Red Hat Enterprise Linux 5 Administration Unleashed

356CHAPTER 17Securing Remote Logins with OpenSSHIf using a default security level in Red Hat Enterprise Linux, use the Security LevelConfiguration tool to allow SSH connections. Start it by selecting Administration,System Settings, Security Level from the System menu on the top panel of the desktopor by executing the system-config-securitylevel command. Enter the root passwordwhen prompted if running as a non-root user. SSH is allowed by default if the defaultsecurity level is enabled as shown in Figure 17.1. On the Firewall Options tab, make surethe SSH service in the Trusted services section has a check mark beside it. If not, click thecheck box beside SSH, and click OK to enable the changes.FIGURE 17.1Allowing SSH RequestsConfiguring the ServerThe openssh-server RPM package is required to configure a Red Hat Enterprise Linuxsystem as an OpenSSH server. If it is not already installed, install it with Red Hat Networkas described in Chapter 3, “Operating System Updates.”After it is installed, start the service as root with the command service sshd start. Thesystem is now an SSH server and can accept connections if the server allows connectionson port 22 as described in the “Allowing Connections” section of this chapter.To configure the server to automatically start the service at boot time, execute thecommand chkconfig sshd on as root. To stop the server, execute the command servicesshd stop. To verify that the server is running, use the command service sshd status.Retaining Keys After ReinstallingWhen the OpenSSH server package is installed, server authentication keys are generated.The keys are generated when the OpenSSH server package is installed and are unique tothe server. They are used to verify that the server being connected to is the intendedserver. The first time a client connects to an OpenSSH server, it must accept the public