How to limit simultaneous connections to SSH Server

If you run an SSH server, you may want to limit the number of simultaneous connections to prevent resource exhaustion and potential security breaches. In this article, we’ll explore various methods to limit simultaneous connections to an SSH server.

What is SSH?

Secure Shell (SSH) is a cryptographic network protocol for secure data communication. SSH is commonly used to log into a remote machine and execute commands, but it also supports tunneling, forwarding TCP ports, and X11 connections. SSH provides confidentiality and integrity of data exchanged between two computers over an insecure network.

Why Limit Simultaneous Connections to an SSH Server?

An SSH server is an attractive target for attackers looking to gain unauthorized access to a system. They may use brute-force attacks or exploit vulnerabilities in the SSH server to gain access to the system. If an attacker gains access to an SSH server, they can potentially gain access to other machines on the network.

Limiting the number of simultaneous connections to an SSH server can prevent resource exhaustion and potential security breaches. It ensures that only authorized users can access the system, and it reduces the risk of an attacker gaining unauthorized access to the system.

Methods to Limit Simultaneous Connections to an SSH Server

Method 1: Using SSH Config File

One way to limit simultaneous connections to an SSH server is by configuring the SSH server to allow a specific number of connections per user. To do this, we can edit the SSH config file.

1. Open the SSH config file using your preferred text editor

sudo nano /etc/ssh/sshd_config

2. Add the following line at the end of the file to allow a maximum of three simultaneous connections per user:

MaxSessions 3

3. Save and close the file.
4. Restart the SSH server to apply the changes:

sudo systemctl restart sshd

This method limits the number of simultaneous connections per user. However, it does not limit the number of simultaneous connections from different IP addresses.

Method 2: Using TCP Wrappers

TCP Wrappers is a host-based network access control system that allows you to restrict access to network services based on the host name, IP address, or domain name of the client machine. You can use TCP Wrappers to limit the number of simultaneous connections to an SSH server.

1. Install TCP Wrappers on your system:

sudo apt-get install tcpd

2. Edit the hosts.allow file using your preferred text editor:

sudo nano /etc/hosts.allow

3. Add the following line at the end of the file to allow a maximum of three simultaneous connections from the IP address 192.168.1.1

sshd: 192.168.1.1 : 3

3. Save and close the file.
4. Edit the hosts.deny file using your preferred text editor:

sudo nano /etc/hosts.deny 

5. Add the following line at the end of the file to deny access to all other hosts

sshd: ALL

6. Save and close the file.
7. Restart the SSH server to apply the changes

sudo systemctl restart sshd

This method limits the number of simultaneous connections from a specific IP address. However, it does not limit the number of simultaneous connections per user.

Conclusion

Limiting simultaneous connections to an SSH server is an important step in securing your system and preventing unauthorized access. We’ve explored two methods to limit simultaneous connections to an SSH server: using the SSH config file to limit the number of connections per user and using TCP Wrappers to limit the number of connections from a specific IP address.

While both methods have their benefits, using TCP Wrappers provides more fine-grained control over access to the SSH server. You can restrict access based on the IP address, domain name, or host name of the client machine, which can be useful in a variety of scenarios.

As always, it’s important to keep your SSH server up-to-date with the latest security patches and configurations. Additionally, you should always use strong passwords and consider implementing two-factor authentication to further secure your SSH server.

For more information on securing your SSH server, check out the following resources:

• OpenSSH Security

By following best practices and limiting simultaneous connections to your SSH server, you can help ensure the security and integrity of your system.

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