Magento: Resolving the "Upstream Sent Too Big Header" Error in Nginx Ingress Controller

If you’ve encountered the following error in your Nginx Ingress Controller logs:

2024/09/02 21:45:47 [error] 9991#9991: *14112990 upstream sent too big header while reading response header from upstream, client: 174.51.159.51, server: shop.example.com, request: "POST /customer/account/loginPost/referer/aHR0cHM6Ly9zaG9wLmdvZ2xpZGVzLmNvbS9jdXN0b21lci9hY2NvdW50L2NyZWF0ZS8%2C/ HTTP/2.0", upstream: "http://10.42.163.29:8080/customer/account/loginPost/referer/aHR0cHM6Ly9zaG9wLmdvZ2xpZGVzLmNvbS9jdXN0b21lci9hY2NvdW50L2NyZWF0ZS8%2C/", host: "shop.example.com", referrer: "https://shop.example.com/customer/account/login/referer/aHR0cHM6Ly9zaG9wLmdvZ2xpZGVzLmNvbS9jdXN0b21lci9hY2NvdW50L2NyZWF0ZS8%2C/"

This error indicates that the Nginx Ingress Controller received a response header from the upstream server (your application) that exceeds the default header buffer size. This often happens in applications with complex or large HTTP headers, such as those involving authentication tokens, cookies, or long URLs.

Why Does This Happen?

Nginx, by default, allocates a limited amount of memory to handle headers from upstream servers. If the headers are too large—whether due to large cookies, complex URL structures, or extensive use of authentication tokens—Nginx can’t process them, resulting in a "502 Bad Gateway" error.

Resolving the Issue

To resolve this issue, you need to increase the buffer sizes that Nginx allocates for handling headers. This can be done by modifying the Nginx Ingress Controller's configuration.

Steps to Fix

• Edit the Nginx ConfigMap: Locate and edit the ConfigMap associated with your Nginx Ingress Controller. This ConfigMap controls various Nginx settings.

kubectl edit configmap -n ingress-nginx nginx-configuration

• Update the Buffer Settings: Add or modify the following settings in the ConfigMap to increase the buffer size:

apiVersion: v1
data:
  proxy-buffer-size: "16k"
kind: ConfigMap
metadata:
  labels:
    app: ingress-nginx
  name: nginx-configuration
  namespace: ingress-nginx

• Apply the Changes: After saving the ConfigMap, the Nginx Ingress Controller should automatically pick up the new configuration. If it doesn’t, you may need to restart the Nginx Ingress Controller pods:

kubectl rollout restart deployment nginx-ingress-controller -n ingress-nginx

Once the changes are applied, try accessing the problematic parts of your application again to ensure the issue is resolved.

The "upstream sent too big header" error is a common issue in environments with complex or large HTTP headers. By adjusting the buffer sizes in the Nginx Ingress Controller configuration, you can ensure that Nginx can handle larger headers without returning a "502 Bad Gateway" error, leading to a more stable and reliable application.