Red Hat Enterprise Linux 5 Administration Unleashed

CHAPTER 17Securing Remote Loginswith OpenSSHIN THIS CHAPTER. Allowing Connections. Configuring the Server. Connecting from the Client. Logging ConnectionsOpenSSH is the open source version of SSH, or Secure Shell.Connectivity tools such as Telnet and FTP are well-known,but they send data in plain text format, which can be interceptedby someone using another system on the samenetwork, including the Internet. On the other hand, all datatransferred using OpenSSH tools is encrypted, making itinherently more secure. The OpenSSH suite of tools includesssh for securely logging in to a remote system and executingremote commands, scp for encrypting files while transferringthem to a remote system, and sftp for secure FTP transfers.OpenSSH uses a server-client relationship. The systembeing connected to is referred to as the server. The systemrequesting the connection is referred to as the client. Asystem can be both an SSH server and a client.OpenSSH also has the added benefits of X11 forwarding andport forwarding. X11 forwarding, if enabled on both theserver and client, allows users to display a graphical applicationfrom the system they are logged in to on the systemthey are logged in from. Port forwarding allows a connectionrequest to be sent to one server but be forwarded toanother server that actually accepts the request.This chapter discusses how to use OpenSSH, both from theserver-side and the client-side.Allowing ConnectionsBy default, the OpenSSH server listens for requests on port22 and port 6010 for X11 forwarding.If custom IPTables rules are being used, refer to Chapter 24,“Configuring a Firewall,” for details on how to allow theseports.