yield minimal results. However, you’ll still see a reference to the embedded version on many default installations.

Checking Log Files

The SSH daemon logs connection attempts, authentication successes or failures, and service restarts. These logs often appear in /var/log/auth.log (Debian/Ubuntu) or /var/log/secure (CentOS/Red Hat/Fedora). You may find lines such as:

sshd: Server listening on 0.0.0.0 port 22.
sshd: Server listening on :: port 22.
OpenSSH_8.4p1, OpenSSL 1.1.1f, protocol version 2.0

If the daemon prints its version upon startup, these entries confirm which version you’re running.

Distribution-Specific Approaches

While the methods above work on most Linux distributions, specific nuances exist among popular systems.

Debian and Ubuntu

Debian and Ubuntu store SSH-related packages under names like openssh-server and openssh-client. Use:

apt show openssh-server

apt show openssh-client

Or a dpkg-based search:

dpkg -l | grep ssh

System logs, including SSH logs, reside in /var/log/auth.log or /var/log/syslog. The configuration files are typically found in /etc/ssh/.

CentOS, Red Hat, and Fedora

CentOS, Red Hat, and Fedora rely on either yum or dnf as their package managers. Check SSH versions with:

yum info openssh-server

yum info openssh-clients

or

dnf info openssh-server

dnf info openssh-clients

System logs storing SSH activity can be found in /var/log/secure (for older or traditional RHEL-based systems) and /var/log/messages for more generalized logging. Configuration files are in /etc/ssh/.

SUSE Linux

SUSE and openSUSE use zypper for package management. To check the SSH package:

zypper info openssh

You can also use RPM commands:

rpm -qa | grep openssh

Depending on the SUSE version, logs may be in /var/log/messages or /var/log/auth.log. Configuration details reside in /etc/ssh/.

Other Distributions

The core principles remain the same for distributions like Arch Linux, Gentoo, or Slackware. You’ll rely on each distribution’s package manager—pacman for Arch or Emerge for Gentoo—and check standard directories or log files. Regardless of the system, commands like ssh -V and sshd -V remain consistent ways to retrieve version information quickly.

Understanding the SSH Version Number Format

When you run ssh -V, you’ll often see output resembling OpenSSH_8.4p1 Ubuntu-5. Let’s break down what this means:

• OpenSSH_8.4p1: Refers to OpenSSH version 8.4 with patch level 1.
• Ubuntu-5: Indicates a distribution-specific release or patch iteration.

Many Linux distributions add their suffixes to note custom patches or packaging differences. For instance, CentOS, Debian, and Fedora may attach strings like el8 or deb11. The critical portion to compare is often the base version (e.g., 8.4p1) because security advisories and feature sets usually refer to that.

Upgrading or Downgrading SSH

In most cases, your goal is to upgrade to the latest stable SSH release that your distribution supports. Occasionally, you may need to downgrade to ensure compatibility with legacy systems. Whichever path you take, approach changes carefully to avoid locking yourself out of remote servers.

Why and When to Upgrade

• Security Patches: New releases often address significant security issues. By upgrading, you protect against known vulnerabilities.
• Performance and Features: Updated versions might introduce new cryptographic ciphers, faster file transfers, or other beneficial features.
• Bug Fixes: Even minor version bumps can fix stability issues, memory leaks, or random disconnections.
• Compliance Requirements: An upgrade might be non-negotiable if your organization mandates specific encryption standards.

Steps to Upgrade

While the exact commands differ per distribution, the general process is:

Debian/Ubuntu

sudo apt update
sudo apt install --only-upgrade openssh-client openssh-server

CentOS/Red Hat/Fedora

sudo yum update openssh

or

sudo dnf update openssh

SUSE Linux

sudo zypper update openssh

After updating, confirm the new version with ssh -V and sudo sshd -V.

Potential Challenges

• Service Restart: Upgrading sshd typically restarts the daemon, potentially dropping active connections. Plan this carefully if you’re administering remote servers.
• Configuration Changes: New versions may deprecate older ciphers or configuration parameters. Always back up /etc/ssh/sshd_config before upgrading.
• Dependency Conflicts: Libraries like OpenSSL may need updating, especially on older distributions. If you encounter issues, consult your distribution’s documentation or use a staging environment to test the upgrade first.

Downgrading Considerations

Downgrading is less common and is usually performed only if you must connect to legacy systems that don’t support newer ciphers. Often, you can find an older version of SSH installed in your distribution’s package manager or compile a prior version from the source. Be careful because older SSH versions may lead you to potential security issues.

Troubleshooting SSH Version Issues

Even with meticulous version management, issues can arise. Common symptoms of SSH version conflicts include failed connections, unexpected warning messages, and inconsistent encryption negotiations. Here are some strategies to tackle these problems:

Common Error Messages

• Protocol Mismatch: Errors like Protocol major versions differ: 1 vs 2 suggest one side is attempting SSH-1 while the other uses SSH-2. Modern systems should disable SSH-1 entirely.
• Cipher Negotiation Failures: If a server only allows strong ciphers (like [email protected]) but your client is outdated, you’ll see messages such as Unable to negotiate with 192.168.1.10: no matching cipher found.
• Deprecated Algorithms: Warnings about host key or key exchange algorithms often appear if your version no longer supports older methods.

Conflicts Between Multiple Installations

On rare occasions, multiple SSH implementations or versions might coexist on a single system. This can happen if you compile OpenSSH from the source without removing your existing package. To resolve:

• Check Paths: Run which ssh and sshd to see which binary is invoked.
• Uninstall or Rename: Remove older packages or rename conflicting binaries if you need to keep them for testing.
• Update PATH: Ensure your system’s PATH variable points to the intended binaries to avoid confusion.

Verifying Configuration Files

After an upgrade, configuration directives in /etc/ssh/sshd_config or /etc/ssh/ssh_config might become unsupported or renamed. Examine any warnings in system logs to pinpoint issues:

grep sshd /var/log/auth.log

or

grep sshd /var/log/secure

If the SSH daemon is unwilling to start, look for syntax errors or deprecated options. Make comments or delete dangerous lines, and then reload the service to make the changes active.

Security Implications of Using Obsolete SSH Versions

Relying on an outdated SSH version poses significant security risks. While you might save time by delaying updates, the potential consequences include data breaches, unauthorized access, or system compromise. Understanding these risks underscores why version checks are essential.

Known Vulnerabilities
Older SSH releases often have documented vulnerabilities accessible in public databases like the Common Vulnerabilities and Exposures (CVE) repository. Attackers target unpatched systems precisely because they know the specific exploits to use.

Weak Encryption
Older SSH versions might still enable deprecated ciphers and algorithms like 3DES or RC4. Modern standards increasingly push for more robust encryption methods like AES in Galois Counter Mode (GCM). Using insecure ciphers leaves sessions open to interception and tampering.

Compliance Risks
Industries with strict regulations (finance, healthcare, government) may require up-to-date cryptographic suites. Noncompliance may lead to penalties or, in worst-case scenarios, disqualification from operating in specific sectors.

Best Practices for SSH Maintenance

Staying ahead in your SSH game involves more than checking versions. Good SSH hygiene ensures that your remote sessions are stable, efficient, and secure. Below are some recommendations:

Regular Version Checks

• Periodic Audits: Integrate SSH version checks into a monthly or quarterly audit schedule.
• Automation: Write scripts that run ssh -V or query package managers, then email you if updates are available.

Secure Configuration

• Disable Root Login: To reduce the attack surface, set PermitRootLogin no in/etc/ssh/sshd_config.
• Use Key-Based Authentication: Turn off password authentication by setting PasswordAuthentication no. SSH keys are far more secure.
• Restrict Access: Use a firewall or TCP wrappers to limit which IP addresses can initiate SSH connections.

Logging and Monitoring

• Enable Detailed Logs: If you suspect suspicious activity, increase the log level in sshd_config by setting LogLevel VERBOSE.
• Centralize Logs: Forward logs to a secure, centralized server or use a Security Information and Event Management (SIEM) tool. This setup enhances visibility and compliance.

Access Control and Key Management

• Least Privilege: Only grant accounts the privileges they need. Remove unused user accounts regularly.
• Key Rotation: If an SSH key is compromised or an employee leaves the organization, rotate keys promptly.
• SSH Certificates: Consider using SSH certificates for environments with many users and servers. This approach simplifies key management and user authentication.

Advanced Tips for Managing SSH Versions

For large-scale environments or specialized use cases, you may need more advanced strategies to maintain control and consistency over your SSH configurations.

Compilation from Source

If the distribution’s repositories lag behind the newest releases—or if you want the absolute bleeding edge—you can compile OpenSSH from source:

1. Download Source: Obtain the tarball from the official OpenSSH website or a secure mirror.
2. Install Dependencies: Ensure tools like gcc, make, and relevant libraries (OpenSSL, Zlib) are available.
3. Compile and Install: Run ./configure, followed by make and sudo make install. Decide whether to overwrite the existing binary or install it on an alternative path.
4. Maintain Configuration: Keep backups of your config files and verify compatibility with new features or changes.

Using Testing Repositories

Some distributions, like Ubuntu or Debian, offer “testing” or “backports” repositories containing newer software versions than in the stable channel. Enabling these repositories can allow you to install more recent OpenSSH releases with minimal effort. However, these packages may not receive the same scrutiny or support as the stable ones.

Automation with Configuration Management

Tools like Ansible, Puppet, and Chef are invaluable for automating SSH management across multiple servers:

• Playbooks and Manifests: Define tasks that install or update SSH, modify configuration files, and restart services in a controlled manner.
• Consistency: Ensure each server maintains the same SSH policies, ciphers, and access rules.
• Error Reduction: Automated processes minimize human errors that often occur when updating multiple servers manually.

Zero Downtime Upgrades

Even a brief SSH restart might be unacceptable in highly critical production environments. One strategy is to run a second SSH daemon on a different port. You can test the new version thoroughly without interrupting existing connections:

1. Duplicate Configuration: Copy /etc/ssh/sshd_config to something like /etc/ssh/sshd_config_alt, then change Port 22 to another port (e.g., 2222).
2. Start Another Daemon:
   sudo /usr/sbin/sshd -f /etc/ssh/sshd_config_alt
3. Test Connectivity: From a separate machine, connect using the alternate port:
   ssh -p 2222 user@server_ip
4. Finalize: Once you confirm the new daemon works, update your primary configuration and gracefully end the old SSH session or service.

Conclusion: 

Maintaining a Secure and Efficient SSH Environment

SSH is the cornerstone of secure remote administration in Linux environments. Whether you’re a seasoned system administrator or just starting, knowing how to check the SSH version is essential. Developing this core skill in Linux enables you to maintain current, up-to-date systems, troubleshoot effectively, and protect against emerging threats. By checking your SSH client and server versions, you can understand your security risks, prevent feature mismatches, and meet legal requirements.

Embrace best practices like turning off root login, enforcing key-based authentication, and restricting cipher suites to modern, robust algorithms. Keep abreast of updates from your distribution and OpenSSH, as these releases frequently address newly discovered security issues. Plan carefully when an update arrives, especially in production environments, to avoid downtime or lockouts.

Staying vigilant about SSH versions creates a stable, high-performing, secure infrastructure. This guide offers all the instrumentation you may need to analyze your current state of affairs in SSH management, enable any improvements or scale back where necessary, and, in general, assume a far more strategic approach to the problem than hitherto. This diligence protects the single servers and helps maintain the reliability of your network in its entirety.

About the writer

Vinayak Baranwal wrote this article. Use the provided link to connect with Vinayak on LinkedIn for more insightful content or collaboration opportunities.