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.

Login via ssh keys

To login to a server without typing in your password every time, you can configure ssh to use public key cryptography. In case you use a linux system start by generating a pair of keys and saving them in the folder .ssh:

ssh-keygen -t ed25519 -a 100 -f .ssh/id_sigma2

Make sure to enter a passphrase to encrypt the key.

To copy and install the public key to the server, for example saga, we use:

ssh-copy-id -i ~/.ssh/id_sigma2 username@saga.sigma2.no

Using ssh keys has the added benefit that you can avoid having to type your password every time. ssh-agent is program installed on virtually all linux versions to manage the keys so that you only have to unlock the key once. We can add the new key with:

ssh-add ~/.ssh/id_sigma2

We recommend you configure your ssh client by adding a section for each Sigma2 system you have access to by editing .ssh/config:

Host saga
Hostname saga.sigma2.no
User myusername
IdentityFile .ssh/id_sigma2

This will let you simply type ssh saga, rather than e.g. ssh myusername@saga.sigma2.no -i .ssh/id_sigma2

For more information see https://www.ssh.com/ssh/keygen/

Windows ssh client

In Windows 10 and newer you can now get a fully functional Linux terminal by installing WSL: https://docs.microsoft.com/en-us/windows/wsl/install-win10

This will not only give you a shell with the ssh client, but also let you install and use all of your favourite Linux software such as vim, emacs, nano, perl, python and so on.

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, example for Fram
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

SHA256 fingerprint

No matter how you login, you will need to confirm that the connection shall be trusted. The first time you log in to a machine via ssh, you will get a message like

The authenticity of host '<hostname>' can't be established.
ECDSA key fingerprint is <fingerprint>.
Are you sure you want to continue connecting (yes/no)?

If the <fingerprint> matches the fingerprint of the login machine you are logging in to (see below), you can confirm by typing yes and press Enter. (Note that the trailing "." is not part of the fingerprint.) If the fingerprint does not match, please contact support@metacenter.no immediately.

For the Fram login nodes, the ECDSA key fingerprint is

SHA256:4z8Jipr50TpYTXH/hpAGZVgMAt0zwT9+hz8L3LLrHF8

Some ssh clients will show it as

MD5:5b:af:a6:1d:94:1c:64:e1:11:54:0e:1f:7d:d2:cd:80

(possibly without the MD5: prefix).

Putty seems to at least sometimes display the ED25519 key instead

SHA256:PNlam6vviiHF0V7o4cGMmBfXAo5UEgfOtIkFQgXt9bM
MD5:55:ae:4a:0d:a6:05:76:bb:e7:59:1a:4a:b2:a4:87:2b

For the Saga login nodes, it is

SHA256:qirKlTjO9QSXuCAiuDQeDPqq+jorMFarCW+0qhpaAEA

sometimes shown as

MD5:13:4e:ae:66:89:0d:24:27:b8:15:87:24:31:ed:32:af

or for ED25519

SHA256:ryqAxpKDjNLLa5VeUPclQRaZBOIjd2HFgufUEnn4Jrw
MD5:55:52:eb:a5:c3:a9:18:be:02:15:ea:60:19:d7:5e:06

For the NIRD login nodes, it is

SHA256:ZkBvlcu4b5QMf1o9nKzoPHTmSTAzVhogZxKYvNw9N9I

sometimes shown as

MD5:02:02:cc:9d:c5:b7:43:42:5b:cd:d2:82:09:48:31:e9

or for ED25519

SHA256:sI/YOUiasD/yA/g8UMc2Isg4imXs7l8x/QQK01XfaOQ
MD5:bc:c9:a9:44:ca:b5:cb:53:56:68:02:d1:f1:6a:1a:78

To display all fingerprints for a certain server, you can use the following command on your local machine (Linux or Mac):

ssh-keygen -lf <(ssh-keyscan **login.nird.sigma2.no** 2>/dev/null)

SSHFS

fram.sigma2.no and login.fram.sigma2.no are round-robin DNS entries, every time you use this name the round-robin configuration will send you to one of the following two login nodes: login1.fram.sigma2.no and login2.fram.sigma2.no

When you use sshfs, to make sure your authentication is valid, you should always specify one of the real login nodes above. You should not use login.fram.sigma2.no or fram.sigma2.no in your sshfs command, otherwise you will risk to get your IP address blacklisted, since your session is authenticated against only one login node not both.

Similarly, saga.sigma2.no and login.saga.sigma2.no are round-robin DNS entries for login-1.saga.sigma2.no, login-2.saga.sigma2.no.

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. ↩