How I Fixed Nginx 502 Bad Gateway Error

Server maintenance is no cakewalk. Besides worrying about keeping your site updated with the latest security patches, you also have to ensure your site never goes down because of server errors. 

The 502 bad gateway error is rather infamous for taking websites down for seemingly no reason. The good news is, finding the cause of the error isn’t difficult, but it requires a bit of trial and error. 

In this post, we explore the various causes of the “502 bad gateway” error and elaborate on the many ways of fixing the issue.

What is the 502 Bad Gateway Error in Nginx?

The 502 Bad Gateway Nginx error is an HTTP status code indicative of a situation where a server, acting as a proxy, fails to receive a response from the backend server. 

In Nginx, this error arises either when a connection cannot be established with the upstream server or when an invalid response is received from it.

This particular error is common when attempting to access a web app or site that first connects the user to a load balancer or a reverse proxy. Interestingly, there are several variations of this error that you might encounter, including:

• 502 Bad Gateway
• 502 Proxy Error
• 502 Service Temporarily Overloaded
• Error 502
• HTTP 502
• HTTP Error 502 – Bad Gateway

What Causes the Nginx 502 Bad Gateway Error?

There are many reasons why a Nginx server might not be able to receive a valid response from an upstream server and then throw this error. Some of the most common causes of the 502 Bad Gateway Error include:

Connectivity Issues

network connection problems between the backend and reverse proxy server can trigger a Nginx 502 error. 

Such issues can stem from misconfigured settings, hardware malfunctions, or network congestion. When a reverse proxy server tries to send a request to the backend but fails to connect to it, the users see this error. 

The cause of this problem lies in the reverse proxy server acting as an intermediary. Taking measures like scrutinizing firewall rules, troubleshooting network settings, and vigilant monitoring of network traffic can effectively address and rectify this matter.

Server Overload

When flooded with a ton of requests, backend servers may encounter overloading issues, leading to failure in responding within the designated timeout period. As a result, users might see the frustrating 502 error as the upstream server cannot meet their requests. 

Implementing effective server sizing, load balancing, resource allocation, and scaling strategies are crucial to preventing server overload.

Software Problems

Bugs and other software-based issues in the backend or reverse proxy servers have been known to cause 502 Bad Gateway errors.

When a bug or misconfiguration is present in either server’s software, it could lead to failure in handling requests or timely responses, thereby causing a 502 error to be returned to the client. 

A thorough examination of both the backend and reverse proxy server’s logs is one of the best ways to find a software error. If you see any warning or error messages, there’s a good chance a software issue is causing the 502 error.

Firewall Problems

A 502 Bad Gateway error in Nginx can also be attributed to firewall restrictions that hinder the connection of the backend and the reverse proxy machines. Such issues arise when firewalls are configured to restrict traffic to specific IP addresses or ports. 

As discussed earlier, the underlying reason is the reverse proxy server’s role as an intermediary. If the proxy server cannot connect the user to the backend, it also cannot respond to the user’s request. 

Resolving firewall issues causing the 502 error entails adjusting firewall rules, permitting an unhampered connection of the client and the backend via the proxy. 

Blocked Service Port in Firewall

Firewalls occasionally become obstacles to connectivity if not properly configured, leading to blocked legitimate requests or service failures. 

Consider a Linux server running the Plesk automation suite, where Nginx operates on port 80 and Apache on port 7080. By default, firewalls tend to block uncommon ports like 7080, causing Nginx to face difficulties in connecting to Apache, eventually resulting in a 502 Bad Gateway error.

So, during migrations, you’re likely to encounter this error when introducing new services in the backend (e.g., caching server, Ruby, etc.). You may even see the error after a server upgrade. The best way to detect issues stemming from blocked ports is by running the following command:

Suppose you detect any service running on non-standard ports. In that case, you must either adjust the service configuration to adopt a standard port or edit the firewall configuration to allow the non-standard port. This is likely to fix the bad gateway error.

PHP-FMP Takes Too Long To Respond

In Nginx, PHP-FPM can trigger a 502 Bad Gateway error when it encounters a critical error or doesn’t respond within the timeout period. The underlying causes for this error may include insufficient resources, misconfiguration, or bugs present in the PHP code.

PHP-FPM serves as a popular tool for running PHP applications within the Nginx framework. The Nginx server forwards the request to PHP-FPM, which then handles the PHP code and sends the result to Nginx. Finally, the server sends a response to the client who requested it. 

Addressing PHP-FPM-related issues requires appropriate adjustments to the PHP-FPM configuration. Adjusting the timeout settings or increasing the number of processes/threads can fix this issue. 

Additionally, scrutinizing the PHP code is essential to identify and resolve any bugs or performance-related concerns. By diligently monitoring the server logs and system resources, you can effectively identify potential patterns that indicate more extensive issues with PHP-FPM.

DNS Issues

The occurrence of a 502 Bad Gateway error in Nginx can be attributed to DNS-related complications, particularly when the DNS resolution for the backend server encounters a breakdown. 

This situation arises from a variety of factors, including DNS server failures, erroneous DNS configurations, and even DNS caching problems. When a client initiates a request to the reverse proxy server, the server’s primary task is to translate the site’s domain name into an IP address. 

So, when the DNS resolution fails, the proxy server is unable to send the request to the backend server, inevitably leading to the return of a 502 error to the client.

Question: How to fix this  Error : nginx error: 502 Bad Gateway  /

Answer :

Your  problem will be fixed by adding two lines to Nginx configuration file /etc/nginx/nginx.conf   

http {
    ...
    fastcgi_buffers 8 16k;
    fastcgi_buffer_size 32k;
    ...
}

Restart   Nginx

is done.

Other Methods of Fixing Nginx 502 Bad Gateway Error

Besides increasing the buffer size, there are a few other ways of fixing the 502 bad gateway error. Some of the fixes are much simpler than the others: 

#1 Reload the Page

One of the simplest and initial fixes to the 502 bad gateway error is to wait for approximately a minute before reloading the page. Sometimes, this error arises due to server overloading, which is temporary. So, there’s a chance that the website will start functioning again on reload. 

#2 Use Another Browser

Using a different browser is the best way to eliminate any browser-related issues, although clearing your browser’s cache could also work. 

In the event that the error disappears when using an alternative browser, it might be a good idea to consider uninstalling and then reinstalling the originally used browser.

There’s also a chance that an extension you installed is causing the error. Instead of disabling the extensions one after another in an attempt to find the culprit, you could try opening the site in incognito mode. If it works, you will need to find the extension causing the issue through trial and error.

#3 Use Another Device

If you think it’s your device that’s having connection issues and not the site, exploring access to the website through a different device is recommended. Performing a quick test involves using the cell phone’s mobile data to access the desired page. 

If successful, the next step is to restart the computer and other networking equipment, adhering to the classic tip of unplugging the wireless router, waiting briefly, and then reconnecting it. This way, you can ascertain whether the error is linked to your networking devices.

#4 Test for DNS Problems

A 502 gateway error could stem from a DNS problem, such as the domain not resolving to the correct IP or unresponsive DNS servers. This typically happens when a site undergoes migration to a new host. 

If you have only recently migrated your site, you must wait for at least 24 hours for the migration to complete fully. 

#5 Restart Your Server

Restarting your Nginx server is the most straightforward solution to resolve the 502 bad gateway error. The simplest way to restart a server is to run a relevant command on your terminal. Here are the nginx restart commands for popular operating systems: 

These commands will restart your server when all the active connections are complete. 

#6 Test PHP-FPM Status and Fix It

In some cases, the 502 Bad Gateway Nginx error can arise due to PHP-FPM not running as expected. Therefore, it becomes essential to verify the status of PHP-FPM to ensure its proper functioning.

Here’s a Linux command that allows you to do this:

If PHP-FPM is running properly, you will see a message indicating its active state. However, if PHP-FPM is not running, you can attempt to resolve the issue by restarting it using the following command:

Executing this command restarts the PHP-FPM service, which can potentially rectify any underlying problem causing the 502 bad gateway Nginx error.

Conclusion

Addressing the 502 Bad Gateway error in Nginx demands a multi-faceted approach. From troubleshooting server settings and optimizing configurations to verifying DNS and PHP-FPM status, each step plays a vital role in resolving the issue. 

With careful monitoring and timely adjustments, users can ensure their Nginx server’s smooth functioning and stability.