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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11To fix 400 Bad Request: Request Header or Cookie Too Large in NGINX, first determine whether one incoming header field—usually Cookie—is larger than a single Nginx header buffer. Clear the affected browser’s site data to confirm a cookie-growth problem, remove or shrink unnecessary application cookies for the durable fix, or raise client_header_buffer_size and large_client_header_buffers in the configuration actually loaded by Nginx. If you are asking “Why does NGINX reject my cookie?” or “Where do I set large_client_header_buffers?”, the answers are in the distinction between the initial buffer, per-field limits, and the active virtual-server configuration.
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What the 400 error means
Nginx returns this 400 response while it is reading the request from the client. The problem is not normally the application’s response and not normally an upstream response header. A request header field cannot exceed the size of one configured large-client-header buffer. A long Cookie field is the most common example, but a custom authorization, tracing, or metadata header can trigger the same rejection.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsThe request line has a separate rule. If the URI and method line cannot fit in one configured large-client-header buffer, Nginx returns 414 Request-URI Too Large, not the 400 header-too-large response. This difference tells you which part of the request to investigate.
| Symptom | What exceeded the limit | First place to investigate |
|---|---|---|
| 400 Bad Request while sending a request | One request-header field, often Cookie, is larger than one large-client-header buffer |
Cookie and custom request headers |
| 414 Request-URI Too Large | The request line, including the URI, is larger than one large-client-header buffer | Path, query string, and URL-generating code |
Nginx’s documented default for large_client_header_buffers is 4 8k. That means four buffers of 8 KiB each can be allocated when needed; it does not mean one header field may use all four. The individual field must fit in one buffer. Buffers are allocated only when a request needs them.
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How Nginx allocates request-header buffers
The initial buffer: client_header_buffer_size
client_header_buffer_size is the initial memory area Nginx uses while reading request headers. Its documented default is 1k. Most ordinary requests fit there. Nginx specifically calls out long cookies and WAP clients as cases that may not fit the initial buffer; it then allocates buffers governed by large_client_header_buffers.
The larger buffers: large_client_header_buffers
The directive takes a count and a size, for example 4 8k. The size is the important limit for an individual header field or the request line. Increasing the count does not make one field larger; it increases the number of large buffers available while processing a request. A field that is 20 KiB cannot fit in an 8 KiB buffer merely because four buffers are configured.
Because large buffers are created only when required, the setting does not reserve the full amount for every idle connection. It still raises the amount of memory a busy request can consume, so choose a bounded value based on legitimate traffic rather than using an arbitrarily large limit.
Find out what is actually too large
1. Capture a failing request
Compare the failing browser or API request with one that succeeds. In browser developer tools, open the Network panel, select the failed request, and inspect its request headers. For an API client, enable its request logging or reproduce the call with an equivalent command that shows headers without exposing secrets in shared logs.
- Inspect the complete
Cookieheader, including repeated or duplicated cookie names. - Check custom headers added by authentication, telemetry, feature flags, or reverse-proxy middleware.
- Look at the request URL. An unusually long query string or path points to the separate 414 condition.
- Compare a request from an unaffected browser, private window, or clean API client.
Do not paste access tokens or session values into tickets or public issue trackers. Measure the header length in a controlled environment and redact values before sharing diagnostic output.
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2. Use the response status as a branch point
A 400 generated immediately by Nginx while the request arrives strongly suggests an oversized field. A 414 points to the request line. If the request reaches your application and fails later, inspect application logs and upstream behavior instead of changing client-header buffers first.
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Test the cookie hypothesis without changing Nginx
Clear site data for the affected host
For an isolated user, clear cookies and other site data for the host that returns the error, close or reload the affected tab, and try the request again. If the request succeeds immediately, the browser’s stored state is a strong indication that one or more application cookies have grown too large or have been duplicated.
This is a diagnostic and a short-term recovery, not a durable application fix. Clearing cookies logs users out and removes client-side preferences. It also does not help API clients, mobile clients, or every user if the server keeps issuing an oversized cookie.
Find the application-side cause
After reproducing the problem with a clean session, inspect the response headers that set cookies and the code that serializes session, experiment, consent, or shopping data. Common corrective actions include:
- Remove duplicate cookies created by a domain, path, or deployment change.
- Store large state server-side and keep only a short identifier in the cookie.
- Drop obsolete feature flags and stale experiments instead of appending new values forever.
- Set an appropriate expiration or maximum age so abandoned state is not retained indefinitely.
- Prevent middleware from copying an entire token or serialized object into every request.
The durable fix is to reduce unnecessary application cookie data or its lifetime. A larger Nginx limit can provide headroom for legitimate headers, but it should not conceal unbounded cookie growth.
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Inspect the configuration Nginx is really using
Why editing the obvious file can fail
Nginx combines a main configuration with included files and chooses a virtual-server context for the request. A setting in a file that is not included, in a different server block, or in a server that does not receive the request will not change the failing path. Some request handling occurs before the final virtual server is selected, so relying on a later server block can be misleading.
Display the effective configuration
Use the configuration test or dump command provided by your installation and inspect the complete, expanded configuration, including included files. A common systemd/Linux sequence is:
nginx -t
nginx -Tnginx -t checks syntax and referenced files. nginx -T prints the configuration Nginx would load, which helps you locate duplicate directives, include order, the default server, and the block that receives the affected hostname. If your package uses a nonstandard binary path, run the commands with that binary or through the service’s normal configuration-test mechanism.
Confirm all of the following before changing values:
- The edited file is included by the running master process.
- The hostname and port in the failing request select the intended server.
- No later or more specific setting overrides the value you expect.
- You are changing the Nginx instance that actually terminates the client connection, not only an internal proxy.
Set larger buffers deliberately
A representative configuration
The following is a starting example, not a universal requirement:
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http {
client_header_buffer_size 4k;
large_client_header_buffers 4 16k;
# server blocks and other settings...
}This changes the initial buffer from the documented 1k default to 4k and changes the large-buffer pool from the documented 4 × 8k default to 4 × 16k. The values are examples; measure the largest legitimate individual header in your application and keep a safety margin without making the limit needlessly broad.
Choose the size from the offending field
- Measure the largest legitimate
Cookieor custom header in a representative request. - Choose a buffer size larger than that one field, accounting for normal variation during login, consent, or feature changes.
- Keep the buffer count and size bounded. A larger limit increases the maximum per-request memory exposure when many clients send large headers.
- Change the directive in the active configuration context shown by the effective configuration dump.
Do not increase only the count when the failing field cannot fit in one buffer. Conversely, increasing the size may be unnecessary if the application can remove a few kilobytes of cookie data safely.
Validate, reload, and verify the result
- Run the platform’s normal Nginx configuration test. On a typical installation, that is
nginx -t. - Fix every syntax, include, permission, or duplicate-context error before reloading.
- Reload the service using the host’s normal mechanism. A systemd host commonly uses
sudo systemctl reload nginx; use the service manager appropriate to your installation. - Repeat the exact browser or API request that failed.
- Confirm that the response is successful and that access and error logs do not show a new upstream, routing, or application failure.
- Test an ordinary request as well as the large-header request so the change has not altered unrelated virtual servers.
A successful reload only proves that Nginx accepted the syntax. It does not prove that the request selected the block you edited, which is why the effective configuration and a real retest are both necessary.
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HTTP/1.1, HTTP/2, and HTTP/3 behavior
For HTTP/2 and HTTP/3, Nginx applies the same current large-client-header limits. Compression changes how headers travel over the connection, not the safety boundary after Nginx decodes them: one buffer limits the compressed field size after HPACK or QPACK processing, and the total header limit is applied after decompression.
Older examples may show http2_max_field_size or http2_max_header_size. Current Nginx documentation marks those directives obsolete and directs administrators to large_client_header_buffers instead. Do not copy an old HTTP/2 snippet without checking the Nginx version and its current documentation.
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Protocol-specific checks
- Reproduce the request over the same protocol the user reported; a browser may negotiate HTTP/2 or HTTP/3 while a command-line test uses HTTP/1.1.
- Check whether a gateway or load balancer terminates HTTP/2 or HTTP/3 before forwarding HTTP/1.1 to Nginx. The component that first rejects the request owns the relevant limit.
- Use the current directive names in the Nginx instance that performs the TLS and protocol termination.
Do not confuse request-header limits with upstream response buffers
proxy_buffer_size controls the first part of a response received from an upstream server. It is relevant to errors such as “upstream sent too big header,” where Nginx is reading a response from an application server. It does not replace client_header_buffer_size or large_client_header_buffers when Nginx rejects an incoming request with 400 before proxying it.
| Directive family | Data direction | Typical failure |
|---|---|---|
client_header_buffer_size and large_client_header_buffers |
Client request to Nginx | 400 for an oversized header field or 414 for an oversized request line |
proxy_buffer_size |
Upstream response to Nginx | Errors about an upstream response header being too large |
Changing the response buffer cannot make a rejected browser cookie acceptable, and changing client-header buffers cannot solve an oversized upstream response header.
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Only one browser or user fails
Clear that host’s site data, retry, and compare the cookie set before and after the reset. If the problem disappears, focus on cookie duplication, serialization, expiration, and per-user state. Keep the Nginx change out of the critical path unless legitimate headers for many users also exceed the configured limit.
Every client fails after a deployment
Inspect newly added response cookies and custom request headers, then inspect the active Nginx configuration. A deployment can both increase cookie size and place the intended directive in an unused server block. Fix the application payload when it is unbounded; adjust the active Nginx limit only when the larger field is legitimate and bounded.
The error changes to 414
You are now dealing with the request line rather than an individual header field. Reduce generated query parameters, avoid putting large state in URLs, or raise the one-buffer size only when long URLs are an intentional, controlled part of the interface.
Nginx reloads but the error remains
Use the effective configuration dump to verify the loaded values, then identify which process terminates the client connection. Check DNS, a load balancer, a container ingress, and multiple Nginx instances. The setting must be changed where the 400 is generated.
The 400 disappears but an upstream error appears
The request-header limit may no longer be the blocker. Read the new error literally: an upstream response-header problem, routing error, or application rejection requires a different fix. In particular, investigate proxy_buffer_size only for an upstream response-header error, not for the original client-request 400.
Operational trade-offs and prevention
- Smaller client headers: reduce bandwidth, parsing work, and exposure to accidental per-request memory growth. This requires application changes when cookies or custom headers carry too much state.
- Higher bounded buffers: preserve a legitimate protocol contract when a known header must be larger, but increase the maximum memory that a busy Nginx worker may handle for large requests.
- Client-side clearing: restores an isolated browser quickly and helps prove cookie causality, but logs the user out and does not correct the server’s cookie policy.
- Configuration-only changes: can be deployed quickly, yet they may mask an application regression if the cookie continues growing.
Monitor cookie and request-header sizes during authentication and major feature changes. Set a review threshold below the Nginx limit so the application can remediate growth before users receive a 400. Keep the count and size of large buffers explicit in configuration management, test them in staging with HTTP/1.1 and negotiated HTTP/2 or HTTP/3 traffic, and document which component terminates each protocol.
Quick Recap
A concise recovery checklist
- Identify the exact failing request and inspect
Cookie, custom headers, and the URI. - Use the status code: 400 indicates an oversized field; 414 indicates an oversized request line.
- Clear site data for an isolated browser failure and retest.
- Measure the largest legitimate field and fix cookie or header growth in the application.
- Dump the effective Nginx configuration, including includes and the selected server.
- Set
client_header_buffer_sizeand, when justified,large_client_header_bufferswith bounded values. - Run the configuration test, reload Nginx, and repeat the original request.
- Use current large-client-header directives for HTTP/2 and HTTP/3; avoid obsolete HTTP/2-specific settings.
- Investigate
proxy_buffer_sizeonly when the error concerns an upstream response header.
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