Common Linux System Problems

You will inevitably encounter problems with your Linux system. You might misconfigure an application, have a hardware problem, or experience a system breach by crackers. Therefore, troubleshooting is an important part of system administration. Unfortunately, quickly identifying a problem and its solution requires years of experience. You can enhance your ability to troubleshoot, though, by generalizing the problem into a category. Once you know the category the problem falls into, you narrow the possible causes down to a manageable number.

In Red Hat Linux, security concerns can arise if files have incorrect or unusual permissions. Permissions are used to control who can access, modify, or execute files and directories. If the permissions are not set correctly, it can leave the system vulnerable to unauthorized access or modification. For example, if a sensitive file has permissions set to allow any user to read or write to it, it could be accessed or modified by unauthorized users. Similarly, if a critical system file has permissions that allow users to execute it without proper authentication, it could be used to compromise the system. To mitigate these security concerns, it is important to regularly audit file and directory permissions and ensure they are set appropriately. This includes setting permissions to restrict access to sensitive files, limiting executable permissions to only authorized users, and ensuring that system files are not writable by unprivileged users. Additionally, using tools such as SELinux can provide an additional layer of security by enforcing mandatory access controls.

The table below lists common problems and their symptoms.

Problems	Symptoms
Boot/shut-down problems	System daemons that do not start or dump core[1] during start-up or shut-down
Login problems	Users can't log in remotely, can't log in at the console, or can't su to another user
File- and directory-access problems	A daemon won't start because it can't read its configuration file Users can't log in because their home directory is unreadable Mail cannot be sent because the mail spool is not writeable
Network problems	Daemons are slow to start, possibly indicating a DNS problem X Windows won't start because it requires networking built into the kernel Users are unable to send or receive mail Users complain that it takes too long to establish or work with incoming connections.
Performance problems	Commands take too long to complete, filesystems become too full, or system memory becomes scarce
Security concerns	Files have incorrect or unusual permissions Logs fill up more frequently than usual Users display unusual login times (e.g., people who normally log in between 8 a.m. and 5 p.m. log in at 2 a.m.) Random program crashes (due perhaps to newly installed libraries)

If you can classify problems into one of these categories, you can solve the problem by applying common solutions appropriate to that category.
The next lesson introduces you to some general troubleshooting strategies.

Before moving on to the next lesson, click the Exercise link below to test your understanding of common Linux system problems and their symptoms.
Common Linux System Problems - Exercise

[1] Dump core: The action a process takes when it encounters a fatal error. A core file (also known as a core dump) is usually created to aid in debugging.