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A 400 response in an Nginx–Varnish stack is usually caused by a malformed request, a missing or invalid Host header, a PROXY-protocol or TLS framing mismatch, or a request header that exceeds the limit at one of the two hops. Do not raise every buffer first. Identify which process emitted the response, inspect the rejected request, remove unnecessary header data, and then increase only the specific limit that legitimate traffic needs.
Start by finding the hop that generated the 400
The public URL alone does not tell you whether Nginx or Varnish rejected the request. A client can receive the same status code even though the two servers use different parsers, limits and logs.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errors- Capture the public response. Record the status, response headers, response body, timestamp and any request or trace ID. A verbose HTTP client is useful for seeing the connection and protocol in use, for example
curl -sv --http1.1 https://your-public-host.example/. Redact cookies, authorization values and other secrets before sharing the output. - Query each reachable hop directly. Test the public listener, the Varnish listener and the origin or Nginx listener separately where your topology permits. Use the same method, path and relevant headers so that the failing request is comparable.
- Correlate timestamps. Check Nginx access and error logs alongside Varnish Shared Log (VSL) records from
varnishlog. If Varnish rejects a client request, its record commonly containsBogoHeader,LostHeader,RespStatus 400or a related request event. A request that never appears in Nginx logs was probably rejected before Nginx received it. - Repeat with a minimal request. Send a request with one ordinary path, a normal
Host, no application cookies and no optional tracing headers. If the minimal request succeeds, add the original headers back in small groups until the offending field or protocol feature is isolated.
Keep the exact software versions and build information with the incident. Nginx defaults and parameter behavior can vary by release and build, and Varnish settings differ between versions and editions.
What a 400 means at each layer
| Failure class | Likely emitting hop | Evidence to look for | Corrective direction |
|---|---|---|---|
| Malformed header syntax | Varnish or Nginx | Varnish BogoHeader; Nginx error log entries about invalid request headers |
Fix the client, proxy or header-generation code; do not simply enlarge buffers |
HTTP/1.1 request without a valid Host |
Varnish or Nginx | Request capture shows a missing, duplicated or malformed Host |
Correct the client or forwarding proxy so exactly one valid authority is sent |
| PROXY-protocol framing mismatch | The listener expecting PROXY protocol | One side is configured for PROXY protocol while its peer sends ordinary HTTP, or the versions do not agree | Enable the same PROXY protocol version on both sides, or disable it on both |
| One oversized request-header field | Nginx or Varnish | Large Cookie, Authorization or forwarded header; limit-related log message |
Reduce the field; otherwise raise the one-field limit at the rejecting hop |
| Total request too large | Varnish, Nginx or an upstream proxy | Combined headers and request line exceed the configured request-size limit | Remove needless data or increase the total request limit narrowly |
| Too many header lines | Varnish or another intermediary | Header count approaches the configured maximum | Remove duplicate and obsolete fields or raise the count limit only when required |
A 400 generated while parsing the client request is different from a 503 generated while parsing a response from the backend. Treat those paths separately; changing client request buffers will not repair an origin response-header failure.
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Validate HTTP syntax, Host and connection framing
Check the HTTP/1.1 Host header
Varnish documents that it returns HTTP 400 when it sees an HTTP/1.1 request without a Host header. The field must also be syntactically valid for the authority being requested. Inspect the request as it arrives at Varnish, not only the headers that an application eventually sees.
- Make sure the client sends exactly one
Hostfield for HTTP/1.1. - Check that an upstream proxy is not deleting the field, adding a second one, or forwarding an empty value.
- Verify that a health check, synthetic monitor or custom script is not using HTTP/1.0-style requests against an HTTP/1.1-only path.
- For a TLS request, confirm that the authority used for certificate selection and routing is the intended host; do not substitute an unrelated internal hostname when testing.
Confirm PROXY protocol agreement
PROXY protocol carries client-connection metadata before the HTTP request. A listener configured to expect that preface cannot parse an ordinary HTTP request as valid framing. The reverse mismatch is also bad: a listener that expects HTTP will see the PROXY preface as invalid bytes. Check both ends of every connection—client to Varnish, Varnish to Nginx, or a load balancer to Varnish—and confirm that the configured version is the one the peer actually sends.
Make sure TLS is terminated at the intended hop
Sending plain HTTP to a TLS-only listener, or encrypted TLS bytes to a listener expecting clear-text HTTP, can produce a parsing error that looks like an application problem. Draw the connection sequence and mark where TLS terminates. Then test each hop with the correct scheme and port. A request that works when sent directly to the TLS terminator but fails on the next clear-text hop points to forwarding or protocol configuration rather than a cookie-size limit.
Look for illegal header characters and continuation lines
Varnish uses BogoHeader diagnostics for malformed headers such as illegal whitespace, control characters, missing names and invalid continuation lines. Inspect the raw request generated by the client and every proxy that rewrites it. Common sources include hand-built HTTP clients, concatenated header values, duplicated separators and middleware that appends a second forwarded-header chain without normalizing the first.
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Understand Nginx request-header limits
The NGINX core documentation states that a request-header field must fit inside one large_client_header_buffers buffer; if an individual field is larger than one buffer, Nginx returns 400. The documented default is four buffers of 8 KB each. That is a per-field rule, not a promise that four buffers combine to permit a 32 KB cookie.
| Directive | What it controls | Important diagnostic detail |
|---|---|---|
client_header_buffer_size |
Initial buffer used while reading request headers | Changing it alone does not make a long individual field fit if the large-buffer rule is still exceeded |
large_client_header_buffers number size |
Number and size of buffers available for large request lines and header fields | One request-header field must fit in one size buffer; increasing number alone does not lengthen a permitted line |
The request line has a related but different failure: when it exceeds one large buffer, Nginx commonly returns 414 (Request-URI Too Large), whereas an individual request-header field that exceeds one buffer produces 400. This distinction helps you decide whether to inspect a long URL or a long header such as Cookie.
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Find the offending field
- Cookie: stale session, consent, experiment and shopping-cart cookies can accumulate across subdomains. Compare the total cookie string and each cookie value, then expire obsolete names at the application.
- Authorization: oversized bearer or signed tokens may exceed a single field buffer. Prefer shorter-lived, compact claims or a server-side session reference when your authentication design allows it.
- X-Forwarded-* and tracing fields: repeated proxy hops can append long address, host, port and trace chains. Normalize them at a trusted boundary and remove duplicate entries.
- Custom application fields: inspect generated JSON-in-header, feature flags and debugging metadata. Headers are a poor place for large payloads; move substantial data into a request body or server-side state when the protocol and application permit.
Inspect Varnish limits and VSL evidence
Varnish checks both the shape and the size of the client request. The Varnish Cache varnishd reference documents these defaults:
| Parameter | Documented default | Meaning |
|---|---|---|
http_max_hdr |
64 header lines | Maximum number of request-header lines Varnish accepts |
http_req_hdr_len |
8 KB | Maximum length of an individual request-header field |
http_req_size |
32 KB | Maximum size of the complete request, including the request line and headers as Varnish parses them |
h2_max_header_list_size |
Version/configuration dependent; inspect the installed HTTP/2 settings | Limit applied to the decoded HTTP/2 header list |
Use VSL to distinguish these cases rather than guessing from the browser’s generic error page. A header-length or request-size rejection points to a limit or excessive data. BogoHeader and LostHeader point toward malformed or discarded fields. For HTTP/2, investigate the configured header-list limit when the VSL record identifies a header-list overflow.
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Varnish’s troubleshooting guidance warns that larger limits consume workspace and memory and do not guarantee flawless operation. A higher ceiling can expose downstream limits, increase per-connection resource use and make abusive requests more expensive to process. Change one parameter at a time, document the reason and observe memory and rejection rates after the reload or restart.
Remove unnecessary request data before raising limits
Request cleanup is safer than making every component accept arbitrarily large headers. Start at the source that creates the data and work toward the public edge.
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- Delete stale cookies. Expire old application versions, duplicate tracking names and abandoned experiments. Set appropriate domain and path scopes so a cookie is not sent to unrelated hosts.
- Collapse forwarded chains. At a trusted proxy, rebuild
X-Forwarded-For,X-Forwarded-Hostand related fields from known connection data instead of blindly appending unbounded values. - Reduce token and tracing overhead. Remove debug baggage in production, shorten unnecessary trace baggage and avoid copying the same identifier into several custom fields.
- Fix malformed emitters. Correct code that creates empty names, illegal control bytes, invalid whitespace or broken continuation lines. A larger buffer cannot make invalid syntax valid.
- Retest the original request. Preserve a safe reproduction of the failing header lengths so that cleanup can be verified and regression-tested.
Apply a narrow Nginx change when the traffic is legitimate
Only change Nginx after logs or a direct-hop test show that Nginx is the rejecting parser and the request is valid. Place the directive at the narrowest appropriate http or server scope. For example:
http {
client_header_buffer_size 4k;
large_client_header_buffers 4 16k;
server {
listen 80;
# proxy_pass and normal Varnish routing here
}
}This example raises the size of each large buffer to 16 KB while retaining four buffers; it is not a universal recommendation. Select a value from the measured maximum field length plus reasonable protocol overhead, and leave headroom for normal variation. If one cookie is 20 KB, four 16 KB buffers still cannot accept it because the field must fit in one buffer.
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- Measure the largest legitimate individual field and the total request at the rejecting hop.
- Change only the relevant Nginx context. Avoid a global increase when one virtual server or endpoint needs the exception.
- Run
nginx -tand stop if configuration validation fails. - Reload through your normal change-control procedure, preserving the previous configuration for rollback.
- Replay the exact failing request and a minimal control request. Check Nginx logs, Varnish VSL and memory behavior.
Increasing the number of buffers can help Nginx hold more separate large fields, but it does not make an individual field longer. If the error is caused by a huge cookie, reduce that cookie or increase the buffer size at the field-rejecting server—not merely the buffer count.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Apply a narrow Varnish change when VSL proves its limit
Varnish parameters are supplied through the startup command or the service configuration used by your installed release. A representative configuration is:
-p http_max_hdr=96
-p http_req_hdr_len=16k
-p http_req_size=64kThese values are examples of syntax, not safe defaults for every site. Use the smallest change supported by measured traffic: raise http_req_hdr_len for one valid long field, http_req_size for a valid but large aggregate request, or http_max_hdr for a high count of legitimate fields. For HTTP/2, adjust the configured header-list limit only when the VSL evidence identifies that condition.
- Confirm the installed Varnish version, edition and current parameter values.
- Identify whether the rejection is per-field, aggregate-size, count or HTTP/2 header-list related.
- Remove needless request data first, then select the one parameter that remains too restrictive.
- Update the service or startup configuration, validate it using the deployment procedure for that installation and restart or reload as required.
- Monitor VSL for the original rejection markers, backend reachability and memory/workspace pressure.
Do not raise all three limits together merely because a 400 occurred. Larger workspaces can increase memory consumption, and accepting a request at Varnish may simply move the failure to Nginx or the origin.
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HTTP/2-specific checks
HTTP/2 transmits a compressed header list rather than the textual HTTP/1.1 lines you see in a basic packet capture. A client can therefore pass an HTTP/1.1 test while an HTTP/2 request is rejected for its decoded header-list size. Compare the protocol explicitly, for example by testing with and without --http2 where your client supports it, and inspect VSL for the HTTP/2 limit named by the installed version. The remedy is still the same sequence: remove excess headers, verify the emitting hop, then raise only the documented HTTP/2 limit if valid traffic requires it.
Use a troubleshooting matrix instead of changing everything
| Observation | Most useful next check | Likely fix |
|---|---|---|
Varnish VSL shows BogoHeader and no Nginx request |
Raw request syntax, Host, control characters and continuation lines | Fix the client or preceding proxy |
| Only requests with a large cookie fail | Measure that one field at Varnish and Nginx | Expire or reduce cookies; otherwise raise the per-field limit at the first rejecting hop |
| Many ordinary fields cause failure | Count lines and inspect duplicate forwarding or tracing fields | Remove duplicates or adjust http_max_hdr narrowly |
| Small HTTP/1.1 request fails before routing | Host header, PROXY preface and TLS termination | Make the connection framing and protocol settings agree |
| HTTP/1.1 works but HTTP/2 fails | Varnish HTTP/2 header-list diagnostics and configured limit | Reduce the decoded header list or adjust the HTTP/2 limit |
| After a request-limit change, Varnish returns 503 with backend header errors | Origin response headers and VSL FetchError/BogoHeader records |
Fix backend response parsing separately; do not treat it as the client-side 400 |
Verify the fix and guard against regression
- Replay the original request through the public endpoint and directly against each available hop.
- Test both HTTP/1.1 and HTTP/2 if both are enabled.
- Include a normal request, the largest legitimate cookie set and a deliberately oversized test in a controlled environment.
- Confirm that the request reaches the intended backend and that Nginx access/error logs and VSL no longer show the rejection markers.
- Watch memory, workspace exhaustion, connection errors and latency after raising any limit.
- Keep configuration changes versioned with the measured header lengths, affected virtual host, software versions and rollback procedure.
If the public status changes from 400 to 503, stop applying client-header changes. The new status indicates a different investigation: inspect backend response headers, origin syntax and Varnish fetch diagnostics.
Prevention checklist
- Set a documented maximum for application cookies and expire old names during releases.
- Normalize forwarded headers at each trusted boundary instead of allowing unbounded appends.
- Keep health checks and synthetic monitors compliant with the protocol expected by the listener.
- Document whether each connection uses TLS, clear-text HTTP, PROXY protocol and which version.
- Alert on rising Varnish
BogoHeader,LostHeaderand 400 counts before users report failures. - Record Nginx and Varnish versions with every limit change because defaults and parameter names can vary by release, build and edition.
Documentation used for the limits and behaviors
The per-field Nginx rule and the four 8 KB large_client_header_buffers default are documented in the NGINX Modules Reference and core-module documentation (2025). Varnish’s missing-Host behavior, malformed-header diagnostics and client-side 400 handling are described in Varnish Software’s built-in VCL and troubleshooting documentation. The http_max_hdr, http_req_hdr_len and http_req_size defaults come from the Varnish Cache varnishd reference. Confirm the corresponding documentation for the exact versions installed in your environment before changing production limits.
