When it comes to managing servers, Secure Shell (SSH) stands as a critical tool in the arsenal of system administrators. It provides a secure channel over an unsecured network, enabling users to log into another computer over a network, execute commands in a remote machine, and move files from one machine to another. However, encountering a “connection refused” error can be a frustrating roadblock. This comprehensive guide aims to demystify this common issue on CentOS 7 systems, providing you with the knowledge to diagnose and resolve the problem efficiently.

Understanding the “Connection Refused” Error in SSH

The “connection refused” message is an indication that your attempt to establish an SSH connection was rejected by the server. This rejection can occur for various reasons, ranging from service issues to firewall configurations. Before diving into troubleshooting methods, it’s essential to understand the underlying causes that might lead to this error.

Common Causes of SSH Connection Refusal

SSH Service Not Running: If the sshd daemon isn’t running on the server, there’s nothing to accept incoming connections.
Firewall Restrictions: A firewall that’s configured to block the default SSH port (22) will prevent connections.
Incorrect SSH Port: If the SSH service is listening on a different port, attempts to connect to port 22 will fail.
SELinux Policies: Security-Enhanced Linux (SELinux) might enforce policies that restrict SSH access.
Max Startups Breach: Exceeding the maximum number of unauthenticated connections can cause additional attempts to be refused.
Host-Based Access Restrictions: Configuration settings may limit which hosts can connect to the SSH server.

Troubleshooting Steps to Resolve “Connection Refused” on CentOS 7

Now that we’ve identified potential culprits, let’s walk through systematic troubleshooting steps to get your SSH connection up and running.

Step 1: Verify SSH Service Status

Firstly, ensure that the SSH daemon is active and running on your CentOS 7 server. You can check the status of the sshd service using the following command:

sudo systemctl status sshd

If the service is not running, start it with:

sudo systemctl start sshd

And enable it to start on boot with:

sudo systemctl enable sshd

Step 2: Inspect Firewall Settings

Next, verify that your firewall allows traffic on the SSH port. The default firewall management tool on CentOS 7 is firewalld, and you can check its settings with:

sudo firewall-cmd --list-all

Look for the SSH service or port 22 in the output. If it’s missing, add it with:

sudo firewall-cmd --zone=public --add-service=ssh --permanent
sudo firewall-cmd --reload

Step 3: Confirm SSH Port Configuration

Ensure that the SSH server is listening on the correct port. Check the configuration file /etc/ssh/sshd_config for the Port directive. If it’s set to a non-standard port, you’ll need to specify that port when connecting:

ssh -p [non-standard-port] user@hostname

Step 4: Review SELinux Policies

SELinux might be enforcing policies that prevent SSH connections. To see if SELinux is the cause, temporarily set it to permissive mode with:

sudo setenforce 0

Try connecting again. If it works, you’ll need to adjust SELinux policies accordingly. Remember to re-enable enforcing mode with sudo setenforce 1 after testing.

Step 5: Check for Host-Based Restrictions

Inspect the /etc/hosts.allow and /etc/hosts.deny files for any rules that might be blocking your IP address. Remove or modify any restrictive entries as needed.

Advanced Diagnostic Tools and Techniques

If the basic checks don’t resolve the issue, it’s time to delve deeper with advanced diagnostic tools and techniques.

Using Verbose Mode in SSH

Running SSH in verbose mode can provide more detailed information about the connection attempt. Use the -v option to increase verbosity:

ssh -v user@hostname

Carefully review the output for clues on where the connection is failing.

Checking SSH Log Files

The SSH daemon logs events which can be helpful for troubleshooting. On CentOS 7, these logs are typically found in /var/log/secure. Look for any error messages related to refused connections.

Analyzing Network Traffic with tcpdump

Sometimes, observing the actual packets being sent and received can pinpoint the issue. Use tcpdump to capture SSH traffic:

sudo tcpdump port 22 -i any -nn

This command listens for traffic on port 22 across all network interfaces. Analyze the output for anomalies or lack of expected traffic.

Frequently Asked Questions

What does “Connection Refused” mean?

It means that the server is reachable over the network but is not accepting connections on the SSH port, usually due to misconfiguration or service issues.

How do I know if my SSH service is running?

Use the command systemctl status sshd to check the status of the SSH service on your CentOS 7 server.

Can firewalls cause SSH connection refusals?

Yes, if a firewall is configured to block the SSH port or has not been set to allow SSH traffic, it can result in a connection refusal.

Is it possible that SELinux is blocking my SSH connection?

Yes, SELinux can enforce strict policies that might inadvertently block SSH connections. Adjusting these policies or setting SELinux to permissive mode can help determine if it’s the cause.

Conclusion

Encountering a “connection refused” error while trying to SSH into a CentOS 7 server can be a perplexing experience. However, by systematically checking the SSH service status, firewall settings, port configurations, SELinux policies, and host-based restrictions, you can identify and fix the root cause. Advanced diagnostics like verbose SSH output, log file analysis, and packet capturing with tcpdump can further aid in resolving stubborn connectivity issues. With this guide in hand, you’re well-equipped to tackle the challenge and restore secure, remote access to your server.

References

CentOS Project. (n.d.). Documentation. Retrieved from https://www.centos.org/docs/
OpenSSH. (n.d.). OpenSSH Manual Pages. Retrieved from https://man.openbsd.org/sshd
Red Hat Enterprise Linux 7: System Administrator’s Guide. (n.d.). Retrieved from https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/system_administrators_guide/chap-Red_Hat_Enterprise_Linux-System_Administrators_Guide-Security_Guide

connection refused ssh centos 7

Unraveling the Mystery Behind “Connection Refused” in SSH on CentOS 7