How to harden an SSH server?


With the sudden rise in SSH brute force attacks, securing SSH is more important than ever. In a shared hosting environment, you options are somewhat limited, but if you have a well defined user group, you can really lock down SSH such that brute-force attacks are no longer a threat. Here are a few methods you can employ to further harden your SSH installation.

Default Settings
Before going over the changes, I just want to highlight some of the issues with the default settings. These apply to Red Hat, Fedora, and CentOS distributions.

Allows Legacy SSH protocol version 1 which has known security issues Allows direct access to root via password authentication Uses low key strength to secure sessions Allows access to all users

Though these items are relatively minor, they can easily be corrected with proper configuration.

Harden SSH
Previously, I commented on some ways to protect S“SH against brute force...

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Secure Shell is a command line interface to access remote Linux server. SSH is based on network protocol and can use to execute various command line operations and data transfer. SSH protocol can also be used to perform scp(Secure Copy) and sftp(Secure File Transfer).

We can use both Windows and Unix based system to create SSH connection to remote system

In this tutorial we will show you some steps to improve the security of SSH connection.

1: Disable Default Port.

By default SSH uses port 22. Majority of automated attacks are based on default port. Once it changed it is bit harder for attackers to find which port you are using for SSH communication.

Steps to change SSH port:

1) Login to server as root user.

2) Make a backup of SSH configuration file ssd_config as shown below.

root@test:/# cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak

3) Open the file sshd_config. You can use text editors such as nano, vim etc. To...

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One of the most important things that you should do immediately after initial server setup is to protect your machine against unauthorized access via SSH. Otherwise network scanning programs will quickly find out that your server port 22 is open. In that case, the only thing a hacker has to do to get the full control of the system is to fit the root user password.

Apparently unprotected SSH poses a grave security threat to your server. Of course, there is no absolute protection on the Internet. So your task is to make the attacker’s work as difficult as possible. Let’s examine a few safety precautions to reduce the risk of VPS cracking via SSH.

Change SSH port number. This is the most obvious and the simplest thing you can do. To do that, edit the configuration file, changing the Port directive value (replace 22 with nonstandard port number). SSH port number changing is the easiest way to get rid of majority of password guessing attempts. Disable root user login. From...
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With the amount of SSH brute force attacks, securing SSH is more important than ever.

Here are a few methods you can consider to further harden your SSH installation.

Default Settings
First of all, we look att the default settings as these apply to many (most?) distributions.

Allows Legacy SSH protocol version 1 (has known security issues) Allows direct access to root via password authentication Uses low key strength to secure sessions Allows SSH access to all users

Four minor items that should be changed at first boot of the server.

Harden SSH

Disable SSH Protocol 1
SSH has two protocols, 1 and 2. Protocol 1 has a number of known flaws and should no longer be used.
Diisable Protocol 1 by editing /etc/ssh/sshd_config

Restrict Root Login
There are two options in restricting access to root logins. You can either disable root logins completly or you can force it to use SSH keys.

Setting the option to “no” disables...

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In this tutorial, we will be looking at securing SSH server from unauthorized access. SSH is the primary way to remote into Linux VPS or cloud servers and also one of the primary attack vector by bad guys. It’s important that we secure it as much as possible.

Your SSH Server is Being Attacked

Run the following command on your Linux server and you will see how often malicious people are trying to get SSH access to your server. To be honest, some of my SSH servers are being attacked every minute.

sudo journalctl -xe | grep sshd

It’s very common for bad guy to brute-force attack the root user password. Here’s a snippet output of the above command on my Linux server.

sshd[27389]: Failed password for invalid user root from port 42803 ssh2 sshd[27389]: Failed password for invalid user root from port 42803 ssh2 sshd[27389]: Failed password for invalid user root from port 42803 ssh2 sshd[27389]: Received disconnect from...
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SSH (Secure SHell) is a commonly used protocol for secure data communications between systems. It is rare to find systems not having this service running. As this opens up a potential gateway into the system, hardening the configuration of the SSH server is an important step in server hardening. In this guide we will focus on several common configuration options of SSH and improve it.

Client and Server

SSH has two parts: the client used for connecting to a server, and the server daemon itself. This latter one is usually the most important part in deciding how “secure” a connection may be. One example is that the server can decide if normal password based logins are allowed or denied. Even if the client has a preference, it is the server to make the final call.

The client configuration settings can be found in /etc/ssh/ssh_config (system wide) or ~/.ssh/config (per user). For the server configuration file: /etc/ssh/sshd_config. Let’s start with the SSH server...

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The following are some of the steps you can take to harden the SSH Server against unauthorized access attempts.

These steps should be pursued only after you have successfully configured the SSH Server, and tested that it serves the mode of use you desire. After applying each hardening step, test that your desired mode of use still works.

If a hardening step has broken your configuration, you can:

If you have configured and successfully tested public key authentication, you can disable password authentication for individual accounts:

You can also disable password authentication in Windows or virtual group settings entries, as a default for multiple accounts:

Note that users will not know that password authentication is disabled. If an SSH client is not able to connect otherwise, it will still display a password prompt. However, the SSH Server will refuse all password-based login attempts for an account, if password authentication for...

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This guide is based on various community forum posts and webpages. Special thanks to all. All comments and improvements are very welcome as this is purely a personal experimental project at this point and must be considered a work in progress.

This guide is intended as a relatively easy step by step guide to:

Harden the security on an Ubuntu 16.04 LTS server by installing and configuring the following:

Install and configure Firewall - ufw Secure shared memory - fstab SSH - Key based login, disable root login and change port Apache SSL - Disable SSL v3 support Protect su by limiting access only to admin group Harden network with sysctl settings Disable Open DNS Recursion and Remove Version Info - Bind9 DNS Prevent IP Spoofing Harden PHP for security Restrict Apache Information Leakage Install and configure Apache application firewall - ModSecurity Protect from DDOS (Denial of Service) attacks with ModEvasive Scan logs and ban...
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If you have recently created a DigitalOcean Droplet, and you are new to working with Linux servers, you will need to learn how to use SSH to connect to and manage it. SSH, which stands for Secure Shell, is an encrypted network protocol that is used to for, among other things, remote server login and command execution. It is the standard method used for accessing and interacting with Linux servers.

This quick tutorial will show you how to connect to your new Linux cloud server for the first time, by logging into it using an SSH client.


The prerequisites section describes everything that you need know about to follow this tutorial. Of course, you will need to have created a new Droplet through the DigitalOcean Control Panel.

Server Information and Login Credentials

In order to connect to a remote Linux server via SSH, you must have following:

User name: The remote user to log in as. The default admin...
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Its good to harden your box which is in DMZ.
What is DMZ?
Ans : DMZ is a De Militarised Zone where we will be keeping our servers, so that they can be access by out side people. Let me explain more about this DMZ. Who are not familiarise(And this activity is most of the time a Network admin work).
1. DMZ is a place where we will be isolate machines from companies local LAN.
2. These DMZ machines will have different IP address range and subnet.
3. The communication between two machines in DMZ is blocked for security reasons.
4. We cannot login to Local LAN machine from a DMZ machine, but we can login to DMZ machine from local LAN(only one way communication from LAN).
5. Ping to these machines will be disabled(most of the companies will do this for security reasons).
6. The way these machines communicate totally depends on network team what type of rule they set on their routers.
7. The security of DMZ machines are more when compared to...

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My approach to SSH hardening is... complex. The following items are in terms of how I do it, from the edge-most border of my network(s) to the servers themselves.

Border-level filtering of traffic through IDS/IPS with known service scanners and signatures in the blocklist. I achieve this with Snort via my border firewall (this is my approach, a pfSense appliance). Sometimes, I can't do this though, such as with my VPSes.

Firewall/Network filtering of the SSH port(s). I explicitly only allow certain systems to reach into my SSH servers. This is either done via a pfSense firewall at the border of my network, or the firewalls on each server explicitly being configured. There are cases where I can't do this, though (which is almost never the case, except in private pen-testing or security testing lab environments where firewalls won't help test things).

In conjunction with my pfSense, or a border firewall NAT-ing the internal network and separating from the...

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Encryption and secure communications are critical to our life on the Internet. Without the ability to authenticate and preserve secrecy, we cannot engage in commerce, nor can we trust the words of our friends and colleagues.

It comes as some surprise then that insufficient attention has been paid in recent years to strong encryption, and many of our "secure" protocols have been easily broken. The recent Heartbleed, POODLE, CRIME and BEAST exploits put at risk our trust in our networks and in one another.

Gathered here are best-practice approaches to close known exploits and strengthen communication security. These recommendations are by no means the final word on the subject—the goal here is to draw focus upon continuing best practice.

Please note that many governments and jurisdictions have declared encryption illegal, and even where allowed, law enforcement has become increasingly desperate with growing opaque content (see the Resources section for...

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I am not completely sure, but you may want to look at the protocol setting in sshd_config.


# Protocol 2,1 Protocol 2

Change Protocol 1 to Protocol 2 and restart. This should already be set to Protocol 2 in Centos 6.5, but you may want to double check.

I found this run down of the different protocol options

Not sure if that is going to be enough to solve your particular issue though.

Do you know what they are using to check the configuration?


This is from running man sshd_config on

Ciphers Specifies the ciphers allowed for protocol version 2 in order of preference. Multiple ciphers must be comma-separated. The supported ciphers are “3des-cbc”, “aes128-cbc”, “aes192-cbc”, “aes256-cbc”, “aes128-ctr”, “aes192-ctr”, “aes256-ctr”, “arcfour128”, “arcfour256”, “arcfour”, “blowfish-cbc”, and “cast128-cbc”....
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Bitvise SSH Server installer - version 7.15, size 13.1 MB. Free for non-commercial personal use. Excellent terminal. Single virtual filesystem for SFTP and SCP with multiple configurable mount points. Cryptography FIPS 140-2 compliant if enabled in Windows. Requires Windows XP, Windows Server 2003, or newer. Supports the latest Windows server and desktop versions, both 32-bit and 64-bit. Alternate installer for users of old Windows versions.

If the link doesn't work, try again - it is Amazon S3 hosted in Ireland and should be very reliable. Users of older Windows versions may need to use the alternate installer, signed using SHA-1 instead of SHA-2 256.

Our installers are cryptographically signed. Our most recent installers use an Extended Validation digital certificate from DigiCert. Do not run any installers for our software that do not carry a valid digital signature by Bitvise Limited.

Bitvise SSH Server is easy to uninstall.

Using Bitvise SSH Server...

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Our SSH server is designed for all Windows NT-series operating systems. Supported platforms include all desktop and server versions of Windows, starting from Windows XP and Windows Server 2003, to the most recent; including Windows 8.x, Windows Server 2012 R2, and Windows 10. Both 32-bit and 64-bit versions of Windows are supported.

Bitvise SSH Server supports the following SSH services:

Secure remote access via console (vt100, xterm and bvterm supported) Secure remote access via GUI (Remote Desktop or WinVNC required) Secure file transfer using SFTP and SCP (compatible with all major clients) Secure, effortless Git integration. Secure TCP/IP connection tunneling (port forwarding)

You can try out Bitvise SSH Server risk-free. To begin, simply download the installation executable - you will find the download links on our download page. After installing, you are free to evaluate Bitvise SSH Server for up to 30 days. If you then decide to continue using it, purchase a...

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If your SSH client supports it, you can use public key authentication to log into Bitvise SSH Server. On Windows, we recommend Bitvise SSH Client, which has strong support for public key authentication, as well as password authentication, and Kerberos single sign-on in domain environments.

If you are new to public key authentication, we first suggest reading Public keys in SSH.

To set up public key authentication, you first need to generate a keypair on the client, or select one or more existing keypairs for use with client authentication. The procedure for generating the keypair depends on the client software being used:

If you are using Bitvise SSH Client, click the link titled Client key manager in the Login tab. You can generate, edit, import and export keypairs in the dialog box that pops up. If you are using a different client, you need to follow its process for generating keypairs. For example, in OpenSSH, keypairs are generated using the ssh-keygen...
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How to Harden SSH with Identities and Certificates


It's 2014 and remote unix shells are as popular as ever. Which is great, except that people don't seem to be using the more advanced security features nearly enough. So, what are they and how can you use them?


Whether you just need to feel in power or you actually use shells for day-to-day tasks, the Secure Shell [SSH] is probably the most important administrative access tool to your servers. It's also one of the least secured mission-critical services on most UNIX servers. Why? Because for some reason people are still using mere passwords to protect their root accounts. That's not quite as bad as using telnet, but not by too much. You might as well be using plain FTP to transfer data to your server... oh, wait, that's another article.

Using passwords for your remote servers exposes you to a whole class of unnecessary security risks, which are easily avoided by either switching...

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Changes in Bitvise SSH Server 7.15: [ 4 September 2016 ]

Updated EULA to make more explicit our licensing and support policies. The policies themselves remain unchanged. Fixed an issue which caused the SSH Server's settings description in textual log files to include all settings fields. It now again properly records only fields that differ from defaults.

Changes in Bitvise SSH Server 7.14: [ 3 August 2016 ]

SSH implementations have a chance of generating RSA signatures slightly smaller than expected with a small probability (e.g. 1:200). Windows CNG has been found to not validate such signatures as presented. With our software versions 7.12, this has resulted in occasional connection or login attempt failures. Our SSH Server, SSH Client, and FlowSsh now re-encode RSA signatures, so that smaller-than-expected ones can verify correctly. Windows CNG, as used by our new cryptographic provider in versions 7.xx, has been found to return an incorrect signature size for...
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Linux already claims a large share of the server market, and forecasts show that this share will increase because of the demands of cloud computing. Enterprise IT shops concerned with security need to take a look at the vulnerabilities these servers pose to the network and how these machines can be secured. This article demonstrates how to tighten Secure Shell (SSH) sessions, configure a firewall, and set up intrusion detection.

Plan the server installation

The first step in hardening a GNU/Linux server is determining the server's function, which determines the services that need to be installed on it. For example, if the server in question is used as a web server, you should install Linux, Apache, MySQL, and Perl/ PHP/ Python (LAMP) services. If the server is used for directory services, the only applications and services that should be permitted to run on ...

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