• Diff for "SSH"

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ToDo: merge (and translate) this page and the french one (more complete)

Introduction

SSH stands for Secure Shell and is a protocol for secure remote login and other secure network services over an insecure network¹. See Wikipedia - Secure Shell for more general information and ssh, lsh-client or dropbear for the SSH software implementations out of which OpenSSH is the most popular and most widely used². SSH replaces the unencrypted telnet,rlogin and rsh and adds many features.

In this document we'll be using the OpenSSH command suite, it will also be assumed that the following two variables are defined:

remote_host=<the remote computer>
remote_user=<your user name on $remote_host>

So, if you want to use the recipes below, first set these variables to the remote computer name and the user name on that remote computer. Then cut and paste of the commands below should work. remote_host may also be an IP-address.

Installation

Installation of the client

Normally the client is installed by default. If not it suffices to run as root:

apt-get install openssh-client

Installation of the server

The server allows to connect remotely and gets installed by running as root:

apt-get install openssh-server

Configuration files

The main configuration files are in the directory /etc/ssh :

• ssh_config : client configuration file

• sshd_config : server configuration file

In addition this directory contains the private/public key pairs identifying your host :

• ssh_host_dsa_key
• ssh_host_dsa_key.pub
• ssh_host_rsa_key
• ssh_host_rsa_key.pub

Since OpenSSH 5.7³, a new private/public key pair is available:

• ssh_host_ecdsa_key
• ssh_host_ecdsa_key.pub

Since OpenSSH 6.5⁴, a new private/public key pair is available:

• ssh_host_ed25519_key
• ssh_host_ed25519_key.pub

Regenerating host keys

rm /etc/ssh/ssh_host_*
dpkg-reconfigure openssh-server

Remote login

With password

If you want to login to $remote_host as user $remote_user simply type

ssh $remote_user@$remote_host

and then type in your password.

If the usernames on the local and the remote computer are identical, you can drop the $remote_user@-part and simply write

ssh $remote_host

If this is the first time you login to the remote computer, ssh will ask you whether you are sure you want to connect to the remote computer. Answer 'yes' after you verified the remote computer's fingerprint, type in your password, and ssh will connect you to the remote host.

Using shared keys

One of the functions of ssh is using a pair of private/public keys to connect to a remote host. This method allows you to login to a remote host without typing your password every time. To do this you must generate a pair of private/public keys on your local machine and deposit the public key on the remote host.

To generate the key, use the program ssh-keygen as follows

ssh-keygen -t rsa

This program generates a pair of private/public keys in the directory ~/.ssh. The program first asks for the destination files for the keys, by default located in ~/.ssh. Afterwards a passphrase is requested.

Note: We recommend not to leave the passphrase empty. An attacker who gets hold of your private key can otherwise connect to the hosts where you deposited you public key since the passphrase is empty. Choose a long and complex passphrase.

Your private key is id_rsa (don't give it to someone else), your public key is id_rsa.pub.

You copy your public key to a remote host with the command ssh-copy-id

ssh-copy-id -i ~/.ssh/id_rsa.pub $remote_user@$remote_host

Now you can connect simply to the remote host and the passphase is asked for. Once done, you get connected to the remote host. In case of a new connection the passphrase does not get asked for again during your entire session.

Securing

By default a SSH server is relatively secure. With the help of some configuration options and external utilities it is possible to make it even harder for crackers.

Using the latest version of package openssh-server allows to protect against known security holes.

Configuration Options

One should edit the file /etc/ssh/sshd_config to change the parameters and then restart the ssh server with

invoke-rc.d ssh restart

• Deactivate using passwords for authentication (PasswordAuthentication no).

• Deactivate using the root account (PermitRootLogin no).

• Only allow login by certain users or groups (AllowUsers and AllowGroups)

The options AllowUsers and AllowGroups do not improve the security of a SSH server. But in certain cases their use allows to resist a brute force attack a little longer.

External Utilities

• fail2ban : allows to automatically blacklist IPs attempting to brute force a SSH server with the help of iptables.

• denyhosts : as fail2ban, denyhosts allows to block IP addresses trying to brute force a connection to ssh. But in contrast to fail2ban it does not use iptables, but the file /etc/hosts.deny.

Additional Functions

Additional Commands

scp

scp is a command line utilty allowing to transfer files between two machines.

• Sending a file:

scp $source_file $remote_user@$remote_host:$destination_file

• Copying a file to the local machine:

scp $remote_user@$remote_host:$source_file $destination_file

sftp

[empty for now]

text mode

[empty for now]

graphical mode

[empty for now]

clusterssh

[empty for now]

ssh-agent and ssh-add

ssh-agent is a useful utility to manage private keys and their passphrases. It should be invoked at the beginning of your session like so on a bourne shell:

eval `ssh-agent -s`

or on a C shell:

eval `ssh-agent -c`

When a private key is first needed, you are prompted for its passphrase and ssh-agent remembers the key. Whenever that private key is used later on, the passphrase doesn't get asked anymore.

ssh-add can be used to manage the remembered keys:

• Adding a key: ssh-add $private_key

• List the added keys: ssh-add -l

• Remove all keys from the knowledge of ssh-agent: ssh-add -D

keychain

Keychain, provided by the package keychain, is a shell script allowing to use the ssh agent in multiple sessions of the same computer. In effect after the first start ssh-agent creates a permanent socket allowing the communication with ssh. This socket is referenced only in the enviromment of the session in which the agent was started. Keychain allows to detect the agent and propagate the access to this agent to other sessions; this allows to use a single instace of ssh-agent per user on a machine.

ssh-askpass

ssh-askpass is an utility to simply the question for the password of a private key when using it. Several implementations exist:

• x11-ssh-askpass : version for X11

• kaskpass : integration of ssh-askpass into the KDE environment

• ssh-askpass-gnome : integration of ssh-askpass into the Gnome environment

libpam-usb

libpam-usb is an utility (only available up to Debian Jessie) allowing authentication with an USB stick. This package includes a useful utilty : pamusb-agent. This utility, once correctly configured, allows to load the SSH keys present on the USB stick once it is connected and to unload them when it is disconnected.

Remote commands

If you just want to run one command on the remote computer, you don't need to login. You can tell ssh to run the command without login, for instance,

ssh $remote_user@$remote_host 'ls *.txt'

lists all files with extension .txt on the remote computer. This works with single tick quotes '...' as shown here, with double tick quotes "...", and without quotes. There may be differences between these three cases, though, not yet documented here.

SSH into Debian from another OS

• PuTTY is a terminal emulator application which can act as a client for ssh. It's widely used by Windows users.

• Wikipedia has Comparison_of_SSH_clients

SSH and security

SSH Server

• Consider using fail2ban which is a log file monitor that automatically bans an ip address after a predefined number of failed login attempts. Guards against brute force attacks.

• Use SSH keys rather than password.

SSH Client

• http://lackof.org/taggart/hacking/ssh/ - Good practices for using ssh

GUI for keys management

Optionally, seahorse is a GNOME application which easily manage encryption keys and passwords through an intuitive Graphical User Interface (GUI). Seahorse is able to do various operations. Such as create SSH keys, configure them, and cache them.

Troubleshooting

OpenSSL version mismatch. Built against 1000105f, you have 10001060

If you get an error message like this when starting the ssh daemon, you need to run:

apt-get install openssh-server openssh-client

Also see the bug report.

Keep SSH connection alive

For security reason, by default a SSH connection is automatically closed after a set period of time. But in some cases you want to keep that connection open. Such as cloud storage over SSH connection.

For Debian 7.x server

Steps to keep SSH connection alive.

1. On the SSH server edit "/etc/ssh/sshd_config" file

2. Add the following at the bottom of that file

   # Keep client SSH connection alive by sending every 300 seconds a small keep-alive packet to the server in order to use ssh connection. 300 seconds equal 5 minutes.
   ClientAliveInterval 300
    
   # Disconnect client after 3333 "ClientAlive" requests. Format is (ClientAliveInterval x ClientAliveCountMax). In this example  (300 seconds x 3333) = ~999,900 seconds = ~16,665 minutes = ~277 hours = ~11 days.
   ClientAliveCountMax 3333

3. As Root user restart the SSH service

   service sshd restart

   Please note that on recent Debian systems (e.g. Wheezy 7 with current updates as of Nov. 2015), the above command no longer works and returns the error:

   sudo service sshd restart
   sshd: unrecognized service

   However, the following works:

   sudo service ssh restart
   [ ok ] Restarting OpenBSD Secure Shell server: sshd.