Rlogin allows an automated (password-free) form of login by means of a file called .rhosts on the server. You put a line in your .rhosts file saying something like jbloggs@pc1.example.com, and then when you make an Rlogin connection the client transmits the username of the user running the Rlogin client. The server checks the username and hostname against .rhosts, and if they match it does not ask for a password.
This only works because Unix systems contain a safeguard to stop a user from pretending to be another user in an Rlogin connection. Rlogin connections have to come from port numbers below 1024, and Unix systems prohibit this to unprivileged processes; so when the server sees a connection from a low-numbered port, it assumes the client end of the connection is held by a privileged (and therefore trusted) process, so it believes the claim of who the user is.
Windows does not have this restriction: any user can initiate an outgoing connection from a low-numbered port. Hence, the Rlogin .rhosts mechanism is completely useless for securely distinguishing several different users on a Windows machine. If you have a .rhosts entry pointing at a Windows PC, you should assume that anyone using that PC can spoof your username in an Rlogin connection and access your account on the server.
The ‘Local username’ control allows you to specify what user name PuTTY should claim you have, in case it doesn't match your Windows user name (or in case you didn't bother to set up a Windows user name).
4.18 The SSH panel
The SSH panel allows you to configure options that only apply to SSH sessions.
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4.18.1 Executing a specific command on the server
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4.18.2 ‘Don't start a shell or command at all’
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4.18.3 ‘Enable compression’
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4.18.4 ‘Preferred SSH protocol version’
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4.18.5 Sharing an SSH connection between PuTTY tools
4.18.1 Executing a specific command on the server
In SSH, you don't have to run a general shell session on the server. Instead, you can choose to run a single specific command (such as a mail user agent, for example). If you want to do this, enter the command in the ‘Remote command’ box.
Note that most servers will close the session after executing the command.
4.18.2 ‘Don't start a shell or command at all’
If you tick this box, PuTTY will not attempt to run a shell or command after connecting to the remote server. You might want to use this option if you are only using the SSH connection for port forwarding, and your user account on the server does not have the ability to run a shell.
This feature is only available in SSH protocol version 2 (since the version 1 protocol assumes you will always want to run a shell).
This feature can also be enabled using the -N command-line option; see
section 3.8.3.13
.
If you use this feature in Plink, you will not be able to terminate the Plink process by any graceful means; the only way to kill it will be by pressing Control-C or sending a kill signal from another program.
4.18.3 ‘Enable compression’
This enables data compression in the SSH connection: data sent by the server is compressed before sending, and decompressed at the client end. Likewise, data sent by PuTTY to the server is compressed first and the server decompresses it at the other end. This can help make the most of a low-bandwidth connection.
4.18.4 ‘Preferred SSH protocol version’
This allows you to select whether you would prefer to use SSH protocol version 1 or version 2, and whether to permit falling back to the other version.