This post will guide you how to configure the /etc/ssh/sshd_config file in your Linux system. How do I configure you SSH client under CentOS/Ubuntu Linux. How to install and configure OpenSSH server in the Linux operating system.
What is OpenSSH?
OpenSSH is a suite of secure networking utilities based on the Secure Shell protocol, which provides a secure channel over an unsecured network in a client–server architecture.
OpenSSH Installation
You need to install the OpenSSH package using yum or apt-get command in the CentOS or Ubuntu Linux.
For CentOS Linux:
You need to use the following yum command to install the package of OpenSSH client and server in your current CentOS Linux. Type:
# yum install openssh-server openssh-clients -y
For Ubuntu/Debian Linux:
You need to use the apt-get command to install the openssh client and server packages, type:
# apt-get install openssh-server openssh-clients
OpenSSH Configuration File
The default configuration file of OpenSSH is located in /etc/ssh/ directory called sshd_config. And this file is the system-wide configuration file. And this file is used to set options to modify the operation of the ssh daemon.
You can add or change the sshd_config file via vim command:
Type:
#vim /etc/ssh/sshd_config
The possible keywords and their meanings are as followss:
Port 22
The option Port specifies on which port number ssh daemon listens for incoming connections. The default port is 22.
ListenAddress 192.168.1.1
The option ListenAddress specifies the IP address of the interface network on which the ssh daemon server socket is bind. The default is 0.0.0.0; to improve security you may specify only the required ones to limit possible addresses.
HostKey /etc/ssh/ssh_host_key
The option HostKey specifies the location containing the private host key.
ServerKeyBits 1024
The option ServerKeyBits specifies how many bits to use in the server key. These bits are used when the daemon starts to generate its RSA key.
LoginGraceTime 600
The option LoginGraceTime specifies how long in seconds after a connection request the server will wait before disconnecting if the user has not successfully logged in.
KeyRegenerationInterval 3600
The option KeyRegenerationInterval specifies how long in seconds the server should wait before automatically regenerated its key. This is a security feature to prevent decrypting captured sessions.
PermitRootLogin no
The option PermitRootLogin specifies whether root can log in using ssh. Never say yes to this option.
IgnoreRhosts yes
The option IgnoreRhosts specifies whether rhosts or shosts files should not be used in authentication. For security reasons it is recommended to no use rhosts or shosts files for authentication.
IgnoreUserKnownHosts yes
The option IgnoreUserKnownHosts specifies whether the ssh daemon should ignore the user’s $HOME/.ssh/known_hosts during RhostsRSAAuthentication.
StrictModes yes
The option StrictModes specifies whether ssh should check user’s permissions in their home directory and rhosts files before accepting login. This option must always be set to yes because sometimes users may accidentally leave their directory or files world-writable.
X11Forwarding no
The option X11Forwarding specifies whether X11 forwarding should be enabled or not on this server. Since we setup a server without GUI installed on it, we can safely turn this option off.
PrintMotd yes
The option PrintMotd specifies whether the ssh daemon should print the contents of the /etc/motd file when a user logs in interactively. The /etc/motd file is also known as the message of the day.
SyslogFacility AUTH
The option SyslogFacility specifies the facility code used when logging messages from sshd. The facility specifies the subsystem that produced the message–in our case, AUTH.
LogLevel INFO
The option LogLevel specifies the level that is used when logging messages from sshd. INFO is a good choice. See the man page for sshd for more information on other possibilities.
RhostsAuthentication no
The option RhostsAuthentication specifies whether sshd can try to use rhosts based authentication. Because rhosts authentication is insecure you shouldn’t use this option.
RhostsRSAAuthentication no
The option RhostsRSAAuthentication specifies whether to try rhosts authentication in concert with RSA host authentication.
RSAAuthentication yes
The option RSAAuthentication specifies whether to try RSA authentication. This option must be set to yes for better security in your sessions. RSA use public and private key pairs created with the ssh-keygen1utility for authentication purposes.
PasswordAuthentication yes
The option PasswordAuthentication specifies whether we should use password-based authentication. For strong security, this option must always be set to yes.
PermitEmptyPasswords no
The option PermitEmptyPasswords specifies whether the server allows logging in to accounts with a null password. If you intend to use the scp utility to make automatic backups over the network, you must set this option to yes.
AllowUsers admin
The option AllowUsers specifies and controls which users can access ssh services. Multiple users can be specified, separated by spaces.
OpenSSH Client Side Configuration
There is another OpenSSH configuration called ~/.ssh/config, and this file is a client-side configuration, and it is only work for the specific login user. This file is not created by default, so you need to create it by yourself.
For example, if you want to override the default system openssh configuration (/etc/ssh/sshd_config) for a user, you just need to create one config file under ~/.ssh/ directory, and then added new config line into it.
Type:
# touch ~/.ssh/config
Appending the following configuration option for a host named osetc1.com:
Host osetc1.com
HostName osetc1.com
User osetc
Port 4345
ServerAliveInterval 120