SSH

What is ssh?
Ssh (Secure Shell) is a program to log into another computer over a network, to execute commands in a remote machine, and to move files from one machine to another. It provides strong authentication and secure communications over unsecured channels. It is intended as a replacement for rlogin, rsh, and rcp. Additionally, ssh provides secure X connections and secure forwarding of arbitrary TCP connections. Why should I use it?
The traditional BSD 'r' - commands (rsh, rlogin, rcp) are vulnerable to different kinds of attacks. Somebody who has root access to machines on the network, or physical access to the wire, can gain unauthorized access to systems in a variety of ways. It is also possible for such a person to log all the traffic to and from your system, including passwords (which ssh never sends in the clear).

The X Window System also has a number of severe vulnerabilities. With ssh, you can create secure remote X sessions which are transparent to the user. As a side effect, using remote X clients with ssh is more convenient for users. Users can continue to use old .rhosts and /etc/hosts.equiv files; changing over to ssh is mostly transparent for them. If a remote site does not support ssh, a fallback mechanism to rsh is included.

What kinds of attacks does ssh protect against?
Ssh protects against:

IP spoofing, where a remote host sends out packets which pretend to come from another, trusted host. Ssh even protects against a spoofer on the local network, who can pretend he is your router to the outside.

IP source routing, where a host can pretend that an IP packet comes from another, trusted host.

DNS spoofing, where an attacker forges name server records

Interception of clear text passwords and other data by intermediate hosts.

Manipulation of data by people in control of intermediate hosts

Attacks based on listening to X authentication data and spoofed connection to the X11 server. support for external authentication methods, including Kerberos, SecurID, Smart Card

In other words, ssh never trusts the net; somebody hostile who has taken over the network can only force ssh to disconnect, but cannot decrypted or play back the traffic, or hijack the connection. The above only holds if you actually use encryption. Ssh does have an option to use encryption of type "none" this is only for debugging purposes, and should not be used.

What kind of attacks does ssh not protect against?
Ssh will not help you with anything that compromises your host's security in some other way. Once an attacker has gained root access to a machine, he can then subvert ssh, too. If somebody malevolent has access to your home directory, then security is nonexistent. This is very much the case if your home directory is exported via NFS.

There are currently two versions of SSH available, SSH Secure Shell Version 1 and Secure Shell Version 2. SSH1 not as secure as SSH2 and is being slowly withdrawn from use.