SSH: Configure additional options described in documentation.

Install the SSH service if it is not already there:

# yum install -y openssh-server

Activate the SSH service at boot:

# chkconfig sshd on

Start the SSH service:

# service sshd start

Add a new rule to the firewall:

# iptables -I INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT

Save the firewall configuration:

# service iptables save

Let’s open the /etc/ssh/sshd_config file and discuss its content:

Port 22                                 # defines listening port for ssh
AddressFamily any                       # accepts IPv4 et IPv6 addresses
ListenAddress 0.0.0.0                   # allows ssh to listen on all network interfaces
ListenAddress ::                        # listens on IPv6 addresses too
Protocol 2                              # defines version of ssh (version 1 is not used anymore)
SyslogFacility AUTHPRIV                 # stores logging attempts in /var/log/secure (see rsyslog.conf file)
LoginGraceTime 2m                       # sets the time to connect
PermitRootLogin yes                     # allows direct login as root: outside lab, this option should be set to 'no'
StrictModes yes                         # allows connection only if the user's home directory is not world-writable
MaxAuthTries 6                          # defines the number of authentication attempts allowed
MaxSessions 10                          # defines the limit of simultaneous open connections
PubKeyAuthentication yes                # enables public key authentication
AuthorizedKeysFile .ssh/authorized_keys # defines the location of the authorized-keys file
HostbasedAuthentication no              # forbids the use of /etc/hosts.equiv
IgnoreUserKnownHosts no                 # reads the .ssh/known_hosts at each connection
IgnoreRhosts yes                        # doesn't read user's ~/.rhosts file
PasswordAuthentication yes              # sets password-based authentication
PermitEmptyPasswords no                 # doesn't allow empty passwords (hopefully!)
ChallengeResponseAuthentication no      # forbids use of one-time passwords
UsePAM yes                              # enables the Pluggable Authentication Module interface
AllowAgentForwarding yes                # allows the ssh-agent to forward private keys
AllowTCPForwarding yes                  # allows TCP communications to be forwarded
GatewayPorts no                         # prevents remote hosts from connecting to ports forwarded for the client
X11Forwarding yes                       # enables X11 forwarding
X11DisplayOffset 10                     # limits the number of GUI display open at the same time
X11UseLocalhost yes                     # defines how the GUI display is bound to the SSH server
PrintMotd yes                           # displays the message of the day
PrintLastLog yes                        # displays the date of the last login
TCPKeepAlive yes                        # allows the system to send TCP keepalive messages
UseLogin no                             # specifies whether login is used for interactive login session
UsePrivilegeSeparation yes              # separates incoming network traffic processing from the rest
PermitUserEnvironment no                # doesn't deal with environment options
Compression delayed                     # specifies that compression is delayed until user authentication
ClientAliveInterval 0                   # doesn't send any message before client deconnection
ClientAliveCountMax 3                   # defines the number of messages before client deconnection
-                                       # if ClientAliveInterval is different from 0
UseDNS yes                              # checks remote hostnames against DNS
PidFile /var/run/sshd.pid               # defines the file where the SSH process ID is stored
MaxStartups 10                          # defines the number of terminals simultaneously allowed
PermitTunnel no                         # doesn't support device forwarding
ChrootDirectory none                    # disables the use of chroot
Subsystem sftp /usr/libexec/openssh/sftp-server # supports the use of SSH encryption for SFTP file transfers