Securing and Optimizing Linux: RedHat Edition -A Hands on Guide
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15.4. Configure the /etc/ssh/sshd_config file

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:

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.

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