# Upstream Sent Too Big Header While Reading Response Header From Upstream

The error `upstream sent too big header while reading response header from upstream` in Nginx indicates that the upstream server (such as a backend application server or FastCGI server) is sending response headers that exceed the maximum allowed size configured in Nginx. This can occur if the upstream server is including large headers, such as cookies or custom headers.

To resolve this issue, you need to adjust the Nginx configuration to increase the maximum allowed size for headers. Here’s a step-by-step guide on how to address this issue:

### **1. Increase the `proxy_buffers` and `proxy_buffer_size` Settings**

If you are using Nginx as a reverse proxy, you can increase the buffer sizes in the Nginx configuration.

### **Configuration Steps:**

1. **Open the Nginx Configuration File**:
    
    This could be in `/etc/nginx/nginx.conf`, or in a site-specific configuration file under `/etc/nginx/sites-available/` or `/etc/nginx/conf.d/`.
    
    ```bash
    sudo nano /etc/nginx/nginx.conf
    ```
    
2. **Add or Update the `http` Block**:
    
    Increase the `proxy_buffer_size` and `proxy_buffers` values. These directives control the size of buffers used to read the response from the upstream server.
    
    ```
    http {
        # Increase buffer size
        proxy_buffer_size 128k;
        proxy_buffers 8 128k;
        proxy_busy_buffers_size 256k;
    
        # Other settings...
    }
    
    ```
    
    Adjust the buffer sizes according to your needs. The `proxy_buffer_size` sets the size of the buffer for reading the response header, while `proxy_buffers` sets the number and size of the buffers used for reading the response body.
    
3. **Save and Close the File**.
4. **Test the Configuration**:
    
    ```bash
    sudo nginx -t
    ```
    
    Ensure there are no syntax errors in the configuration.
    
5. **Reload Nginx**:
    
    ```bash
    sudo systemctl reload nginx
    ```
    
    This applies the new configuration.
    

### **2. Increase the `fastcgi_buffers` and `fastcgi_buffer_size` Settings**

If you are using FastCGI with Nginx, you can increase the buffer sizes related to FastCGI.

### **Configuration Steps:**

1. **Open the Nginx Configuration File**:
    
    ```bash
    sudo nano /etc/nginx/nginx.conf
    ```
    
2. **Add or Update the `http` Block**:
    
    Increase the `fastcgi_buffer_size` and `fastcgi_buffers` values.
    
    ```
    http {
        # Increase FastCGI buffer size
        fastcgi_buffer_size 128k;
        fastcgi_buffers 8 128k;
    
        # Other settings...
    }
    
    ```
    
3. **Save and Close the File**.
4. **Test and Reload Nginx**:
    
    ```bash
    sudo nginx -t
    sudo systemctl reload nginx
    ```
    

### **3. Adjust `client_header_buffer_size`**

If the issue is related to large client headers, you may need to adjust the `client_header_buffer_size` directive.

### **Configuration Steps:**

1. **Open the Nginx Configuration File**:
    
    ```bash
    sudo nano /etc/nginx/nginx.conf
    ```
    
2. **Add or Update the `http` Block**:
    
    ```
    http {
        # Increase client header buffer size
        client_header_buffer_size 1k;
        large_client_header_buffers 4 4k;
    
        # Other settings...
    }
    
    ```
    
    Adjust `client_header_buffer_size` and `large_client_header_buffers` based on the header sizes you expect.
    
3. **Save and Close the File**.
4. **Test and Reload Nginx**:
    
    ```bash
    sudo nginx -t
    sudo systemctl reload nginx
    ```
    

### **4. Check Upstream Server Configuration**

Sometimes the upstream server configuration itself may be the cause of oversized headers. Check the configuration of your upstream application or server to ensure it is not sending unnecessarily large headers.

### **Summary**

- **For Reverse Proxy Configurations**:
    - Increase `proxy_buffer_size`, `proxy_buffers`, and `proxy_busy_buffers_size`.
- **For FastCGI Configurations**:
    - Increase `fastcgi_buffer_size` and `fastcgi_buffers`.
- **For Large Client Headers**:
    - Adjust `client_header_buffer_size` and `large_client_header_buffers`.
- **Review and Adjust Upstream Server**:
    - Ensure the upstream server is configured correctly and is not sending excessively large headers.

By adjusting these settings, you should be able to resolve the `upstream sent too big header while reading response header from upstream` error and handle large headers from your upstream server.