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Troubleshooting CentOS

By : Jonathan Hobson

4 (3)

Troubleshooting CentOS

4 (3)

By: Jonathan Hobson

Overview of this book

It is assumed that you will already have a server up and running, you have a good working knowledge of CentOS, and you are comfortable with the concept of working with those services used by your server.

Preface

Preface

What this book covers

What you need for this book

Who this book is for

Conventions

Reader feedback

Customer support

Free Chapter

1. Basics of Troubleshooting CentOS

1. Basics of Troubleshooting CentOS

Installing some basic tools

Gathering hardware information

Understanding dmesg

Understanding log files

Reading log files and affecting the output

Using tail to monitor log files

Using cat, less, and more

Using grep

Using diff

Using truncation

Summary

References

2. Troubleshooting Active Processes

2. Troubleshooting Active Processes

Tuning server performance with memory management and swap

Managing memory with vmstat

Checking the system load with the top command

Monitoring disk I/O with iotop

Checking processes with the ps command

Checking performance with iostat and lsof

Calculating the system load

Discovering process IDs with pgrep and systemctl

More about systemd

Issuing the kill signal

Dealing with an orphaned process

Summary

References

3. Troubleshooting the Network Environment

3. Troubleshooting the Network Environment

Using ping, dig, host, traceroute, and mtr

Monitoring network connections with the ss command

Packet analysis with tcpdump

Summary

References

4. Troubleshooting Package Management and System Upgrades

4. Troubleshooting Package Management and System Upgrades

Gathering software information

Using Yum plugins

Fixing Yum operations

Installing additional Yum repositories

Downloading an RPM package with Yum

Diagnosing a corrupt RPM database

Minor release upgrades

Summary

References

5. Troubleshooting Users, Directories, and Files

5. Troubleshooting Users, Directories, and Files

Users

Monitoring user activity with utmpdump

Resetting the root password and enhancing logging

Recovering lost or deleted files with Scalpel

Restoring file and directory permissions

Working with and extending the XFS filesystem

Running repairs on XFS

Investigating fragmentation on XFS

Auditing directories and files

Visualizing directories and files

Summary

References

6. Troubleshooting Shared Resources

6. Troubleshooting Shared Resources

Providing NFS shares on a CentOS 7 server

About NFS exports

Mounting NFS shares on a CentOS client

Mounting an external drive with CIFS

Using autofs to mount an external drive

Summary

References

7. Troubleshooting Security Issues

7. Troubleshooting Security Issues

Auditing SELinux with aureport and setroubleshoot

SSH banners

Tuning SSH

Intrusion detection with Tripwire

Firewalld – zone, service, and port management

Removing Firewalld and returning to iptables

Summary

References

8. Troubleshooting Database Services

8. Troubleshooting Database Services

Getting up-and-running with MariaDB

Resetting and recovering a root password with MariaDB

Tuning MariaDB and MySQL

Obtaining metrics from MariaDB and MySQL

Returning to MySQL

Installing and configuring PostgreSQL 9

Summary

References

9. Troubleshooting Web Services

9. Troubleshooting Web Services

Auditing the server with cURL

Debugging Akamai headers with cURL

Adding Varnish to Apache

Testing Varnish with cURL

Using cURL to access an FTP directory

Enabling mod_status in Apache

Summary

References

10. Troubleshooting DNS Services

10. Troubleshooting DNS Services

Changing the hostname and managing the FQDN

Performing system sanity checks with BIND

Monitoring bandwidth with iftop

Flushing the cache

Summary

References

Index

Index

Installing some basic tools

During the course of this book, it is assumed that you will already have access to the basic tools associated with troubleshooting your server. Some of the more obscure tools will be mentioned and instructions will be given; however, for those who may or may not have access to the basic toolbox, as the root user you may want to begin by running the following command:

# yum groupinstall "Base" "Development Libraries" "Development Tools"

This action, if and when confirmed, will begin to download and install the common development tools, libraries, and base components of a CentOS server system. It also contains the relevant utilities required by RPM, additional text editors, and packages required to compile custom packages.

Note

The practice of installing these packages at the outset is optional and all of these packages can be installed individually (as and when required). However, in an environment where disaster recovery planning has a vital role to play, it is worth considering the notion that a server has everything in place before any issues arise.

So, having prepared the system with the necessary tools and utilities, we shall begin in earnest by taking a closer look at the hardware. To do this, it is recommended that you continue with root access to the system in question.

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