Explore information related to troubleshooting

Troubleshoot server down issues - How to do it

This article covers how to troubleshoot server down issues. Basically, network issues might cause the Datacenter to go down. This can lead us into some unlucky instances. Troubleshooting server down issues is never an easy task. Whether you have a small home network, or a super connection of thousands of computers, there are meticulous steps you need to take to get your server back up running.


Steps to take when troubleshooting server down issues:

1. ANALYZE YOUR NETWORK INFRASTRUCTURE

You will have a better chance at troubleshooting network problems if you first figure out where everything is connected in the infrastructure.

2. STUDY YOUR NETWORK

If you don't have an infrastructure design to go by, you will have to learn your network’s layout when analyzing your connectivity. Several tools can help you to map out the entire network infrastructure. Tools such as IPCONFIG can aid in finding the problem.

3. CONNECTION IS DOWN

From the network troubleshooting application, find out from the OSI model if all the seven layers are working correctly. Usually, if the first layer doesn’t work the whole connection will be down. Check whether the network cable is plugged in.

4. NO IP ADDRESS

Your server could be down just because of unknown IP address settings. Anon IP address such as 0.0.0.0 or an automatic one that starts with 169.254 will typically result in server down problems. You will need to obtain a valid IP address before you can get your server back up. 

5. NO DNS SERVERS

Without DNS servers configured on your network, all communication will only be possible through an IP address. A server down issue, in this case, might be a broken a line between the router and the internet. 

6. NO DEFAULT GATEWAY

Your servers could be down because there is no default gateway IP address. This breaks the communication between the subnet and the local area network. You will still be able to work as usual on your local servers. 

7. MISCONFIGURED IP SUBNET MASK

A misconfigured subnet mask IP can impede server communication. You can manually configure this IP subnet mask or work with the DHCP server to identify the source if there is a misconfiguration.

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PXE Boot or DHCP Failure on Guest - Fix it now

This article covers how to fix PXE Boot (or DHCP) Failure on Guest.

Nature of this error:

A guest virtual machine starts successfully, but is then either unable to acquire an IP address from DHCP or boot using the PXE protocol, or both. There are two common causes of this error: having a long forward delay time set for the bridge, and when the iptables package and kernel do not support checksum mangling rules.


Cause of PXE BOOT (OR DHCP) ON GUEST FAILED:

Long forward delay time on bridge.

This is the most common cause of this error. If the guest network interface is connecting to a bridge device that has STP (Spanning Tree Protocol) enabled, as well as a long forward delay set, the bridge will not forward network packets from the guest virtual machine onto the bridge until at least that number of forward delay seconds have elapsed since the guest connected to the bridge. This delay allows the bridge time to watch traffic from the interface and determine the MAC addresses behind it, and prevent forwarding loops in the network topology. If the forward delay is longer than the timeout of the guest's PXE or DHCP client, then the client's operation will fail, and the guest will either fail to boot (in the case of PXE) or fail to acquire an IP address (in the case of DHCP).


Fix to PXE BOOT (OR DHCP) ON GUEST FAILED:

If this is the case, change the forward delay on the bridge to 0, or disable STP on the bridge.

This solution applies only if the bridge is not used to connect multiple networks, but just to connect multiple endpoints to a single network (the most common use case for bridges used by libvirt).


If the guest has interfaces connecting to a libvirt-managed virtual network, edit the definition for the network, and restart it. 

For example, edit the default network with the following command:

# virsh net-edit default

Add the following attributes to the <bridge> element:

<name_of_bridge='virbr0' delay='0' stp='on'/>

XML


If this problem is still not resolved, the issue may be due to a conflict between firewalld and the default libvirt network.

To fix this, stop firewalld with the service firewalld stop command, then restart libvirt with the service libvirtd restart command.

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Troubleshoot DNS issues - Step by Step tips to resolve it

This article covers how to troubleshoot DNS issues. One of the handiest tools for troubleshooting DNS failures is the NSLOOKUP command, which you can access from a Windows Command Prompt window. Simply type NSLOOKUP followed by the name of the host for which you want to test the name resolution.

Basically, DNS errors are caused by problems on the user end, whether that's with a network or internet connection, misconfigured DNS settings, or an outdated browser. They can also be attributed to a temporary server outage that renders the DNS unavailable.


DNS: online name resolution:

The domain name system (DNS) is a directory service used for transforming alphanumeric domain names into numeric IP addresses. 

A decentralized process, name resolution generally takes place on DNS servers’ networks distributed throughout the world. 

Every internet address you enter into your web browser’s search bar is then forwarded by your router to a DNS server. This server then dissolves the domain name into a numeric sequence and returns a corresponding IP address. 

Should the DNS server fail to produce an answer, then it won’t be possible to access the desired website; the result is the error message ‘DNS server not responding’.


To  clear your DNS cache:

1. On your keyboard, press Win+X to open the WinX Menu.

2. Right-click Command Prompt and select Run as Administrator.

3. Run the following command: ipconfig /flushdns.


To Troubleshoot DNS issues:

The root of such irritating messages can often be traced back to the server outage. In such cases, the DNS server is temporarily unavailable. Most of the time, these problems can be corrected by changing browsers, switching a few of your firewall settings, or restarting your router.

1. In order to rule out that the connection problem isn’t being caused by your web browser, carry out a test by attempting to logon on to the desired web page with alternative applications.

2. In case you aren’t able to achieve your desired results simply by changing browsers, then the next step is to rule out Windows Firewall as the possible culprit.

3. Connection problems can often be solved by restarting the server. Most devices include a power button specifically for this purpose. Should this fail to yield any results, then it looks like a hard reboot may be in store; this is done simply by pulling out the power plug. 

4. If you have ruled out common causes of error such as the router software crashes or conflicts with Windows Firewall, then changing your DNS server could be the solution.



How to Check DNS server?

You can find out whether changing DNS server has solved the problem by carrying out a simple test. 

Enter the URL of a well-known site in your browser (e.g. www.google.com). 

If the site can be accessed it means the DNS server is functioning properly.

If the site can't be accessed, you can enter the following IP address into your browser: 172.217.16.195. 

This is one of Google’s IP addresses. If Google doesn't appear after entering the address, it probably means there's a general internet problem rather than a problem with the DNS server.

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MySQL crash – Causes Recovery and Prevention methods

This article covers methods to fix and prevent MySQL downtime. Basically, many issues such as resource limits, #database table errors, file system errors, and so on, can lead to a MySQL crash.


To fix MySQL errors in cPanel?

You can also restart the MySQL service from WHM.

1. Login to WHM.

2. Navigate to Restart Services.

3. Now you can restart the MySQL service by clicking the icon “SQL Server(MySQL)”.


To use mysqlcheck, follow these steps:

1. As the root user, type the following command: cd /var/lib/mysql.

2. Replace the database with the name of the database that you want to check.

3. Mysqlcheck checks the specified database and tables.


How do I know if MySQL is running?

We check the status with the service mysql status command. We use the mysqladmin tool to check if MySQL server is running. 

The -u option specifies the user which pings the server. 

The -p option is a password for the user.


What does repair table do MySQL?

REPAIR TABLE repairs a possibly corrupted table, for certain storage engines only. 

This statement requires SELECT and INSERT privileges for the table. Although normally you should never have to run REPAIR TABLE , if disaster strikes, this statement is very likely to get back all your data from a MyISAM table.


What causes MySQL #tables to crash?

The major cause of the database corruption is the lack of free disk space on the server.

Other possible causes of MySQL table crashes include operating system problems, power failures, hardware issues, unexpected MySQL server termination, data corruption due to external programs, and so on.


To access your #MySQL database in Linux, please follow these steps:

1. Log into your Linux web server via Secure Shell.

2. Open the MySQL client program on the server in the /usr/bin directory.

3. Type in the following syntax to access your database: $ mysql -h {hostname} -u username -p {databasename} Password: {your password}

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No guest machines present libvirtd - Fix it now

This article covers how to troubleshoot and fix No guest machines present libvirtd for our customers. 

The virsh program is the main interface for managing virsh guest domains. The program can be used to create, pause, and shutdown domains. 

It can also be used to list current domains. Libvirt is a C toolkit to interact with the virtualization capabilities of recent versions of Linux (and other OSes).

The libvirt daemon is successfully started, but no guest virtual machines appear to be present.


There are various possible causes of this problem.

Performing these tests will help to determine the cause of this situation:

1. Verify KVM kernel modules

Verify that KVM kernel modules are inserted in the kernel:

$ lsmod | grep kvm

If you are using an AMD machine, verify the kvm_amd kernel modules are inserted in the kernel instead, using the similar command lsmod | grep kvm_amd in the root shell.

If the modules are not present, insert them using the modprobe <modulename> command.

Note: Although it is uncommon, KVM virtualization support may be compiled into the kernel. In this case, modules are not needed.

2. Verify virtualization extensions

Verify that virtualization extensions are supported and enabled on the host:

# egrep "(vmx|svm)" /proc/cpuinfo

Enable virtualization extensions in your hardware's firmware configuration within the BIOS setup.

3. Verify client URI configuration

Verify that the URI of the client is configured as desired:

# virsh uri


How to fix No guest machines present #libvirtd #error:

After performing these tests, use the following command to view a list of guest virtual machines:

# virsh list --all

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Troubleshoot KVM Virtualization Problem

This article covers how to troubleshoot KVM virtualization problem.


Log file locations and tools used to track down #KVM #problems are:
1. $HOME/.virtinst/virt-install.log – virt-install tool log file.
2. $HOME/.virt-manager/virt-manager.log – virt-manager tool log file.
3. /var/log/libvirt/qemu/ – Log files for each running virtual machine. If centos is virtual machine name, than log file is /var/log/libvirt/qemu/centos.log.

You can use the grep and other Linux tools to view this files:
# tail -f /var/log/libvirt/qemu/freebsd.log
# grep something $HOME/.virtinst/virt-install.log
$ sudo tail -f /var/log/libvirt/qemu/openbsd.log

Hyper-V backups can fail for any number of reasons, but there are some things to look for when backups don’t work the way that they are supposed to.
When backups fail, the first thing that you should do is to check the backup logs in an effort to learn more about the problem.
Specifically, you need to determine if the problem is confined to a particular host, a particular virtual machine, or perhaps related to the backup target itself.

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Troubleshoot Azure Cache for Redis timeouts

This article will guide you on how to troubleshoot Azure #Cache for Redis #timeouts. Azure Cache for Redis regularly updates its server software as part of the managed service functionality that it provides. 

Azure Cache for #Redis is a fully managed, in-memory cache that enables high-performance and scalable architectures. Use it to create cloud or hybrid deployments that handle millions of requests per second at sub-millisecond latency—all with the configuration, security, and availability benefits of a managed service.

This patching activity takes place largely behind the scene. During the failovers when Redis server nodes are being patched, Redis clients connected to these nodes may experience temporary timeouts as connections are switched between these nodes.


To help mitigate #Azure memory issues:

1. Upgrade the cache to a larger size so that you aren't running against memory limitations on the system.

2. Set expiration times on the keys so that older values are evicted proactively.

3. Monitor the used_memory_rss cache metric. When this value approaches the size of their cache, you're likely to start seeing performance issues. Distribute the data across multiple shards if you're using a premium cache, or upgrade to a larger cache size.


To fix #CPU bound on the server or on the client:

i. Check if you're getting bound by CPU on your client. High CPU could cause the request to not be processed within the synctimeout interval and cause a request to time out. 

ii. Moving to a larger client size or distributing the load can help to control this problem.

iii. Check if you're getting CPU bound on the server by monitoring the CPU cache performance metric. Requests coming in while Redis is CPU bound can cause those requests to time out. To address this condition, you can distribute the load across multiple shards in a premium cache, or upgrade to a larger size or pricing tier.

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Troubleshoot Redis latency Issues

This article will guide you on steps to troubleshoot Redis #latency problems.

#Amazon #ElastiCache allows you to seamlessly set up, run, and scale popular open-source compatible in-memory data stores in the cloud. 

Build data-intensive apps or boost the performance of your existing databases by retrieving data from high throughput and low latency in-memory data stores.

#Redis #performance monitoring metrics:

1. Used Memory

"memory_used" gives the total amount of memory in bytes that is used by the Redis server. If it exceeds physical memory, system will start swapping causing severe performance degradation.

2. Peak used memory

"memory_used_peak" metric calculates and displays the highest amount of memory in bytes consumed by the Redis server.

3. Used CPU system

The metric "cpu_used_sys" gets the total amount of system CPU consumed by the Redis server. High CPU usage is not bad as long as it doesn't exceed CPU limit.

4. Used CPU user

The metric "cpu_used_user" records the total amount of user CPU consumed by the Redis server.

5. Used CPU user children

The metric "cpu_used_user_children" records the total amount of user CPU consumed by other background processes.

6. Used CPU system children

Get the total amount of system #CPU consumed by background processes with the metric "cpu_used_sys_children".

7. Rejected connections

Use the metric "total_conn_rec" and get the total number of connections accepted by the Redis server.

8. Connected clients

Get the total number of slave connections made to the Redis server with the metric "conn_slaves".

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Are You Sure You Want to Do This Error in WordPress

This article will guide you on how to go about fixing "Are You Sure You Want to Do This" Error in WordPress. 

Critical errors damage your website's credibility, usability and accessibility. An important part of website optimization is therefore to detect critical errors and correct them as quickly as possible. The most common critical errors include server errors, broken pages, and broken images.

To fix WordPress #errors:

1. Check Your .htaccess File.

2. Switch To The Default Theme. If you are seeing the 500 Internal Server Error message, then you probably can't access the dashboard.

3. Increase #Memory Limit.

4. Deactivate All Your #WordPress #Plugins.

To get rid of warnings in WordPress:

i. Access your website by clicking the “public_html” folder in the directory.

ii. Select the wp-config.

iii. Click the “Edit” button on the new window.

iv. You may see “true” instead of false.

v. Click the “Save Changes” button in the top right.

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Azure Cache for Redis client-side issues

This article will guide you on troubleshoot Azure #Cache for Redis client for issues and fix common Redis #error.

Azure Cache for Redis gives you the ability to use a secure open source Redis cache. It is a dedicated offering managed by Microsoft, to build highly scalable and responsive applications by providing you super-fast access to your data.

All #Redis #data resides in the server's main memory, in contrast to databases such as PostgreSQL, Cassandra, MongoDB and others that store most data on disk or on SSDs.

The way to test for this in Redis is to simply query the key. If the key is empty, populate it. If it is a string use get (or exists).

To configure a storage account for your cache metrics:

1. In the Azure Cache for Redis page, under the Monitoring heading, select Diagnostics.

2. Select + Add diagnostic setting.

3. Name the settings.

4. Check Archive to a storage account.

5. Select Configure to choose the storage account in which to store the cache metrics.

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Troubleshooting DHCP client issues

This article will guide you on troubleshooting different DHCP #client issues. Whenever a DHCP server gets a request from a client it responds with a DHCP offer containing IP address being offered, #network mask offered, the amount of time that the client can use and keep it, the ip address of the DHCP server making this offer.

To #Troubleshoot DHCP:

1. Check for IP Address Conflicts. #DHCP clients connect to the network using a leased IP address.

2. Check Physical Connectivity.

3. Test connections with other Client using a Static #IP Address.

4. Confirm Switch #Port #Configuration.

5. Verify the source of Clients IP Address.

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Windows Live Mail Error 0x800ccc0e

This article will guide you on steps to fix #Outlook #Error #0x800ccc0e. Usually, this error occurs due to bad #email settings or because of minor problems with the email server or internet connection. Also, the customer may experience this error while receiving the mail on the email #client.

Try to run Windows Live Mail as Administrator in compatibility mode. Try to re-configure the Windows Live Mail account. Remove the existing WLM account and create a new one. Try reinstalling Windows Essentials 2012 on your Windows 10.

To get rid of Windows Live Mail error:

1. Open your Windows Live mail program.

2. Select the Home tab at the top right of your Windows Live Mail.

3. Click the Outbox to the left. Find any emails that are not sending out.

4. Once you send the email again, you can delete the email in the Outbox. 

The error will go away after the sent email is deleted.

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NFS Data ISO and Export Storage Domain to oVirt RHEV

This article will guide you on steps to add Export Storage #Domain & #NFS #Data to an oVirt/RHEV environment. NFS is the most common protocol for sharing #files between #Unix #systems over a #network. NFS servers export directories from their local hard disks to NFS clients, which mount them so that they can be accessed like any other #directory.

To modify etc #exports:

1. Open the /etc/exports file in a text editor on an NFS client that has root access to the storage system.

2. Make your changes.

3. Save the file.

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How to monitor Azure Cache for Redis

This article will guide you on how to monitor Azure Cache for Redis. Azure Cache for #Redis uses #Azure Monitor to provide several options for monitoring #cache instances. These include view metrics, pin #metrics charts to the Startboard, customize the date and time range of monitoring charts, and so on.

To Use Azure Redis Cache:

1. Log into Azure port, go to #Databases >> Redis Cache.

2. Create a news Redis Cache.

3. Get the Access Keys to connect with the newly created Redis Cache.

4. Install the #StackExchange. 

5. You can add Redis Cache connection string into the appsettings.

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WordPress White Screen of Death error Tips to resolve

This article is about will guide you on steps to resolve WordPress White Screen of Death which is very frustrating as there is no error message, and you are locked out of your WordPress website. 

This error can also happen due to a poorly coded theme or plugin installed on your site. Sometimes it can happen if there is an issue with your web hosting server. Since the white screen error can be caused by any number of things, it requires methodical #troubleshooting to fix.

To fix WordPress White Screen of Death:

1. Disable Your WordPress Plugins.

2. Switch to a Default WordPress Theme.

3. Clear Browser and #WordPress #Plugin Cache.

4. Switch on Debugging Mode.

5. Increase Your #Memory Limit.

6. Check File Permission Issues.

7. Check for Failed Auto-Update Issues.

8. Resolve #Syntax #Errors or Restore a #Backup.

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Redis Latency Monitoring How to enable it

This article will guide you on the aspects of Latency Monitoring and how it helps Redis which helps the user to check and troubleshoot possible latency problems.

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Setting up Zimbra Amavis Spam filtering

This article will guide you on the steps to configure whitelist and blacklist on Zimbra Amavis for Zimbra Amavis Spam filtering.

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Tips to fix unable to unmount delete VMFS Datastore the resource is in use

This guide will help to fix VMWare error "unable to unmount/delete vmfs datastore: the resource is in use" which occurs when trying to remove an empty VMFS Datastore in VMWare vSphere.

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