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Access Ubuntu Server Remote

Server / May 7, 2018

First, make a backup of your sshd_config file by copying it to your home directory, or by making a read-only copy in /etc/ssh by doing:

sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.factory-defaults sudo chmod a-w /etc/ssh/sshd_config.factory-defaults

Creating a read-only backup in /etc/ssh means you'll always be able to find a known-good configuration when you need it.

Once you've backed up your sshd_config file, you can make changes with any text editor, for example;

sudo gedit /etc/ssh/sshd_config

runs the standard text editor in Ubuntu 12.04 or more recent. For older versions replace "sudo" with "gksudo". Once you've made your changes (see the suggestions in the rest of this page), you can apply them by saving the file then doing:

sudo restart ssh

If you get the error, "Unable to connect to Upstart", restart ssh with the following:

sudo systemctl restart ssh

Configuring OpenSSH means striking a balance between security and ease-of-use. Ubuntu's default configuration tries to be as secure as possible without making it impossible to use in common use cases. This page discusses some changes you can make, and how they affect the balance between security and ease-of-use. When reading each section, you should decide what balance is right for your specific situation.

Because a lot of people with SSH servers use weak passwords, many online attackers will look for an SSH server, then start guessing passwords at random. An attacker can try thousands of passwords in an hour, and guess even the strongest password given enough time. The recommended solution is to use SSH keys instead of passwords. To be as hard to guess as a normal SSH key, a password would have to contain 634 random letters and numbers. If you'll always be able to log in to your computer with an SSH key, you should disable password authentication altogether.

If you disable password authentication, it will only be possible to connect from computers you have specifically approved. This massively improves your security, but makes it impossible for you to connect to your own computer from a friend's PC without pre-approving the PC, or from your own laptop when you accidentally delete your key.

It's recommended to disable password authentication unless you have a specific reason not to.

To disable password authentication, look for the following line in your sshd_config file:

#PasswordAuthentication yes

replace it with a line that looks like this:

PasswordAuthentication no

Once you have saved the file and restarted your SSH server, you shouldn't even be asked for a password when you log in.

By default, you can tunnel network connections through an SSH session. For example, you could connect over the Internet to your PC, tunnel a remote desktop connection, and access your desktop. This is known as "port forwarding".

By default, you can also tunnel specific graphical applications through an SSH session. For example, you could connect over the Internet to your PC and run nautilus "file://$HOME" to see your PC's home folder. This is known as "X11 forwarding".

While both of these are very useful, they also give more options to an attacker who has already guessed your password. Disabling these options gives you a little security, but not as much as you'd think. With access to a normal shell, a resourceful attacker can replicate both of these techniques and a specially-modified SSH client.

If either of the above lines don't exist, just add the replacement to the bottom of the file. You can disable each of these independently if you prefer.

You can explicitly allow or deny access for certain users or groups. For example, if you have a family PC where most people have weak passwords, you might want to allow SSH access just for yourself.

Allowing or denying SSH access for specific users can significantly improve your security if users with poor security practices don't need SSH access.

It's recommended to specify which accounts can use SSH if only a few users want (not) to use SSH.

To allow only the users Fred and Wilma to connect to your computer, add the following line to the bottom of the sshd_config file:

To allow everyone except the users Dino and Pebbles to connect to your computer, add the following line to the bottom of the sshd_config file:

It's possible to create very complex rules about who can use SSH - you can allow or deny specific groups of users, or users whose names match a specific pattern, or who are logging in from a specific location. For more details about how to create complex rules, see the sshd_config man page

It's possible to limit the rate at which one IP address can establish new SSH connections by configuring the uncomplicated firewall (ufw). If an IP address is tries to connect more than 10 times in 30 seconds, all the following attempts will fail since the connections will be DROPped. The rule is added to the firewall by running a single command:

sudo ufw limit ssh

On a single-user or low-powered system, such as a laptop, the number of total simultaneous pending (not yet authorized) login connections to the system can also be limited. This example will allow two pending connections. Between the third and tenth connection the system will start randomly dropping connections from 30% up to 100% at the tenth simultaneous connection. This should be set in sshd_config.

In a multi-user or server environment, these numbers should be set significantly higher depending on resources and demand to alleviate denial-of-access attacks. Setting a lower the login grace time (time to keep pending connections alive while waiting for authorization) can be a good idea as it frees up pending connections quicker but at the expense of convenience.

By default, the OpenSSH server logs to the AUTH facility of syslog, at the INFO level. If you want to record more information - such as failed login attempts - you should increase the logging level to VERBOSE.

It's recommended to log more information if you're curious about malicious SSH traffic.

To increase the level, find the following line in your sshd_config:

and change it to this:

Now all the details of ssh login attempts will be saved in your /var/log/auth.log file.

If you have started using a different port, or if you think your server is well-enough hidden not to need much security, you should increase your logging level and examine your auth.log file every so often. If you find a significant number of spurious login attempts, then your computer is under attack and you need more security.

Whatever security precautions you've taken, you might want to set the logging level to VERBOSE for a week, and see how much spurious traffic you get. It can be a sobering experience to see just how much your computer gets attacked.

If you want to try to scare novice attackers, it can be funny to display a banner containing legalese. This doesn't add any security, because anyone that's managed to break in won't care about a "no trespassing" sign-but it might give a bad guy a chuckle.

To add a banner that will be displayed before authentication, find this line:

and replace it with:

Source: help.ubuntu.com