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Linux Services Organization : Linux SSH Linux Server

OpenSSH is the open source version of SSH secure shell protocol. It provides secure (encrypted) communication between systems using a client/server architecture. It allows users to log into remote systems or execute remote commands in a safe way because of all data transferred between ssh client and server is encrypted. It also allows secure (encrypted) data transfers between systems using scp or sftp the encrypted secure FTP version.

By default SSH tools are installed on RHEL6 systems, the openssh* rpms are included on the default installation. The server daemon sshd listens on port 22 TCP/IP and the configuration files are located in the /etc/ssh directory.

Introduction to Encryption

Encryption with SSH requires a private key and a public key, generated by 'ssh-keygen' command. In order to establish an SSH encrypted communication between you and others the first step is send your public key to others keeping always your private key private. When others want to send data to you through SSH, their messages are encrypted with your public key that you have send previously. Your computer can decrypt the message with your private key. As can be seen the public and private keys are related with not easy to guess mathematical algorithms.

Private keys

Private key must be secure and used only by you to decrypt messages encrypted with you public key. Secure SSH encrypted communications are based on keeping the private key secure.

Public keys

Public key is publicly available. The recipient of your messages will encrypt the data with your public key that previously you have send. Only you using your private key will be able to decrypt that message.

SSH Tools

These are the most basic SSH tools than a Linux user must be aware.


The daemon service that implements the ssh server. By default it must be listening on port 22 TCP/IP.


The Secure Shell command ssh is a secure way to log and execute commands in to a remote machine using the private/public key encryption method replacing the insecure tools traditionally used for it: telnet, rlogin, rexec, rsh, etc.


The Secure Copy command is a secure way to transfer files between computers using the private/public key encryption method replacing the insecure tool traditionally used for it: ftp.


This utility is used to create the public/private keys with the command 'ssh-keygen -t keytype' where keytype can be DSA (Digital Secure Algorithm) or RSA1 (RSA Security). Later in this lesson will be shown how to use it.


This utility holds private keys used for RSA authentication. The idea is that the ssh-agent command is started in the beginning of an X session or a login session, and all other windows or programs are started as clients to the ssh-agent program. In this way all clients of the ssh-agent can remember through the use of environment variables the public/private keys used when ssh-agent was started, so the user will not be ask for this in all these client sessions.


Adds RSA identities to the authentication agent ssh-agent.

SSH Server

The SSH server configuration file is /etc/ssh/sshd_config. This file is well commented so just having a look on it one can understand the meaning of the main directives.

# cat /etc/ssh/sshd_config

# This directive configures SSH version 2, which is more secure that version 1
Protocol 2

# The following sends all logging attempts to the appropriate log file /var/log/secure
SyslogFacility AUTHPRIV

# This directive authorizes authentication based on local user passwords
PasswordAuthentication yes

# Set this to 'yes' to enable PAM authentication, account processing, and session processing
UsePAM yes

# The following directive allows to open remote GUI tools executed through SSH using the local X Server
X11Forwarding yes

# This directive supports the use of SSH encryption for secure FTP file transfers
Subsystem sftp /usr/libexec/openssh/sftp-server

Once the configuration file has been set lets start the ssh server and make sure it will start at system boot.

# /etc/init.d/sshd restart
# chkconfig sshd on

SSH client

The SSH client standard configuration file for all system is /etc/ssh/ssh_config. Each user can have custom SSH client configurations in their ~/.ssh/config files.

Some examples of SSH client tools can be :


Allows to login and execute shell commands on remote systems.

node01> ssh rhel6 -l john
It will login as john on rhel6 system.

node01> ssh rhel6 "ls -lrt /home/john".
It will execute the command 'ls -lrt /home/john' as user john on rhel6 system. The command output is displayed on node01 the SSH client from where are launched the connection


Used to transfer data between computer systems using SSH.

node01> scp /tmp/file.txt john@rhel6:/tmp/file.txt
This command will transfer file /tmp/file.txt from SSH client node01 to SSH server rhel6 on /tmp directory using 'john' account.

node01> scp -r john@rhel6:/tmp/dir /tmp/
This command will transfer from SSH client rhel6 the directory /tmp/dir to the SSH server node01 on /tmp dir using 'john' account. In this case node01 receives the data so node01 is the SSH server, sshd daemon must be running on node01.

SSH Security


As has been commented the sshd server listen on port 22 TCP/IP so this port must be open in order to allow ssh server service through a firewall.

-A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT

User and Host Based Security

Some additional directives can be added to /etc/sshd/sshd_config file in order to make the access to ssh server more restrictive.

# Do not allow empty passwords
PermitEmptyPasswords no

# The following directive will not allow to root user to log on the system using ssh. (Do not allow remote root logins)
PermitRootLogin no

# Limit the users allowed to access a system via SSH. In this case only users 'john' and 'charles' are allowed to login on the system using SSH
AllowUsers john charles

# Or even more restrictive, only allow login through SSH users 'john' and 'charles' from node.
AllowUsers john@ charles@

# In addition you can restrict the access to users. In this case all users less 'john' are allowed to connect to the SSH server.
DenyUsers john

SSH using only public/private keys

If the system where SSH server is running is directly connected to the Internet it will be a good idea to disable password authentication on the SSH server and allow only public/private keys authentication. This will reduce dramatically the chance that a cracker has login on your system because the probability that he has to guess the pair user/private_key is much lower that user/password pair. In order to accomplish this the following directives must be changed/added to /etc/ssh/sshd_config file.

# cat /etc/ssh/sshd_config
# Do not allow password authentication
PasswordAuthentication no

# Allow public/private key authentication
PubkeyAuthentication yes
AuthorizedKeysFile .ssh/authorized_keys


Next step is create the public/private key pair on the ssh client node01 from where are going to connect to the SSH server (rhel6).

node01> su - john
john-$>ssh-keygen -t dsa

(It will ask for a passphrase in order to protect your private key on the local node)

This command will create john private key on /home/john/.ssh/id_dsa (permissions 600) and john public key on /home/john/.ssh/ (permissions 644)

Change de .ssh directory permissions to 755.

john-$> chmod 755 .ssh

Copy the content of /home/john/.ssh/ (john public key) generated on node01 (the node from we want to login as john on SSH server) to /home/john/.ssh/authorized_keys on SSH server. If necessary create /home/john/.ssh directory with permission 755 on SSH server.

john-$> cat /home/john/.ssh/ --> >> SSH server(rhel6):/home/john/.ssh/authorized_keys

On SSH server (rhel6) change the permissions of /home/john/.ssh/authorized_keys to 644.

# chmod 644 /home/john/.ssh/authorized_keys

The final step is restart the ssh server and verify that you can connect from SSH client (node01) to SSH server (rhel6) only using public/private key and not using the user password. Have a look on Lab2.

# /etc/init.d/sshd reload

Note: In order to use the private key on SSH client to connect to SSH server the passphrase introduced when the private key has been created with 'ssh-keygen' is asked. If you have left this passphrase empty you will be able to login to SSH server directly without passphrase BUT using your public/private keys. We do not recommend to left this passphrase empty but in any case this method is more secure that using standard password because in this case the cracker must guess the public/private keys that normaly are random strings with at least 512K of size !!!

Using ssh-agent

When we are running a graphical environment on SSH client as gnome or kde we can use the ssh-add utility in order to do not have to enter the passphrase every time we try to connect to the SSH server.

john-$> exec /usr/bin/ssh-agent $SHELL
john-$> ssh-add

(--> Enter john passphrase)

The john passphrase now is stored in the environment variables for 'john' graphical session, so john must not be to retype his passphrase any time that try to login to the SSH server from this graphical environment on SSH client.

SSH Port Forwarding

SSH can secure insecure TCP/IP protocols via port forwarding, SSH server becomes an encrypted conduit to the SSH client. Port forwarding maps a local port on the SSH client to a remote port on the SSH server.

client> ssh -l john -L

Once the user john has been logged on through this ssh connection an SSH encrypted Tunnel has been established between port 25 TCP/IP on and port 2525 TCP/IP on In this way if you execute the command 'telnet localhost 2525' on you are making the telnet directly to port 25 TCP/IP on

client> telnet localhost 2525
Trying ::1...
Connected to localhost.
Escape character is '^]'.
220 ESMTP Sendmail 8.13.8/8.13.8; Fri, 25 Mar 2011 13:18:29 +0100

!!! IT IS MAGIC !!!

If you want forward a port from a machine that is not running an SSH server, but another machine on the same network is, SSH can still be used to secure a SSH tunnel.

client> ssh -l john -L

With this command you are making a ssh tunnel from (that is not running an SSH server) to your local machine connecting as user 'john' on that is in the same LAN as As POP service does not encrypt the data itself, with the SSH tunnel the data is encrypted by SSH, so you are making more secure the connection to your pop service.

Note: SSH Tunnels can be used to skip firewalls. Imagine that there is a firewall that blocks the connection between your local machine and your POP service on port 110 TCP/IP. If the firewall is not blocking access to the SSH server on (or a machine in the same LAN running SSH server) you can establish an SSH tunnel from and and skip the firewall. !!! In reality you can forward any port and skip the firewall if you can connect through ssh!!!

Maybe for security reasons you want to disable port forwarding through your SSH server. In this case the following directive must be configured on the SSH server configuration file /etc/ssh/sshd_config and then reload the SSH server service.

AllowTcpForwarding no


1.- By default SSH server runs on port 23 TCP/IP (true/false).

2.- OpenSSH can be used to encrypt remote X Windows applications (true/false).

3.- OpenSSH can be used to encrypt the traffic generated by any network application (true/false).

4.- Which command must be used in order to log as user 'kate' on node using ssh?.

5.- Which command must be used in order to copy the local file /root/ on node on /root as root using scp?.

6.- Which command must be used in order to generate user charles RSA public/private keys used by SSH?.

7.- Which configuration parameter must be configured on SSH Server configuration file in order to not allow root logins through ssh?.

8.- Which configuration parameter must be configured on SSH Server configuration file in order to allow only user 'kate' login through ssh?.

9.- Which of the following is the SSH server configuration file?.

A - /etc/ssh_config
B - /etc/ssh/ssh_config
C - /etc/sshd_config
D - /etc/ssh/sshd_config

10.- Which of the following commands will open an SSH Tunnel between and localhost:8080. (SSH Server is running on

A - ssh -l root -L
B - ssh -l root -L
C - Both of them
D - None of them


1.- Configure a SSH server on rhel6 ( Do not allow root and john users to login to it and allow the rest of users. Verify that SSH data transfers are encrypted.

2.- Re-configure SSH server on rhel6 ( to allow logins only using public/private keys. Generate the keys for user 'charles' on node01 ( with an empty passphrase and configure 'charles' account on rhel6 in order to allow 'charles' ssh login from node01.

3.- Using SSH Tunnel (Port forwarding) redirect rhel6 port 23 TCP/IP (telnet) to node01 port 2323 TCP/IP. Verify that you are able to loggin on rhel6 from node01 through telnet skipping the firewall on rhel6.

-- This page is part of Linux Server online tutorial --