How to Fix “ssh_exchange_identification: read: Connection reset by peer” Error

Introduction

A remote machine has prevented an SSH connection you were attempting to establish or maintain. The “ssh_exchange_identification: read: Connection reset by peer” message is not specific enough to immediately explain what triggered the error.

To be able to resolve the issue successfully, we first need to identify its cause. This article provides an in-depth analysis of the likely causes and provides the most effective solutions.

By reading this tutorial, you will learn how to fix the “ssh_exchange_identification: read: Connection reset by peer” Error.

Prerequisites

• Necessary permissions to access remote server
• A user account with root or sudo privileges

What Causes the “Connection reset by peer” SSH Error?

The “ssh_exchange_identification: read: Connection reset by peer” error indicates that the remote machine abruptly closed the Transition Control Protocol (TCP) stream. In most instances, a quick reboot of a remote server might solve a temporary outage or connectivity issue.

Learning how to troubleshoot this issue, and determining the underlying cause, helps you prevent future occurrences on your system. The most common causes of the “ssh_exchange_identification: read: Connection reset by peer” error are:

• The connection is being blocked due to the Host-Based Access Control Lists.
• Intrusion prevention software is blocking your IP by updating firewall rules (Fail2ban, DenyHosts, etc.).
• Changes to the SSH daemon configuration file.

Check the hosts.deny and hosts.allow File

The hosts.deny and hosts.allow files are TCP wrappers. As a security feature, these files are used to limit which IP address or hostname can establish a connection to the remote machine.

How to Edit hosts.deny File

Access your remote server and open the hosts.deny file using your preferred text editor. If you are using nano on a Debian based system, enter the following command:

sudo nano /etc/hosts.deny

Empty lines and lines starting with the ‘#’ symbol are comments. Check if you can locate your local IP or host-name in the file. If it is present, it should be removed or commented out, or else it prevents you from establishing a remote connection.

After making the necessary changes, save the file and exit. Attempt to reconnect via SSH.

How to Edit hosts.allow File

As an additional precaution, edit the hosts.allow file. Access rules within the hosts.allow are applied first. They take precedence over rules specified in hosts.deny file. Enter the following command to access the hosts.allow file:

sudo nano /etc/hosts.allow

Adding host-names and IPs to the file defines exceptions to the settings in the hosts.deny file.

For example, a strict security policy within the etc/hosts.deny file, would deny access to all hosts:

sshd : ALL
ALL : ALL

Subsequently, you can add a single IP address, an IP range, or a hostname to the etc/hosts.allow file. By adding the following line, only the following IP would be allowed to establish an SSH connection with your remote server:

sshd : 10.10.0.5, LOCAL

Keep in mind that such a limiting security setting can affect administering capabilities on your remote servers.

Check if fail2ban Banned Your IP Address

If you’ve tried to connect on multiple occasions, your IP might be blocked by an intrusion prevention software. Fail2ban is a service designed to protect you from brute force attacks, and it can misinterpret your authentication attempts as an attack.

Fail2ban monitors and dynamically alters firewall rules to ban IP addresses that exhibit suspicious behavior. It monitors logs, like the hosts.deny and hosts.allow files we edited previously.

In our example, we used the following command to check if the iptables tool is rejecting your attempted connections:

sudo iptables -L --line-number

The output in your terminal window is going to list all authentication attempts. If you find that a firewall is indeed preventing your SSH connection, you can white-list your IP with fail2ban. Otherwise, the service is going to block all future attempts continuously. To access the fail2ban configuration file, enter the following command:

sudo nano /etc/fail2ban/jail.conf

Edit the file by uncommenting the line that contains "ignoreip =" add the IP or IP range you want to white-list.

Fail2ban is now going to make an exception and not report suspicious behavior for the IP in question.

Check the sshd_config File

If you are continuing to experience the ‘ssh_exchange_identification: read: Connection reset by peer’ error, examine the authentication log entry. By default, the SSH daemon sends logging information to the system logs. Access the /var/log/auth.log file after your failed attempt to login. To review the latest log entries type:

tail -f /var/log/auth.log

The output presents the results of your authentication attempts, information about your user account, authentication key, or password.

The log provides you with information that can help you find possible issues in the sshd configuration file, sshd_config. Any changes made to the file can affect the terms under which an ssh connection is established and lead the remote server to treat the client as incompatible. To access the sshd_config file type:

sudo nano /etc/ssh/sshd_config

The sshd configuration file enables you to change basic settings, such as the default TCP port or SSH key pairs for authentication, as well as more advanced functions such as port-forwarding.

For example, the MaxStartups variable defines how many connections a system accepts in a predefined period. If you have a system that makes a large number of connections in a short timeframe, it might be necessary to increase the default values for this variable. Otherwise, the remote system might refuse additional attempted ssh connections.

Anytime you edit the sshd_config file, restart the sshd service for the changes to take effect:

service sshd restart

Only edit the variables that you are familiar with. A server can become unreachable as a result of a faulty configuration file.

Conclusion

You have thoroughly checked the most common reasons behind the “ssh_exchange_identification: read: Connection reset by peer” error. By looking at each possibility, in turn, you have successfully solved the issue and now know how to deal with similar problems going forward.

The number of potential causes is vast and difficult to troubleshoot in every respect. Ultimately, if the error persists, it might be necessary to contact your host.