# The config file

```{admonition} Learning Objectives
- Write your first SSH config file!
- Learn how to avoid typing long SSH commands
- Learn how to automatically hop via an intermediate server
- Learn how to set and remember advanced settings
```

Now you can connect to a server but, if your username or the server name
is long, you will have to type all of this every time you want to connect. Luckily, we can
automate this using the SSH configuration file, `.ssh/config`, in your home
directory. Usually, this file doesn't exist but you can simply create an empty
file with that name.

The syntax of the configuration file is very simple. It's just the name of a
configuration option followed by it's value. For example, to send periodic
status updates which might help keep connections from disconnecting we can
simply write the following in the file:

```
# try to keep the connection alive, this avoids connection timeouts
ServerAliveInterval 60
```

But, more importantly, we can also define "hosts" to connect to and settings that
should only apply for these hosts. For example:

```
Host desy
    User YOUR_DESY_USERNAME
    Hostname bastion.desy.de
```

This now allows us to just execute `ssh desy` and the correct username and
full hostname are taken from the configuration file.

This will also work with `scp` so, now you can just use the shorter version of the previous exercise in *Basics*
to - let's say - copy a file from the login server

```bash
scp <hostname>:/etc/mtod bastion-message
```
where `<hostname>` is whatever name you gave to the server configuration in the config file.

In the case for DESY users, this now also allows us to automate the login to KEKCC via the gateway server

```
Host kekcc
    User YOUR_KEKCC_USERNAME
    Hostname login.cc.kek.jp
    Compression yes
    # Don't connect directly but rather via the gateway server
    ProxyJump sshcc1.kek.jp

Host sshcc1.kek.jp
    User YOUR_SSHLOGIN_USERNAME
```

The line containing `ProxyJump` tells ssh to not directly connect to the host
but first connect to the gateway host and then connect to it from there. We could make
this more complicated if needed by also adding a ProxyJump to the gateway server
configuration if we need to perform even more jumps.
You should now be able to
login to KEKCC by just typing `ssh kekcc` and also copy files directly with
`scp`. But you will have to enter your password two times: once, when
connecting to the gateway server and then, when connecting to the KEKCC machine.

```{note} In case of `ProxyJump` trouble
The `ProxyJump` directive was introduced in OpenSSH 7.3. If you get an
error message `Bad configuration option: proxyjump`, please check if
you can update your SSH client.

While we definitely recommend you to get an up-to-date system that can use
the newer version, a quick workaround is to replace the `ProxyJump` line
with the following (using `ProxyCommand`):

```
ProxyCommand ssh hostname -W %h:%p
```
Where `hostname` should be the server you jump through, so
`sshcc1.kek.jp` in this example.
```

````{admonition} Exercise
:class: tip
Add a working server configuration to your config file and verify that
you can log into it.
```{dropdown} Hint
You can take the above snippet to create the config file. Make sure to replace
your own usernames and servers.
```
````

## Debugging


When debugging, it can sometimes be helpful to disable the config file
to rule it out as a source of error. This can be done
by using the `-F` option to specify a blank config file:

```bash
ssh -F /dev/null <username>@<servername>
```

```{admonition} Key Points
:class: tip
- SSH configuration file is in `.ssh/config`
- The configuration file allows us to automate and configure ssh
- We can define short names to connect to servers
- We can automate jumping between hosts
- We can disable a config file for debugging
```