SSH

Some SSH related frequently asked questions are documented down below. For more in-depth details, other options, please consult the man pages: man ssh and man ssh_config.

X11 forwarding

X11 forwarding should be used with caution due to security implications. Please note that if someone can read your X authorization database 1, that person would be able to access the local X11 display through the forwarded connection.

We suggest switching it on only when needed, with the use of options (-X or -Y) passed to the ssh command. Whenever possible, use -X option to mark remote X11 clients untrusted.

In some cases -X will fail to work and either the use of -Y option or setting ForwardX11Trusted in your ssh config file to "yes" is required. In this case remote X11 clients will have full access to the original X11 display.

Alternatively, if X11 forwarding is always needed, you can configure it on a per-host basis in your .ssh/config file.

.ssh/config example:

# global settings
ForwardX11        no               # disable X11 forwarding
ForwardX11Trusted no               # do not trust remote X11 clients

# per-host based settings
Host fram                            # alias, you may run "ssh fram" only
    HostName fram.sigma2.no          # actual hostname for Fram
    User my_username                 # replace with your username on Fram
    IdentityFile ~/.ssh/id_rsa_fram  # pointer to your private SSH key
    ForwardX11          yes          # enable X11 forwarding
    ForwardX11Trusted    no           # do not trust remote X11 clients

Poor connection

In case of poor connection to the server (likely from a very remote area), usually noticeable with X11 forwarding enabled, you may request data compression by using the -C option.

Please note that the compression uses the CPU to compress-decompress all data sent over ssh and will actually have negative impact, slow down things on a fast network.

1. By default your X authority database is stored in the ~/.Xauthority file. This file contains records with authorization information used in connecting to the X server.

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