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

Banner

Source: https://help.ubuntu.com/community/StricterDefaults#SSH_Welcome_Banner

> To enable login messages uncomment the following line in "/etc/ssh/sshd_config"

Banner /etc/issue.net

Then, create / update the "/etc/issue.net" file:

vim /etc/issue.net

The following example is taken from the Advanced OpenSSH page:

***************************************************************************
                            NOTICE TO USERS


This computer system is the private property of its owner, whether
individual, corporate or government.  It is for authorized use only.
Users (authorized or unauthorized) have no explicit or implicit
expectation of privacy.

Any or all uses of this system and all files on this system may be
intercepted, monitored, recorded, copied, audited, inspected, and
disclosed to your employer, to authorized site, government, and law
enforcement personnel, as well as authorized officials of government
agencies, both domestic and foreign.

By using this system, the user consents to such interception, monitoring,
recording, copying, auditing, inspection, and disclosure at the
discretion of such personnel or officials.  Unauthorized or improper use
of this system may result in civil and criminal penalties and
administrative or disciplinary action, as appropriate. By continuing to
use this system you indicate your awareness of and consent to these terms
and conditions of use. LOG OFF IMMEDIATELY if you do not agree to the
conditions stated in this warning.

****************************************************************************

Once this is in place, restart sshd and all users will see this warning before they get the login prompt. This will obviously not dissuade automated SSH attacks, and will potentially worsen DoS effects, but it may tip off a human attacker that the system is being looked after closely, and that they should move on to some other system on the network

Restart SSH server

/etc/init.d/ssh restart

Security

Firewall

See Firewall#SSH

Fail2ban

see Fail2ban#SSH_configuration

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_key

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_keys file must be write/readable only by that user

chmod 640 authorized_keys
cd ..
chmod 700 .ssh

Configuration changes

vim /etc/ssh/sshd_config

Allow empty password

PermitEmptyPassword yes	        # allow empty password in favor of RSA keys

Do not allow password authentication

Once you've checked that the key authentication is working you can disable the standard authentication with password. :) With that option enabled all brute-force attacks will failed.

!!BE CAREFUL !!

Once that setting is enable there is no rolling back from it. You might get kicked for good from your own server.

PermitEmptyPassword yes	                # That must be set to YES
PasswordAuthentication no	        # Do not allow standard login + password anymore. Only key authentication is allowed.

Restart SSH server

/etc/init.d/ssh restart

SSH server - Authentication using LDAP server

Requirement: LDAP server

Principle

The idea is to use a LDAP server to manage users and groups to ease the maintenance and administration.

• Only 1 group of users is allowed to connect

• Access can be dynamically and easily granted

Configuration

!! TO BE DONE !!

How-to generate private / public keys

Linux

Create key

1. Log-in with the user you want to use.

2. Generate a pair of authentication keys.

# Generating public/private rsa key pair
ssh-keygen -t rsa

>> Enter file in which to save the key: /home/user/.ssh/id_rsa

Log sample:

Created directory '/home/a/.ssh'.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/a/.ssh/id_rsa.
Your public key has been saved in /home/a/.ssh/id_rsa.pub.
The key fingerprint is:
3e:4f:05:79:3a:9f:96:7c:3b:ad:e9:58:37:bc:37:e4

Summary

• id_rsa == private key
• id_rsa.pub == public key

Convert OpenSSH key to Windows Putty

See: http://meinit.nl/using-your-openssh-private-key-in-putty

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.

SSH client

Linux

Standard login

# syntax
ssh user@server -p portNumber

# example
ssh root@daxiongmao.eu -p 4422

Using RSA key

Key points:

• The key must belongs to the current user
• The key rights must be "500"

Then you can log-in using the following command:

ssh -i Guillaume_OpenSSH.private -p 2200 guillaume@dev.daxiongmao.eu

Where:

• -i myFile = the private key you have to use
• -p port = specific port number (if not default 22)

Windows

You have to use Putty to perform SSH login.

How to add a public / private key in Putty ?

1.Create profile

2. Auto-login

3. Attach private key

4. Save profile

References

Source:

• Public / private key theory: http://en.wikipedia.org/wiki/Public_Key_Cryptography
• http://www.howtoforge.com/ssh_key_based_logins_putty

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

How to transform a SSH key into Windows Putty: http://meinit.nl/using-your-openssh-private-key-in-putty