Increasing the security of the ssh service

Nowadays, many bots or hackers look for port 22 on servers and try to log in. Usually, the login attempt is made as the standard linuxe root user. In this short article, I will describe how to create a user that will be able to log in as root and change the default ssh port 22 to 2222. Let’s go:

This way we created the user ‘soban’ and assigned it the default shell ‘/bin/bash’.

We still need to set a password for the user ‘soban’:

In the next step, let’s add it to ‘/etc/sudoers’ so that it can become root. Keep in mind that once the user can get root, he will be able to do anything on the machine!

Please add this entry below:

How can we test whether the user has the ability to log in as root? Nothing easier, first we’ll switch to the user we just created:

To list the possible sudo commands, just type the command:

Finally, to confirm whether it is possible to log in as root, you should issue the command:

Now that we have a root user ready, let’s try disabling ssh logon directly and change the default port. To do this, go to the default configuration of the ssh service, which is located in ‘/etc/ssh/sshd_config’:

We are looking for a line containing ‘Port’ – it can be hashed, so it should be unhashed and ‘PermitRootLogin’. Then set them as below:

In this way, we changed the default port 22 to 2222 and disallowed the possibility of logging in directly to the root user. However, the ssh service still needs to be reloaded, in debian or kali linux we do it like this:

In this way, we have managed to create a user who can safely log into the ssh service and become root. In addition, after changing the port, we will not go out on port 22 scans, which by default is set and scanned by a potential burglar. Installing the fail2ban service is also a very good improvement in security.

Securely Copy Files (scp) tool to copying files by ssh

A very good tool for securely copying files via the ssh protocol between machines is scp. It allows you to transfer files to the target machine as well as download from a given source. The tool is usually built into the system so it works on many distributions. Below I will present how you can send and download files. For correct file transfer, running ssh service is required, because it is the basis of scp operation. Of course, when using the tool, you can specify the port as the parameter, provided that it has been changed. The standard port used by the ssh daemon is 22. 

Let’s start by creating an example file that we will transfer: 

in the next step, let’s move on to uploading the file. In my case, the port from ssh has been changed to 2222:

The first time you connect, you will be asked for a fingerprint. 
As you can see, the file has been sent correctly. 

Instead of the sign at the end of ‘~‘ we can specify where the target file should be placed (/tmp/example-path): 

There are many combinations, you can send, for example, all files containing the ending (*.tar.gz) to the user’s home directory, which is just symbolized by ‘~‘: 

An interesting parameter is the ‘-r‘ in scp where we can transfer entire folders, example using copying a folder from local machine to remote machine: 

OK, after the file has been successfully sent to the target machine, let’s delete the local file we created above and try to download it back: 

Next, let’s move on to downloading the file from the remote server to the local machine: 

Above I gave an example of how to send an entire folder from a local machine to a remote machine. The other way around, of course, we can also do it. To download a remote folder to a local machine, use the ‘-r‘ parameter:

The scp utility has more parameters, you can get them by reading the man page: 

It is worth paying attention to the ‘-l‘ parameter where we can set the limit of transferred files. This is useful when transferring larger files so as not to overload your connection. 

If you are tired of constantly entering your password, I encourage you to read how you can connect to ssh without providing a password. Then copying files using scp will become more: generate ssh key pair in linux.

In my opinion, scp is good for transferring files quickly one time. However, as often you exchange files between machines a more convenient way is to use sshfs as described here: sshfs great tool to mount remote file system.

sshfs great tool to mount remote file system

SSH Filesystem (sshfs) is a very useful tool for remotely transferring files over the ssh protocol. An additional advantage of the whole is encryption. This is a convenient way to mount a remote folder to delete files. Below I will try to briefly introduce how to install sshfs and how to mount the folder remotely. Additionally, we will make an entry in /etc/fstab at the end, so that the resource itself is mounted after restarting the system. Let’s move on to installing the tool itself:

In this case, as you can see, the installation was done on kali linuxe, however the procedure is the same on debian.

Let’s move on to the file mounting itself, at this point I will point out that the default port is 22. In my case, however, the port has been changed to 2222. For services such as ssh, I try to change the default ports so as not to get caught by bots and not end up in the database such as shodan.io. The command itself in this case is very simple, but first we need to create a folder:

Let’s try to mount a remote folder:

During mounting, we will be asked if the fingerprint is correct. Then for the system password. The command itself can be disassembled into ‘soban‘ – this is the username. Then ‘soban.pl‘ is the domain name, you can also put the IP address here. The next ‘/home/soban‘ element is the folder that will be mounted. And after the space ‘/home/kali/myremotedir‘ we give the folder where the remote folder should be mounted. If everything went as planned, we can list ‘/home/kali/myremotedir‘ and it should list the contents of the remotely mounted folder ‘/home/soban‘. Let’s list the contents of the ‘/home/kali/myremotedir‘ folder:

Let’s create a remote file:

Now let’s unmount the remote folder and try listing it again:

As expected, the folder is empty and the file we created was created on a remotely mounted drive. After unmounting as you can see the file ‘/home/kali/myremotedir/example‘.

The next step is to create a private key to mount the folder without entering a password. It is very important not to send nikmou your private key. How we can generate and add a public key to a remote server can be read here: “Generate SSH key pair in Linux“.

Now we will try to add an entry to /etc/fstab which will allow automatic mounting on startup of the remote folder system.
To do this, edit the /etc/fstab entry and add this entry:

It is important that all data is correct, in order to verify the parameters, you can use the command for this ‘id‘:

Now we can move on to mounting the resource:

When mounting for the first time, we may be asked to accept and confirm that the fingerprint is correct. After verifying the correctness of mounting the remote resource, we can restart the system. One note here, the system may get up longer.

Generate SSH key pair in Linux

A very convenient way to log into remote systems via ssh is without the use of passwords. Here it is very important not to share your private key with anyone. Currently, when trying to connect, I am asked for the password to the server:

During the connection, we will be asked if the fingerprint is correct. Then enter the user password that is set on the remote server. During the ssh command ‘soban@soban.pl -p2222‘ I gave the username ‘soban‘ then the domain ‘soban.pl‘ and ‘-p2222‘ port ‘2222’. The default port after ssh is 22, but in this case I changed it so that it does not come out on scans – this increases security as often bots / hackers look for port 22, which is the default ssh port set.

Let’s move on to generating the key and copying it to the server:

This is how the key generation looks like, I hit enter for each question:

As a result, a private key was generated: (/home/kali/.ssh/id_rsa) and a public key (/home/kali/.ssh/id_rsa) that we will place on the remote server:

The last time we log in to the server by entering the password. When logging in, we will not be asked for a password now. This way we are able to add our public key (.ssh / authorized_keys) to the remote server.