You can run graphical applications that are installed on a remote machine on your local computer. X11Forwarding Yes must be set in the /etc/ssh/sshd_config file on the remote machine. Then, when you run ssh with the -X option, the DISPLAY variable is automatically set on the remote machine, and all X output is exported to the local machine over the SSH connection. At the same time, X applications started remotely cannot be intercepted by unauthorized individuals.

A quick test is to run a simple game from the remote machine, such as GNOME Mines:

> ssh tux@sun
Password:
Last login: Tue May 10 11:29:06 2022 from 192.168.163.13
Have a lot of fun...

tux@sun gnome-mines

The remote application should appear on your local machine just as though it were installed locally. (Note that network lag may affect performance.) Close the remote application in the usual way, such as clicking the close button. This closes only the application, and your SSH session remains open.

Important: X11 forwarding does not work on Wayland

X11 forwarding requires the X Windows System, which is the default on SLE, and not the Wayland display server protocol. The X Windows System has built-in networking, while Wayland does not.

Use the following command to learn if your system runs X or Wayland:

> echo $XDG_SESSION_TYPE
x11

If Wayland is in use, it looks like the following example:

> echo $XDG_SESSION_TYPE
wayland

By adding the -A option, the ssh-agent authentication mechanism is carried over to the next machine. This way, you can work from different machines without having to enter a password, but only if you have distributed your public key to the destination hosts and properly saved it there. (Refer to Section 23.7.2, “Copying an SSH key” to learn the safe way to copy your public keys to other hosts.)

If you want to copy several files from or to different locations, sftp is a convenient alternative to scp. It opens a shell with a set of commands similar to a regular FTP shell. Type help at the sftp-prompt to get a list of available commands. More details are available from the sftp man page.

> sftp sun
Enter passphrase for key '/home/tux/.ssh/id_rsa':
Connected to sun.
sftp> help
Available commands:
bye                                Quit sftp
cd path                            Change remote directory to 'path'
[...]

As with a regular FTP server, a user cannot only download, but also upload files to a remote machine running an SFTP server by using the put command. By default the files will be uploaded to the remote host with the same permissions as on the local host. There are two options to automatically alter these permissions:

It is recommended to back up the private and public keys stored in /etc/ssh/ in a secure, external location. In this way, key modifications can be detected or the old ones can be used again after having installed a new system.

Tip: Existing SSH host keys

If you install openSUSE Leap on a machine with existing Linux installations, the installation routine automatically imports the SSH host key with the most recent access time from an existing installation.

When establishing a secure connection with a remote host for the first time, the client stores all public host keys in ~/.ssh/known_hosts. This prevents any man-in-the-middle attacks—attempts by foreign SSH servers to use spoofed names and IP addresses. Such attacks are detected either by a host key that is not included in ~/.ssh/known_hosts, or by the server's inability to decrypt the session key in the absence of an appropriate private counterpart.

If the public keys of a host have changed (that needs to be verified before connecting to such a server), the offending keys can be removed with ssh-keygen -r HOSTNAME.

As of version 6.8, OpenSSH includes a protocol extension that supports host key rotation. Server admins must periodically retire old host keys and create new keys, for example if a key has been compromised, or it is time to upgrade to stronger keys. Before OpenSSH 6.8, if StrictHostKeyChecking was set to yes in ssh_config on client machines, clients would see a warning that the host key had changed, and were not allowed to connect. Then the users would have to manually delete the server's public key from their known_hosts file, reconnect, and manually accept the new key. Any automated SSH connections, such as scheduled backups, would fail.

The new host key rotation scheme provides a method to distribute new keys without service interruptions. When clients connect, the server sends them a list of new keys. Then the next time they log in they are asked if they wish to accept the changes. Give users a few days to connect and receive the new keys, and then remove the old keys. The known_hosts files on the clients are automatically updated, with new keys added and the old keys removed.

Setting up host key rotations requires creating new keys on the server, some changes to /etc/ssh/sshd_config on the server, and to /etc/ssh/ssh_config on the clients.

First create your new key or keys. The following example creates a new RSA key and a new Ed25519 key, with comments. The new keys must have unique names. A useful convention is to name them with the creation date. Remember, a host key must not have a passphrase:

# ssh-keygen -t rsa -f ssh_host_rsa_2022-01 -C "main server"
Generating public/private rsa key pair.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in ssh_host_rsa_2022-01
Your public key has been saved in ssh_host_rsa_2022-01.pub
The key fingerprint is:
SHA256:F1FIF2aqOz7D3mGdsjzHpH/kjUWZehBN3uG7FM4taAQ main server
The key's randomart image is:
+---[RSA 3072]----+
|         .Eo*.oo |
|          .B .o.o|
|          o . .++|
|         . o ooo=|
|        S . o +*.|
|         o o.oooo|
|       .o ++oo.= |
|       .+=o+o + .|
|       .oo++..   |
+----[SHA256]-----+

# ssh-keygen -t ed25519 -f ssh_host_ed25519_2022-01 -C "main server"
Generating public/private ed25519 key pair.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in ssh_host_ed25519_2022-01
Your public key has been saved in ssh_host_ed25519_2022-01.pub
The key fingerprint is:
SHA256:2p9K0giXv7WsRnLjwjs4hJ8EFcoX1FWR4nQz6fxnjxg root@server2
The key's randomart image is:
+--[ED25519 256]--+
|   .+o ...o+     |
| . .... o *      |
|  o..  o = o     |
|  ..   .. o      |
|   o. o S  .     |
|  . oo.*+   E o  |
|   + ++==..  = o |
|    = +oo= o. . .|
|     ..=+o=      |
+----[SHA256]-----+

Record the fingerprints, for users to verify the new keys.

Add the new key names to /etc/ssh/sshd_config, and uncomment any existing keys that are in use:

## Old keys
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_ecdsa_key

## New replacement keys
HostKey /etc/ssh/ssh_host_rsa_2022-01
HostKey /etc/ssh/ssh_host_ed25519_2022-01

Save your changes, then restart sshd:

# systemctl restart sshd.service

The /etc/ssh/ssh_config file on the clients must include the following settings:

UpdateHostKeys ask
StrictHostKeyChecking yes

Test connecting from a client by opening an SSH session to the server to receive the new keys list. Log out, then log back in. When you log back in you should see something like the following message:

The server has updated its host keys.
These changes were verified by the server's existing trusted key.
Deprecating obsolete hostkey: ED25519
SHA256:V28d3VpHgjsCoV04RBCZpLo5c0kEslCZDVdIUnCvqPI
Deprecating obsolete hostkey:
RSA SHA256:+NR4DVdbsUNsqJPIhISzx+eqD4x/awCCwijZ4a9eP8I
Accept updated hostkeys? (yes/no):

You may set UpdateHostKeys ask to UpdateHostKeys yes to apply the changes automatically, and avoid asking the user to approve the changes, though asking them to review the changes and compare fingerprints is safer.

For more information:

There are several key types to choose from: DSA, RSA, ECDSA, ECDSA-SK, Ed25519, and Ed25519-SK. DSA was deprecated several years, and disabled in OpenSSH 7.0 and should not be used. RSA is the most universal as it is older, and more widely used.

Ed25519 and ECDSA are stronger and faster. Ed25519 is considered to be the strongest. If you must support older clients that do not support Ed25519, create both Ed25519 and RSA host keys.

Note: Older clients may be unsafe

Some older SSH clients do not support ECDSA and ED25519. ECDSA and ED25519 were released with OpenSSH 6.5 in 2014. It is important to keep security services updated, and, if possible, to not allow unsafe old clients.

SSH keys serve two purposes: authenticating servers with host keys, and authenticating clients. Host keys are stored in /etc/ssh. Client keys are specific to users, and are stored in /home/user/.ssh.

Host keys must not have passphrases.

In most cases, private client keys should have strong passwords.

The following procedure shows how to create client keys.

Host keys are managed a little differently. A host key must not have a passphrase, and the key pairs are stored in /etc/ssh. OpenSSH automatically generates a set of host keys when it is installed, like the following example:

> ls -l /etc/ssh
total 608
-rw------- 1 root root 577834 2021-05-06 04:48 moduli
-rw-r--r-- 1 root root   2403 2021-05-06 04:48 ssh_config
-rw-r----- 1 root root   3420 2021-05-06 04:48 sshd_config
-rw------- 1 root root   1381 2022-02-10 06:55 ssh_host_dsa_key
-rw-r--r-- 1 root root    604 2022-02-10 06:55 ssh_host_dsa_key.pub
-rw------- 1 root root    505 2022-02-10 06:55 ssh_host_ecdsa_key
-rw-r--r-- 1 root root    176 2022-02-10 06:55 ssh_host_ecdsa_key.pub
-rw------- 1 root root    411 2022-02-10 06:55 ssh_host_ed25519_key
-rw-r--r-- 1 root root     96 2022-02-10 06:55 ssh_host_ed25519_key.pub
-rw------- 1 root root   2602 2022-02-10 06:55 ssh_host_rsa_key
-rw-r--r-- 1 root root    568 2022-02-10 06:55 ssh_host_rsa_key.pub

ssh-keygen has a special option, -A, for creating new host keys. This creates new keys for each of the key types for which host keys do not exist, with the default key file path, an empty passphrase, default bit size for the key type, and an empty comment. The following example creates a complete new set of keys by first deleting the existing keys, then creating a new set:

> sudo rm /etc/ssh/ssh_host*
> sudo ssh-keygen -A

You can replace selected key pairs by first deleting only the keys you want to replace, because ssh-keygen -A does not replace existing keys.

Important

ssh-keygen -A still creates DSA keys, even though they have been deprecated as unsafe for several years. In OpenSSH 7.0 they are still created, but disabled by not being listed in sshd_config. You may safely delete DSA keys.

When you want to rotate host keys, you must create the new keys individually, because they must exist at the same time as your old host keys. Your clients will authenticate with the old keys, and then receive the list of new keys. They need unique names, to not conflict with the old keys. The following example creates new RSA and Ed25519 host keys, labeled with the year and month they were created. Remember, the new host ukeys must not have passphrases:

> cd /etc/ssh
> sudo ssh-keygen -b 4096 -f "ssh_host_rsa_2022_02"
> sudo ssh-keygen -t ed25519 -f "ssh_host_ed25519_2022_02"

You may name your new keys whatever you want.

To copy a public SSH key to ~/.ssh/authorized_keys of a user on a remote machine, use the command ssh-copy-id. To copy your personal key stored under ~/.ssh/id_rsa.pub you may use the short form. To copy DSA keys or keys of other users, you need to specify the path:

> ~/.ssh/id_rsa.pub
ssh-copy-id -i tux@sun

> ~/.ssh/id_dsa.pub
ssh-copy-id -i ~/.ssh/id_dsa.pub  tux@sun

> ~notme/.ssh/id_rsa.pub
ssh-copy-id -i ~notme/.ssh/id_rsa.pub  tux@sun

To successfully copy the key, you need to enter the remote user's password. To remove an existing key, manually edit ~/.ssh/authorized_keys.

When doing lots of secure shell operations it is cumbersome to type the SSH passphrase for each such operation. Therefore, the SSH package provides another tool, ssh-agent, which retains the private keys for the duration of an X or terminal session. All other windows or programs are started as clients to the ssh-agent. By starting the agent, a set of environment variables is set, which will be used by ssh, scp, or sftp to locate the agent for automatic login. See the ssh-agent man page for details.

After the ssh-agent is started, you need to add your keys by using ssh-add. It will prompt for the passphrase. After the password has been provided once, you can use the secure shell commands within the running session without having to authenticate again.

> ssh-agent -s /bin/bash
eval $(ssh-agent)