|Securing and Optimizing Linux: RedHat Edition -A Hands on Guide|
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The /etc/ssh/sshd_config file is the system-wide configuration file for OpenSSH which allows you to set options that modify the operation of the daemon. This file contains keyword-value pairs, one per line, with keywords being case insensitive. Here are the most important keywords to configure your sshd for top security; a complete listing and/or special requirements are available in the man page for sshd(8).
Edit the sshd_config file, vi /etc/ssh/sshd_config and add/or change, if necessary, the following parameters:
# This is ssh server systemwide configuration file. Port 22 ListenAddress 192.168.1.1 HostKey /etc/ssh/ssh_host_key ServerKeyBits 1024 LoginGraceTime 600 KeyRegenerationInterval 3600 PermitRootLogin no IgnoreRhosts yes IgnoreUserKnownHosts yes StrictModes yes X11Forwarding no PrintMotd yes SyslogFacility AUTH LogLevel INFO RhostsAuthentication no RhostsRSAAuthentication no RSAAuthentication yes PasswordAuthentication yes PermitEmptyPasswords no AllowUsers admin
This tells sshd_config file to set itself up for this particular configuration setup with:
The option Port specifies on which port number ssh daemon listens for incoming connections. The default port is 22.
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.
The option HostKey specifies the location containing the private host key.
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.
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.
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.
The option PermitRootLogin specifies whether root can log in using ssh. Never say yes to this option.
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.
The option IgnoreUserKnownHosts specifies whether the ssh daemon should ignore the user's $HOME/.ssh/known_hosts during RhostsRSAAuthentication.
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.
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.
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.
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.
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.
The option RhostsAuthentication specifies whether sshd can try to use rhosts based authentication. Because rhosts authentication is insecure you shouldn't use this option.
The option RhostsRSAAuthentication specifies whether to try rhosts authentication in concert with RSA host authentication.
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.
The option PasswordAuthentication specifies whether we should use password-based authentication. For strong security, this option must always be set to yes.
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.
The option AllowUsers specifies and controls which users can access ssh services. Multiple users can be specified, separated by spaces.