Client Features: ssh, scp, and sftp

OpenSSH clients have new features pertaining to keyboard-interactive authentication, connection sharing, known-hosts handling, port forwarding, and command-line editing and history.

KbdInteractiveDevices Keyword

The KbdInteractiveDevices keyword was undocumented in OpenSSH 3.9, but now it's officially supported. It determines the devices that the client will try for keyboard-interactive authentication.

    KbdInteractiveDevices = pam,skey,bsdauth

More Control for Connection Sharing

If you're using the connection-sharing feature of ssh, you can now control the master process of that connection with the -O option. To check whether you're using connection sharing, run the following:

    $ ssh -O check server.example.com

To request the master process to exit, run the following:

    $ ssh -O exit server.example.com

Hashing of Hostnames

In previous versions of OpenSSH, known_host files contain the hostnames and IP addresses of the computer's you've visited via SSH. If you'd like to keep this information more private, use the new HashKnownHosts configuration keyword in your client configuration file:

    HashKnownHosts yes

SSH clients will now hash the hostnames so they look like random strings—for example:

    |1|Un5Q61BdVPCq65Yj3ec/HH6r+zI=|2pPQE/qjP7rrPLblvS1epjYbUOs=

This feature is experimental at the moment, so use it at your own risk.

Port Forwarding

When you construct a port forwarding, you can now specify a bind address: the address on which the accepting side of the forwarding will listen. This is useful either for controlling whether a forwarding is available off-host (not listening on only on the loopback), or distinguishing among multiple addresses if the listening host is multi-homed. You give the bind address on the command line, preceding the usual -L or -R value. For example, to set up a local forwarding from local port 2001 to remote server port 143 (IMAP), listening on 192.168.100.66:

    $ ssh -L 192.168.100.66:2001:localhost:143 server.example.com

or for a remote forwarding:

    $ ssh -R 192.168.100.66:2001:localhost:143 server.example.com

You can also do this with the LocalForward and RemoteForward configuration keywords, prepending the bind address to the second argument:

    LocalForward 2001 192.168.100.66:localhost:143
    RemoteForward 2001 192.168.100.66:localhost:143

Note that this forwarding will not be listening on the loopback address. You need to connect to 192.168.100.66:2001, even on the server itself; trying to connect to localhost:2001 will result in "connection refused." Also note that the bind address refers to the client for local forwarding and to the server for remote forwarding.

For local forwarding, the default binding is determined by the GatewayPorts keyword. For remote forwarding, the server may choose to honor or ignore a client's binding request using a new GatewayPorts value, clientspecified:

    GatewayPorts clientspecified

This means the SSH client can select the binding address for the forwarding. This permits clients to bind addresses for remote forwardings

An empty binding address, or the special value *, indicates that the client or server should listen on all interfaces (including real ones and the loopback interface for localhost).

sftp Command-Line Features

The sftp client now supports command-line history and editing using Emacs-like keystrokes. You'll need the libedit library installed on your computer, available from http://sourceforge.net/projects/libedit. This feature is controlled at compile time with the flag --with-libedit.