Linux Security

Overview

System security is a complicated topic that individuals study for many years. It would be impractical to impart even a fraction of that knowledge in a Wiki article. What this page will attempt to do is provide a primer in the most basic elements of Linux security and identify common pitfalls for beginners

Users

User accounts are used to log into the system and provide one of the basic building blocks for permissions. You could loosely categorize users into a few categories:

• Regular user accounts like the one created for you during install.
• Accounts used to run specific processes. These users are often named after the service they run. For example the dbus users is user to run the master dbus process.
• The root account.

The root account is an administrator or superuser account. This account to everything in the system and be used with extreme care. In most cases, it shouldn't be used at all. Instead use sudo.

sudo

The command sudo lets you run a command as the root user without actually switching to the root user. In many cases this is safer than using the root user directly as only a single command is being run as root. For example, your normal user account would not be able the file /etc/fstab because it is owned by root. However, you can edit it with sudo like this:

sudo nano /etc/fstab

When you run this command, you will be asked for a password, this will be the password of your normal user account.
For more information about editing configuration files owned as root see this article on configuration files.

sudo vs su

While sudo and su look similar and both involve root access they are very different. sudo runs a single command as another user and requests the password of your normal user account. su lets you *become* root and requests the password of the root user. In general, it is usually safer to use sudo than to use su.

Warning

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Never run a graphical program as root or with sudo, it should only be used with command line programs

Why am I Asked for a Password

Sometimes you will take an action in the terminal or through a GUI application and will get prompted for your password. This is because the action you are trying to take cannot be completed by you user and requires elevated rights. Whenever you get a password prompt like this it is important to pause and think if the action you are taking *should* be asking for elevated rights before entering your password.

Note

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Usually these password prompts will be looking for the password of your normal user account but occasionally they will need the password of the root account

Changing Passwords

To change the password of the user account you are logged in as you can use the command:

passwd

To change the password of a different user on the same system you can use sudo:

sudo passwd USERNAME

Groups

Users on a Linux system are commonly arranged in groups. A user group is a convenient way of assigning more users access to a common task like sound, media, printing and mounting of removable drives etc.

A list of the current groups can be seen on the system with the command:

getent group | awk -F : '{print $1}'

To see which groups a given user belongs to use the command

groups USERNAME

Primary Groups

A user can be a member of any number of groups but they have only one primary group. The primary group is the group used when files are created.

File Permissions

At the most basic level, files are designated as read, write or execute to the user(owner), the group and other. To understand how this works let's look at a real world example.

To get the permissions on the file we can use the command ls -l.

ls -l /etc/fstab
-rw-r--r-- 1 root root 539 Dec 26 23:07 /etc/fstab

That first group of letters and dashes indicate the permissions. It is 10 characters long and the dashes indicate a lack of permissions.

• The first character "-", represents the file type, "-" indicates that it is a normal files.
• The next three characters "rw-" indicate the permissions for the user or owner of the file. In this case reading and writing are allowed but not executing.
• The next three characters "rw-" indicate the permissions for members of the group who owns the file. In this case reading and writing are allowed but not executing.
• The next three characters "r--" indicate the permissions for other users. In this case reading is allowed but not writing or executing.

From more detailed information on how file permissions are broken down take a look at this Wikipedia article

Changing File Permissions

The command chmod can be used to change permissions on a file or directory. It is probably easier to demonstrate than explain.

Add read rights to the user(owner) of the file

chmod u+r filename

Remove execute rights to members of the group owner of filename

chmod g-x filename

Set the rights for the other group to read only

chmod o=r filename

Of course, in normal use you would combine everything like this:

chmod u+rw,g=r,o-rwx filename

This adds read and write to the owner, set the group as read only and remove read, write and execute from other users

The chmod command can do a lot more than that. For more information take a look at Wikipedia's chmod reference

Internet and Network Security

Firewalls

The Firewalls article has a full description of the Firewall solutions available on Manjaro.

Checksums

Checksums are used to validate processed files. By checking the file checksum, you can detect changes to the file, but the method is not perfect.

• The larger the file, the greater the probability of receiving the same checksum. This phenomenon is called collision. This is a disadvantage for checking whether a file is identical and it is also a security defect.
• Certain algorithms (MD5, SHA-0, SHA-1) are prone to collision, generally antiquated for security purposes, and thus not modernly recommended for use.
• Servers can make use of checksums instead of passwords. One advantage is this makes it possible to generate several passwords that will match the same checksum.
• Checksums can also be used to detect changes in files rather than for security. Sample programs for checking file integrity Tripwire, AIDE.

List of hash functions https://en.wikipedia.org/wiki/List_of_hash_functions#Unkeyed_cryptographic_hash_functions

GPG keys

• GPG keys can be used to encrypt messages in asynchronous encryption. In theory, a thief can steal the key message and still will not be able to decrypt the message. Therefore, this method is considered the most secure method.
• Keys are used to sign the files, but does not give it a high level of safety. Because you can copy, edit the key from file. But it is useful for checking from which the package repository comes from, if we use several repositories and if we are able to verify.

Understanding processes

• You can change the priorities of processes.
• You can display the process tree. pstree command
• You can check what file the process comes from. You can also check to which package a file belongs.
• You can detect "zombie" processes and delete them.
• You can limit the maximum number of processes. This is protection against fork bomb attack, but it does not guarantee system stability, if the user's process will be important for the stability of the system.
• "Sandbox" is a security mechanism for separating running programs, usually in an effort to mitigate system failures or software vulnerabilities from spreading. I heard about two. The first is to set up a separate account with restricted rights for programs. Second this is Firejail, but when I tested it it does not work efficiently. During the system start, all rules are loaded instead of just for only programs which will running. You can also use virtual machines, but this is not their main purpose.