Just the commands - fast setup for a secure Linux server

last modified: 2017-04-10

Clément Levallois

What?

  • Debian jessie 8.7

  • vi

→ change for your favorite text editor, and probably valid for Ubuntu as well.

Make sure you have the latest version of all packages:

sudo apt-get update && sudo apt-get upgrade

Network Time Protocol

aptitude install ntp

Then define your time zone (the one where your server is located):

dpkg-reconfigure tzdata

harden the kernel

vi /etc/sysctl.d/local.conf

  • Paste the contents of this file:

  • Close the file

  • reboot the server

changing SSH port

vi /etc/ssh/sshd_config

Text to change in the file:

  • change port SSH 22 by a new port (let’s say 1234), write the new port down somewhere

  • ChallengeResponseAuthentication no

  • UsePAM no

    service sshd restart

Installing the sudo command:

apt-get install sudo

Adding a new user (let’s call it "myUser")

adduser myUser -s /bin/bash passwd myUser vi /etc/sudoers

and place the following line:

myUser ALL=(ALL)

Enabling server connections via myUser

vi /etc/ssh/sshd_config

AllowUsers myUser

Then restart the SSH service:

service sshd restart

Disabling connection through root

vi /etc/ssh/sshd_config

Text to change in the file:

PermitRootLogin no

From there on, you cannot login to the server from root, only from myUser.

To switch to root privileges:

su -

enable SSH key auth

  • Generate a key with puttygen (SSH-2 RSA 1024).

  • Parameters to change in /etc/ssh/sshd_config:

ChallengeResponseAuthentication no

X11Forwarding no

UsePAM no

LogLevel DEBUG3 (this should be added, the parameter is not listed by default)

  • Save the file, then:

    service sshd restart

  • Add your public key to /home/myUser/.ssd/authorized_keys

Make sure that:

  • you have put the keys in /home/myUser/.ssd/authorized_keys (not just in the root user folder)

  • your key starts with "the "ssh-rsa" (the first "s" might be missing …​)

  • the key doesn’t break in several lines

  • do chmod 700 ~/.ssh on the home folder

  • use tail -f /var/log/auth.log for debugging

When SSH key login works, go back to /etc/ssh/sshd_config and do:

PasswordAuthentication no

then: service sshd restart

Things will not work the first time, useful tips:

installing the undifficult firewall

sudo apt-get update
apt-get install ufw

denying all incoming traffic except for SSH port

ufw default deny incoming
sudo ufw allow 1234/tcp
ufw enable

install and config of Psad

First, making sure the firewall logs the traffic:

iptables -A INPUT -j LOG
iptables -A FORWARD -j LOG
apt-get install psad

Then modify some options in the config file, which is situated here:

vi /etc/psad/psad.conf

Here are some options I modified: my psad config file

Then we whitelist our own server:

vi /etc/psad/auto_dl

where I put just 2 values:

127.0.0.1 0; # localhost

xx.xx.xxx.xxx 0; # Server IP (replace xx.xx.xxx.xxx by your actual server IP)

to be continued

the end

Author of this tutorial: Clement Levallois

All resources on linux security: https://seinecle.github.io/linux-security-tutorials/

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