Resolving "Response Buffer Limit Exceeded" Errors by Tuning Nginx Proxy Buffers

If you‘ve ever encountered a dreaded "502 Bad Gateway" error page when accessing a website proxied through Nginx, the underlying cause may be revealed in the following ominous message in the Nginx error logs:

upstream sent too big header while reading response header from upstream

This cryptic error is Nginx‘s way of saying the HTTP response headers it received from the upstream server (e.g. the backend web app) exceeded the allocated buffer size. To understand why this happens and how to fix it, we need to take a closer look at how Nginx handles proxied requests and the role of its various proxy buffer settings.

Understanding Nginx Reverse Proxying and Proxy Buffers

Nginx is commonly deployed as a reverse proxy sitting in front of web applications. In this setup, Nginx receives HTTP requests from clients and forwards them to one or more upstream backend servers to generate the response. This allows Nginx to handle tasks like SSL termination, load balancing, caching, and more.

When Nginx proxies a request, it uses a small amount of memory called a "proxy buffer" to temporarily store the response it gets back from the upstream server. This buffering allows Nginx to start sending the response to the client before the upstream finishes generating the complete response. The proxy buffer is key to Nginx‘s high performance proxying capabilities.

There are two main aspects involved in the proxy buffering process:

1. Buffering the response headers
2. Buffering the response body

The "response buffer limit exceeded" error we‘re discussing relates to the first part – buffering the response headers. For every proxied request, Nginx allocates a chunk of memory as the proxy buffer to store the HTTP response headers it gets from the upstream.

If the response headers are too large and exceed the size of this proxy buffer, Nginx returns the 502 Bad Gateway error and logs the "upstream sent too big header" message.

Several Nginx configuration directives control the proxy buffering behavior and can be tuned to avoid this error. The most important one is proxy_buffer_size.

The proxy_buffer_size Setting

The proxy_buffer_size directive specifies the size of the buffer that Nginx uses for storing the HTTP response headers received from the upstream server. This is the primary setting we need to tune to resolve "response buffer limit exceeded" errors.

By default, Nginx sets proxy_buffer_size to either 4KB or 8KB depending on the platform. This is usually sufficient for most web applications. However, some apps may generate response headers that exceed this default size, especially during authenticated requests where multiple Set-Cookie headers get returned.

When the response headers are too big for the configured proxy_buffer_size, Nginx returns the 502 error. The solution is to increase the proxy_buffer_size value to accommodate the maximum size of response headers the application will generate.

You can set proxy_buffer_size in the http, server, or location context of the Nginx configuration:

proxy_buffer_size 16k;

Here we‘ve bumped up the buffer size to 16KB, which should be more than enough for most web apps. If you still get "response buffer limit exceeded" errors after this, you may need to increase it further.

Determining the Right proxy_buffer_size Value

So how do you know what value to use for proxy_buffer_size? The optimal setting depends on the specifics of your web application and the maximum size of response headers it will realistically generate.

To determine this, you can use curl to request pages on your site that are likely to return the largest headers, like an authenticated page that sets session cookies.

Use the -s flag to hide the response body, -o /dev/null to discard it entirely, and -w "%{size_header}" to print just the size of the response headers:

curl -s -o /dev/null -w "%{size_header}" https://example.com/login

This will output a number indicating the total size of the response headers in bytes. You‘ll want to test a few different pages and note the largest header size to determine a suitable proxy_buffer_size.

As a rule of thumb, it‘s best to set proxy_buffer_size to the next highest multiple of your system‘s page size that accommodates your largest headers. You can get the page size with:

getconf PAGESIZE

On most systems it will be 4096 bytes (4KB). So if your maximum response header size was around 9KB, you‘d bump the proxy_buffer_size up to the next 4KB chunk which is 12KB:

proxy_buffer_size 12k; 

Other Proxy Buffer Settings

While proxy_buffer_size is the key directive for solving "response buffer limit exceeded", Nginx has a few other proxy buffer settings that are worth understanding:

• proxy_buffers – Controls the number and size of buffers used for proxying the response body. Consists of the number of buffers and the size of each buffer (e.g. proxy_buffers 8 4k;). Increasing these can help with proxying large responses.

• proxy_busy_buffers_size – Limits the total size of buffers that can be receiving response body from the upstream while still sending data to the client. Buffers in excess of this are freed.

• proxy_buffer_size_max – Specifies the maximum size of a single buffer used for reading the response body from the upstream. If a response doesn‘t fit into a single buffer of this size, it will be written to the temporary file on disk.

In most cases, you won‘t need to modify these additional settings. Stick with the defaults and only increase them if proxying requests with very large response bodies.

Verifying Proxy Buffer Behavior

After updating your proxy_buffer_size and reloading the Nginx configuration, you can verify it‘s taking effect by rechecking the response header size on the same request:

curl -s -o /dev/null -w "%{size_header}" https://example.com/login

If the reported header size exceeds your configured proxy_buffer_size, you‘ll still get the 502 error. You may need to bump the buffer size higher or look into reducing the number/size of headers returned by the application.

It‘s also useful to tail the Nginx error log when making test requests to watch for any "upstream sent too big header" messages:

tail -f /var/log/nginx/error.log

If you no longer see this error after tuning proxy_buffer_size properly, you‘ve successfully resolved the "response buffer limit exceeded" issue.

Nginx Performance and Proxy Buffers

Increasing proxy_buffer_size and the other proxy buffer settings can help avoid errors, but it‘s important to find the right balance. Larger buffers consume more memory per request which can negatively impact performance and scalability if set excessively high.

The goal is to find the smallest proxy_buffer_size that will still accommodate the response headers generated by your upstream application. Don‘t just set it to an enormous value out of caution.

If your app has different response header sizes for different locations, you can set proxy_buffer_size to a lower default and only increase it for the specific location blocks that need it. This optimizes memory usage.

Also be aware that some web frameworks and applications are prone to generating excessively large response headers due to lack of proper defaults or bugs. Rather than working around this in Nginx, it‘s better to fix the issue at the source and modify the application to return fewer/smaller headers where possible.

Regular monitoring and review of response header and body sizes for your application can help you spot potential issues and keep your proxy buffer settings optimized.

Conclusion

While encountering "response buffer limit exceeded" errors and cryptic "upstream sent too big header" messages in Nginx can be frustrating, the solution is often quite straightforward. Understanding how Nginx proxy buffers work and tuning the proxy_buffer_size directive is usually all it takes to resolve these issues.

By carefully testing your application to determine the maximum response header size and setting proxy_buffer_size accordingly, you can avoid dreaded 502 errors and keep your Nginx reverse proxy humming along smoothly.

The key is to find the optimal balance – large enough proxy buffers to accommodate your application‘s headers and body without going overboard and wasting memory. Monitor and tweak your buffer settings over time as the application evolves.

Proper Nginx performance tuning is an ongoing process, but with an understanding of key directives like proxy_buffer_size, you‘ll be well-equipped to keep your web applications fast, reliable, and error-free. Happy proxying!