What a Nagios host DOWN alert means and how to fix it

A Nagios host DOWN alert means that the monitoring system could not get a response from the host it checks. The server or website may still be working, so start by checking whether your site loads. This article explains what the alert means and the steps to take.

What the alert means

Nagios is third-party monitoring software. The person or team that runs the Nagios instance sets it up and receives its alerts. Hostking does not send these alerts.

Nagios marks a host as DOWN only after several checks fail. A single failed check is logged as a SOFT state. After the retry limit (called "max check attempts") is reached, the state becomes HARD and the alert is sent.

A host marked UNREACHABLE is different. Nagios cannot reach it because a device on the path, such as a router, is down. Check the host named in the alert first, then any device between Nagios and that host.

Common causes include:

• The site or server is really down or unreachable.
• Ping (ICMP) is blocked, so the host check fails even though services are running.
• A security tool is blocking the monitoring server's IP address.
• The check is misconfigured, for example an IPv4 and IPv6 mix-up.
• A short network problem happened when the check ran.

How to check and fix a host DOWN alert

1. Note the time of the alert and the host name or IP address shown in it.
2. Open your website in a browser on a different network, such as a mobile connection.
3. If the site loads, the server is probably working. Continue to the next steps to find out why Nagios reports it as down.
4. If the site does not load, check your client area at https://my.hostking.host for any service messages.
5. Confirm that the host name or IP address in the alert matches the address your site uses. Your welcome email or the client area shows the details for your hosting.
6. Ask the person who runs Nagios whether ping (ICMP) checks are used for this host.
7. If ping is used, ask them to test an HTTP or HTTPS check as well. A web check is more reliable when ping is blocked.
8. Check whether a security tool has blocked the monitoring server's IP address. Follow the steps for your control panel below.
9. Ask the Nagios administrator to run the check manually, using the same user account and arguments that Nagios uses.
10. If the problem continues, contact support using the steps in the Troubleshooting section.

cPanel

1. Log in to cPanel.
2. Find the security section and open the tool that manages blocked or allowed IP addresses.
3. Check whether the monitoring server's IP address is listed as blocked. If it is, remove it or add it to the allow list.
4. Check Imunify360 for blocked addresses. Imunify360 is the security suite on Hostking servers.

DirectAdmin

1. Log in to DirectAdmin.
2. Open the tool for managing blocked or allowed IP addresses.
3. Check whether the monitoring server's IP address is blocked. If it is, add it to the allow list.
4. Check Imunify360 for blocked addresses. Imunify360 is the security suite on Hostking servers.

Troubleshooting

The site loads but the alert continues. Ping may be blocked, or the check may be set up wrongly. Ask the Nagios administrator to test an HTTP check.

The site does not load. Check your client area for service messages. Note the time and any error text from your browser.

Only the monitoring server cannot connect. A security block may be the cause. Check the blocked IP list in your control panel and in Imunify360.

The alert appears once and then clears. It may have been a short network problem. Keep a note of the times it happens.

When you contact support, send the time of the alert and the exact alert text. Do not post your account details or passwords in public places.

If you are still stuck, contact support by live chat on the website or by opening a ticket from the client area at https://my.hostking.host.