ssh: improved structure and linking

wiki/linux/openssh.md

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+# OpenSSH
+
+[OpenSSH](https://www.openssh.com) is an implementation of the [SSH protocol](/wiki/ssh.md).
+
+## Usage
+
+This section addresses the usage of OpenSSH.
+
+### Generate New Keys
+
+To generate new ssh keys simply run `ssh-keygen -t ed25519` or
+`ssh-keygen -t rsa -b 4096`.
+
+The keys can then be added to the authentication agent by as described in the
+[corresponding article](#adding-keys-to-authentication-agent)
+
+### Adding Keys to Authentication Agent
+
+The following line will add a key to the authentication agent.
+
+```ssh
+ssh-add id_rsa
+```
+
+Depending on the name of the certificate inside the `~/.ssh` folder the name
+`id_rsa` can vary.
+For example `id_ed25519` is the name if the first generation option was chosen.
+
+### Enable Root Login via SSH
+
+Edit the `/etc/ssh/sshd_config` and change the line containing `PermitRootLogin`
+to `PermitRootLogin yes`.
+
+### Add Login via SSH Public Key
+
+To enable easy login without password you can add the contents of the file
+`~/.ssh/id_rsa.pub` from your local machine to the file `~/.ssh/authorized_keys`
+on the machine you want to log into.
+
+There are two options to add the public key to the remote machine.
+The first is a more manual approach.
+`<path-to-public-key>` is the path to the public key (for example `~/.ssh/id_rsa.pub`) and `<host>`
+is the (user and) server to add the key to (for example `user@192.168.178.16`).
+
+```sh
+cat <path-to-public-key> | ssh <host> 'cat >> ~/.ssh/authorized_keys'
+```
+
+This can also be more or less fully automated using the `-G` flag of SSH as described in 
+[a YouTube video by nixhero](https://www.youtube.com/watch?v=xCX14u9XzE8).
+
+The second option is a bit safer, due to using OpenSSHs tools, was described in a
+[StackOverflow comment by Boy](https://stackoverflow.com/questions/18690691/how-to-add-a-ssh-key-to-remote-server).
+It functions similar to the first and uses the following command.
+
+```sh 
+ssh-copy-id -f -i <path-to-public-key> <host>
+```
+
+The `-f` flag can alos be omittet to check if the key is already installed.
+For being very safe is important, a dry run can be performed using the `-n` flag.
+
+### Mount Directory With SSHFS
+
+This section is loosely based on
+[a GitHub comment by mlopezgva](https://github.com/VSCodium/vscodium/discussions/693).
+
+To mount a directory (in this case called `directory`) from a remote server
+(in this case called `server` also can be substituted by something like
+`user@server-ip`) on a local mount (in this case `mountpoint`) run:
+
+```sh
+sshfs server:/directory mountpoint
+```
+
+This can be very useful when trying to code on a remote machine but wanting to use the local
+instance of a text editor.
+
+Services that are hosted and use ports on a remote server can be tunneled as described in
+[the corresponding section](#port-tunneling) to setup complete remote development.
+
+### Shorten SSH Connection Commands
+
+By adding the following lines to the file `~/.ssh/config` the command of an SSH
+connection can be shortened from `ssh user@server.tld` to `ssh server`.
+
+```txt
+Host server
+	HostName server.tld
+	User user
+```
+
+### X-Forwarding
+
+The following sections address the usage of X-forwarding which makes it possible to use graphical
+programs remotely over SSH.
+
+#### Enabling X-Forwarding on a Server
+
+This section is based on a
+[blog by rc nectar](https://tutorials.rc.nectar.org.au/x11forwarding/02-enable-x11-on-virtual-machine).
+For the server setup of X-forwarding edit the file `/etc/ssh/sshd_config` and make sure
+X-forwarding is enabled by editing a line to look like the following.
+
+```txt
+X11Forwarding yes
+```
+
+Make sure to restart the SSH daemon afterwards by running the following command.
+
+```sh
+service sshd restart
+```
+
+Afterwards X-forwarding should be enabled on the server.
+For full functionality install the `x11-apps` package aswell.
+
+#### Enabling X-Forwarding on a Client
+
+To use X-forwarding when connecting use the `-C` flag as shown in the following example.
+
+```sh
+ssh -C <host>
+```
+
+The `-C` flag can also be noted in the config file so that it does not have to be specified each
+time.
+This can be achieved by editing the config file which is located at `~/.ssh/config` and include the
+following lines.
+
+```
+    ForwardX11 yes
+    ForwardX11Trusted yes
+```
+
+The functionality of X-forwarding can easily be tested by running a graphical program when remotely
+connected.
+An example for this could be the simple clock program `xclock`.
+
+### Port Tunneling
+
+This section is based on the
+[documentation of PostgreSQL](https://www.postgresql.org/docs/current/ssh-tunnels.html).
+
+A specific port can be tunneled from a remote host to a client via SSH.
+This could be especially useful when coding remotely and wanting to access a database like
+PostgreSQL or other services.
+For the forwarding of a port run the following command.
+`<local address>` (for example `localhost`) and  `<local port>` (for example `63333`) are the
+target (and most of the time local) address and port to tunnel the service to.
+`<address of server>` (for example `joe@foo.com` or an alias) specifies the address and username of
+the server that hosts the service and `<port of service>` (for example `5432`) describes the port
+it is running at.
+
+```sh
+ssh -f -N -L <local port>:<local address>:<port of service> <address of server>
+```
+
+After running the command the tunnel will stay established until the process is killed.
+This can be done by running a command identical or similar to the following which is used to find
+the process.
+
+```sh
+ps aux | grep "ssh -f -N -L <local port>"
+```
+
+Afterwards it can be killed by running the follwing command where `<process-id>` is the id of the
+process found with the previous command.
+
+```sh
+kill <process-id>
+```
+
+Files that are based on a remote server can be mounted as described in
+[the corresponding section](#mount-directory-with-sshfs) to setup complete remote development.