Guides - Troubleshooting Web Servers, Databases, and Other Services
Linux virtual machines equipped with a tailored set of resources designed to run any cloud-based workload.
This guide presents troubleshooting strategies for when you can’t connect to your web server, database, or other services running on your Compute Instance. This guide assumes that you have access to SSH. If you can’t log in with SSH, review Troubleshooting SSH and then return to this guide.
This guide explains how to use different troubleshooting commands on your Compute Instance. These commands can produce diagnostic information and logs that may expose the root of your connection issues. For some specific examples of diagnostic information, this guide also explains the corresponding cause of the issue and presents solutions for it.
If the information and logs you gather do not match a solution outlined here, consider searching the Community Site for posts that match your system’s symptoms. Or, post a new question in the Community Site and include your commands’ output.
Linode is not responsible for the configuration or installation of software on your Compute Instance. Refer to Linode’s Scope of Support for a description of which issues Linode Support can help with.
General Troubleshooting Strategies
This section highlights troubleshooting strategies that apply to every service.
Check if the Service is Running
The service may not be running. Check the status of the service:
|systemd systems (Arch, Ubuntu 16.04+, Debian 8+, CentOS 7+, etc)|
|sysvinit systems (CentOS 6, Ubuntu 14.04, Debian 7, etc)|
Restart the Service
If the service isn’t running, try restarting it:
Enable the Service
If your system was recently rebooted, and the service didn’t start automatically at boot, then it may not be enabled. Enable the service to prevent this from happening in the future:
Check your Service’s Bound IP Address and Ports
Your service may be listening on an unexpected port, or it may not be bound to your public IP address (or whatever address is desirable). To view which address and ports a service is bound on, run the
ss command with these options:
sudo ss -atpu
Review the application’s documentation for help determining the address and port your service should bind to.
Analyze Service Logs
If your service doesn’t start normally, review your system logs for the service. Your system logs may be in the following locations:
|systemd systems||Run |
|Ubuntu 14.04, Debian 7|
Your service’s log location will vary by the application, but they are often stored in
less command is a useful tool for browsing through your logs.
Try pasting your log messages into a search engine or searching for your messages in the Community Site to see if anyone else has run into similar issues. If you don’t find any results, you can try asking about your issues in a new post on the Community Site. If it becomes difficult to find a solution, you may need to rebuild your Compute Instance.
Review Firewall Rules
If your service is running but your connections still fail, your firewall (which is likely implemented by the
iptables software) may be blocking the connections. To review your current firewall ruleset, run:
sudo iptables -L # displays IPv4 rules sudo ip6tables -L # displays IPv6 rules
Your deployment may be running FirewallD or UFW, which are frontends used to more easily manage your iptables rules. Run these commands to find out if you are running either package:
sudo ufw status sudo firewall-cmd --state
Review How to Configure a Firewall with UFW and Introduction to FirewallD on CentOS to learn how to manage and inspect your firewall rules with those packages.
Firewall rulesets can vary widely. Review the Control Network Traffic with iptables guide to analyze your rules and determine if they are blocking connections. For example, a rule which allows incoming HTTP traffic could look like this:
-A INPUT -p tcp -m tcp --dport 80 -m conntrack --ctstate NEW -j ACCEPT
Disable Firewall Rules
In addition to analyzing your firewall ruleset, you can also temporarily disable your firewall to test if it is interfering with your connections. Leaving your firewall disabled increases your security risk, so we recommend re-enabling it afterward with a modified ruleset that will accept your connections. Review Control Network Traffic with iptables for help with this subject.
Create a temporary backup of your current iptables:
sudo iptables-save > ~/iptables.txt
OUTPUTpacket policies as
sudo iptables -P INPUT ACCEPT sudo iptables -P FORWARD ACCEPT sudo iptables -P OUTPUT ACCEPT
nattable that is consulted when a packet that creates a new connection is encountered:
sudo iptables -t nat -F
mangletable that is used for specialized packet alteration:
sudo iptables -t mangle -F
Flush all the chains in the table:
sudo iptables -F
Delete every non-built-in chain in the table:
sudo iptables -X
Repeat these steps with the
ip6tablescommand to flush your IPv6 rules. Be sure to assign a different name to the IPv6 rules file (e.g.
Troubleshoot Web Servers
If your web server is not running or if connections are timing out, review the general troubleshooting strategies.
If your web server is responding with an error code, your troubleshooting will vary by what code is returned. For more detailed information about each request that’s failing, read your web server’s logs. Here are some commands that can help you find your web server’s logs:
grep ErrorLog -r /etc/apache2 # On Ubuntu, Debian grep ErrorLog -r /etc/httpd # On CentOS, Fedora, RHEL
grep error_log -r /etc/nginx
Frequent Error Codes
HTTP 401 Unauthorized, HTTP 403 Forbidden
The requesting user did not have sufficient permission or access to the requested URL. Review your web server authorization and access control configuration:
HTTP 404 Not Found
The URL that a user requested could not be found by the web server. Review your web server configuration and make sure your website files are stored in the right location on your filesystem:
HTTP 500, 502, 503, 504
The web server requested a resource from a process it depends on, but the process did not respond as expected. For example, if a database query needs to be performed for a web request, but the database isn’t running, then a 50X code will be returned. To troubleshoot these issues, investigate the service that the web server depends on.
Is your Disk Full?
One common reason that a database may not start is if your disk is full. To check how much disk space you are using, run:
df -hshows how full those disks are.
You have several options for resolving disk space issues:
Free up space on your disk by locating and removing files you don’t need, using a tool like ncdu.
If you have any unallocated space on your Compute Instance (storage that you pay for already but which isn’t assigned to your disk), resize your disk to take advantage of the space.
Upgrade your Compute Instance to a higher-tier resource plan and then resize your disk to use the newly available space. If your Compute Instance has a pending free upgrade for your storage space, you can choose to take this free upgrade to solve the issue.
Database Performance Troubleshooting
If your database is running but returning slowly, research how to optimize the database software for the resources your Compute Instance has. If you run MySQL or MariaDB, read How to Optimize MySQL Performance Using MySQLTuner.
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