Learn how to fix the “Upstream Sent Too Big Header” error in Nginx. Our Nginx Support team is here to help you with your questions and concerns.
How to Fix “Upstream Sent Too Big Header” Error in Nginx
According to our Experts, the Nginx error “upstream sent too big header” indicates that the response headers from the backend server exceed the buffer limits configured in Nginx. Unfortunately, this can result in failed requests, service disruptions, and challenging user interactions.
In this blog, we will break down the causes of this error, its impact, and provide step-by-step instructions on how to fix it.
An Overview:
What Does the Error Mean?
The full error message usually looks like:
Upstream sent too big header while reading response header from upstream
This error tells us that:
- Nginx cannot process the response headers from the upstream server.
- The headers are larger than the configured buffer size.
- The request fails and cannot be completed successfully.
Facing other upstream-related issues? You may also find this guide on NGINX upstream errors helpful.
Key Impacts of the Error
- Users may experience difficulties accessing certain web pages or features.
- Generates `502 Bad Gateway` errors.
- Drops or terminates requests prematurely.
- In some cases, can even trigger NGINX timeout errors.
- Wastes system resources on failed requests.
- Causes potential bottlenecks.
- Leads to timeouts and broken pages.
- Could expose misconfigurations.
- It might be exploited if not addressed promptly.
Common Causes and Fixes
1. Insufficient Nginx Buffer Size
The default Nginx buffer settings are too small for complex applications.
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In this scenario, edit the Nginx config (usually `/etc/Nginx/Nginx.conf`) and add or update these directives in the `http`, `server`, or `location` block:
proxy_buffer_size 16k;
proxy_buffers 4 16k;
proxy_busy_buffers_size 32k;
large_client_header_buffers 4 16k;
Copy CodeOur Experts suggest matching buffer sizes to the system page size. It is also a good idea to increase incrementally to avoid memory waste. Furthermore, test thoroughly before deployment.
2. Large Cookies and Session Data
Oversized cookies or session data-inflating headers.
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We can optimize cookies by removing unused cookies and using compression and `HttpOnly`/`Secure` flags.
Additionally, move sessions to server-side stores (e.g., Redis, Memcached). We can also use lightweight, compact session formats.
3. Multiple Large Headers
Several large headers exceed the total allowed space.
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In this case, reduce and compress headers and consolidate multiple headers when possible.
We can do some additional Nginx tuning as well:
proxy_headers_hash_max_size 1024;
proxy_headers_hash_bucket_size 128;
Copy Code4. FastCGI or PHP Configuration Issues
Backend (like PHP-FPM) is sending oversized headers.
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To fix this cause, add these to the Nginx config if using FastCGI:
fastcgi_buffer_size 128k;
fastcgi_buffers 4 256k;
fastcgi_busy_buffers_size 256k;
fastcgi_temp_file_write_size 256k;
Copy CodeAlso, review the `php.ini` and FPM pool settings for better optimization.
If you’re seeing odd behavior like unexpected connection drops, you might be hitting the “closed keepalive connection” error in NGINX, which can also be tied to misbehaving headers or upstream timeouts.
5. Excessive Redirects with Long URLs
Redirects with long query strings or paths can bloat the header size.
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- Shorten URLs in redirects.
- Use routing tokens or hash-based IDs.
- Limit redirect lengths:
rewrite_limit 200;Copy Code
6. Complex Application Header Logic
Apps are passing too much data through headers.
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- Optimize how headers are generated.
- Move data to query parameters or the request body.
- Monitor and log the header size actively.
Here is an example of a logging config:
log_format detailed_headers '$remote_addr - $remote_user [$time_local] '
'"$request" $status $body_bytes_sent '
'"$http_referer" "$http_user_agent" '
'headers_size=$http_header_size';
Copy CodePrevention Tips
- Monitor Nginx logs regularly.
- Use compression for headers when possible.
- Optimize application logic for cookies and sessions.
- Periodically tune buffer-related Nginx directives.
- Use proper server-side session management tools.
[Need assistance with a different issue? Our team is available 24/7.]
Conclusion
The “upstream sent too big header” error in Nginx can be fixed by tuning buffer settings, optimizing the backend, tweaking configuration, and periodic checks.
In brief, our Support Experts demonstrated how to fix “Upstream Sent Too Big Header” error in Nginx.
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