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Harden Your SSH Security on Linux Servers

In this post we wil be focusing on increasing / hardening our security by adjusting our ssh configuration and applying some iptables firewall rules.

This will be the list of things that we will do:

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  - Change the SSH Port
  - Don't allow root to SSH
  - Disable password based authentication
  - Enable key based authentication and only for a singular user
  - Allow our user to sudo
  - Use iptables to block sources trying to DDoS your server

Packages

First let’s install the packages that we need, I’m using Debian so I will be using the apt package manager:

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$ apt update && apt upgrade -y
$ apt install sudo -y

Dedicated User

Let’s create our user james:

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$ useradd -m -s /bin/bash james

Allow our user to sudo without a password, by running visudo then append the following line:

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james ALL=(ALL:ALL) NOPASSWD: ALL

SSH Authorized Keys

If you don’t already have a private SSH key, generate one on your client side:

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$ ssh-keygen -f ~/.ssh/james -t rsa -C "james" -q -N ""

Then copy the public key:

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$ cat ~/.ssh/james.pub | pbcopy

On your server create the SSH directories:

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$ mkdir /home/james/.ssh

Now paste your public key into /home/james/.ssh/authorized_keys

Then change the ownership:

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$ chmod 700 /home/james/.ssh
$ chmod 644 /home/james/.ssh/authorized_keys
$ chown -R james:james /home/james

SSH Config

Backup your SSH config:

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$ cp /etc/ssh/sshd_config /etc/ssh_sshd_config.bak

We will be using the SSH port 2914, replace your SSH config with the following and make your adjustments where you need to:

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# /etc/ssh/sshd_config
Port 2914
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key
LoginGraceTime 1m
PermitRootLogin no
MaxAuthTries 3
MaxSessions 5
AuthenticationMethods publickey
PubkeyAuthentication yes
AuthorizedKeysFile      /home/james/.ssh/authorized_keys
PasswordAuthentication no
PermitEmptyPasswords no
ChallengeResponseAuthentication no
UsePAM yes
AllowUsers james
DenyUsers root
X11Forwarding yes
PrintMotd no
UseDNS no
PidFile /var/run/sshd.pid
AcceptEnv LANG LC_*
Subsystem       sftp    /usr/lib/openssh/sftp-server

Then save the file and restart SSH:

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$ systemctl restart sshd

While you are still connected to the shell session, open up a new terminal and try to connect with your new user and private SSH key to ensure that you can connect to your server.

Iptables

We want to drop incoming connections which make more than 10 connection attempts to SSH within 60 seconds.

The tokens get refilled into buckets at 3 per minute and maximum of 3 tokens that can be filled into the bucket.

Let’s create our script:

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$ mkdir -p /opt/scripts
$ touch /opt/scripts/fw.sh

In our script we will place the following content:

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#!/usr/bin/env bash
INTERFACE=eth0 # check ifconfig to determine the correct interface
SSH_PORT=2914
CONNECTION_ATTEMPTS=10
CONNECTION_TIME=60
#WHITELIST_IP=x.x.x.x/32 # replace ip and uncomment if you want to whitelist a ip
#iptables -I INPUT -s ${WHITELIST_IP} -p tcp --dport ${SSH_PORT} -i ${INTERFACE} -j ACCEPT # uncomment if you want to use whitelisting
iptables -A INPUT -p tcp --dport ${SSH_PORT} -i ${INTERFACE} -m state --state NEW -m recent  --set
iptables -A INPUT -p tcp --dport ${SSH_PORT} -i ${INTERFACE} -m state --state NEW -m recent  --update --seconds ${CONNECTION_TIME} --hitcount ${CONNECTION_ATTEMPTS} -j DROP
iptables -A INPUT  -i ${INTERFACE} -p tcp --dport ${SSH_PORT} -m state --state NEW -m limit --limit 3/min --limit-burst 3 -j ACCEPT
iptables -A INPUT  -i ${INTERFACE} -p tcp --dport ${SSH_PORT} -m state --state ESTABLISHED -j ACCEPT
iptables -A OUTPUT -o ${INTERFACE} -p tcp --sport ${SSH_PORT} -m state --state ESTABLISHED -j ACCEPT

Now we want to execute this script whenever the server boots, open up /etc/rc.local and append the following line, so that the file looks more or less like:

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#!/bin/bash
/opt/scripts/fw.sh
exit 0

Ensure both files are executable:

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$ chmod +x /opt/scripts/fw.sh
$ chmod +x /etc/rc.local

When you are sure everything is in place, reboot:

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$ reboot