SSH Client

Installation

By default Debian | Ubuntu doesn't include any SSH server.

apt-get install ssh openssh-server

SSH server configuration

Edit the configuration file:

vim /etc/ssh/sshd_config

X11 forwarding

In the configuration file, uncomment and set:

ForwardAgent yes
ForwardX11 yes
ForwardX11Trusted yes

Enable | Disable the forwarding:

# This server doesn’t have a XServer. Therefore do not forward graphical data.
X11Forwarding no

Port(s) number

You can listen on multiple port. Just do the following:

Port 22
Port 2200

Security psycho mode:

# The default port SSH is 22. You may want to change that port to another one so your server will be more discreet.
# NB: if your server is hosted the provider might need access for maintenance purposes.
Port XXXXX

Restart SSH server

/etc/init.d/ssh restart

SSH server configuration - Authentication by Linux user login / password

Principle

This is the default authentication system.

Each user that has a local account on the server and member is allowed to access the SSH server with its login / password.

Configuration changes

vim /etc/ssh/sshd_config

Protocol and password enforcement

Protocol 2			# only use SSH v2
PermitRootLogin no		# Avoid root connections
PermitEmptyPassword no	        # Forbidden user with empty passwords

Login time

# Time to log
LoginGraceTime 30

Restart SSH server

/etc/init.d/ssh restart

SSH server configuration - Authentication with RSA keys

Introduction

If you’d like to increase the authentication process you can use authentication by private/public key.

• Generate new private / public keys on your own computer
• Put the public key on the remote SSH server
• Only the person with the private key can be authenticate on the server

For instance, this is how hosting company such as OVH can log on your system.

Security improvement: remove password authentication

When the key authentication is working you can remove the default access by login / password. Then, only people with a valid private/public key pair can log in.

That way, there is no way for brute-force attacks to be successful, so your system is more secure.

Declare the public key on the server

You have to:

• log in to your SSH server with the user that’s gonna use this key
• Go to user's home directory
• Create a .ssh folder (if there was none before).

cd ~
mkdir .ssh
cd .ssh

Add the new public key to the list of allowed keys:

vim authorized_key2

Prefix your key with:

• RSA: ssh-rsa
• DSA: ssh-dss

Then paste the public key in one line - the public key mustn't be change or separated in 2 lines!

# Example: 
ssh-rsa AAAAB3NzaC1yc2EA[...]Lg5whU0zMuYE5IZu8ZudnP6ds= myname@example.com
ssh-dss AAAAB3NzaC1yc2EA[...]Lg5whU0zMuYE5IZu8ZudnP6ds= myname@example.com

Adjust file rights, the authorized_keys2 file must be write/readable only by that user

chmod 600 authorized_keys2
cd ..
chmod 700 .ssh

How-to generate private / public keys in Windows

You can generate private / public keys with PuttyGen = Putty key generator.

When the keys are OK, you have to enter a key pass-phrase. Your pass-phrase must be:

• long (> 15 characters)
• hard to guess
• with letters + signs + numbers

Reminder

how to choose your passphrase and protect it: http://www.alcf.anl.gov/resource-guides/user-authentication-policies

Then, save your keys! You should be the only one to access the save location.

Configuration changes

vim /etc/ssh/sshd_config

Restart SSH server

/etc/init.d/ssh restart

Requirements – windows Download the following software: • PuTTY • PuTTYgen • Pageant

Windows - Generate new private / public keys pair Start PuTTYgen

Create a 4096 bits key, DSA algorithm.

Then, click on generate When the keys are OK, you have to enter a key passphrase.  You passphrase must be long (> 15 characters), hard to guess, with letters + signs + numbers

 Reminder: how to choose your passphrase and protect it: http://www.alcf.anl.gov/resource-guides/user-authentication-policies

Then, save your keys! You should be the only one to access the save location.

Declare the public key on the server You have to log in to your SSH server with the standard user that’s gonna use this key. Go to your home directory, and create a .ssh folder (if there was none before).

1. cd ~
2. mkdir .ssh
3. cd .ssh
4. vim authorized_key2

Prefix your key with: RSA: ssh-rsa DSA: ssh-dss Then paste the public key into the file in one line!

Copy the text as shown on the previous image.

Example: ssh-rsa AAAAB3NzaC1yc2EA[...]Lg5whU0zMuYE5IZu8ZudnP6ds= myname@example.com ssh-dss AAAAB3NzaC1yc2EA[...]Lg5whU0zMuYE5IZu8ZudnP6ds= myname@example.com

Adjust file rights  The authorized_keys2 file must be write/readable only by that user

1. chmod 600 authorized_keys2
2. cd ..
3. chmod 700 .ssh

Windows – configure PuTTY client

You have to configure your PuTTY SSH client with this new key.

Create profile

Auto-login

Attach private key

Save profile Go back to the main screen Session and save your changes.

Click on “open” to initialize connection.

Login procedure

 Type your passphrase on system request

Access is granted! 

Disable standard username / password login

Edit the configuration file

1. vim /etc/ssh/sshd_config

Adjust the line:

to:

Restart SSH server:

1. /etc/init.d/ssh restart

OVH server: root access

OVH requires a root access for maintenance. OVH uses a RSA key for authentication. You have to let the following settings: SSH port : 22 Root login : enable UsePam: yes

 Important If this access is removed then OVH will stop your server in case of DoS. More details: http://guide.ovh.com/InstallClefOVH

Fail2ban

see Fail2ban#SSH_configuration

SSH client

Linux

# syntax
ssh user@server -p portNumber

# example
ssh root@daxiongmao.eu -p 4422

References

Source: http://www.howtoforge.com/ssh_key_based_logins_putty

Windows - putty software: http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html