BlackhawkHub

HTTP

HTTP 304Not Modified

HTTP 304 Not Modified is the server's answer to a conditional request: the client sent If-None-Match or If-Modified-Since with its cached copy's validator, and the resource has not changed, so the server sends no body and the client reuses its cache. It is normal, healthy behaviour, not an error.

BlackhawkHub Editorial · Updated

How a 304 happens

  1. The client fetches a resource and receives 200 OK with a validator: ETag: "abc123" and/or Last-Modified: Tue, 15 Sep 2026 10:00:00 GMT.
  2. Later, when its cached copy is stale (per Cache-Control: max-age) or the user reloads, the client asks again with If-None-Match: "abc123" or If-Modified-Since: ….
  3. The server compares. Unchanged → 304 Not Modified with no body, and the client keeps using its copy. Changed → 200 OK with the new content and a new validator.

The cost of a 304 is one small round trip; the saving is the entire body.

What the client can rely on

A 304 may include updated Cache-Control, Expires, ETag and Vary headers, which the client applies to its stored response. It must not include a body.

Configuring validators

Static files: Apache and nginx generate ETag and Last-Modified automatically. Apache's default ETag once included the inode, which broke across servers; FileETag MTime Size avoids that.

Dynamic responses: the application must compute an ETag (a hash of the output or a version number) and compare it to If-None-Match before rendering, otherwise the server does all the work and only saves transfer.

Testing

cmd
curl -I https://example.com/style.css
curl -I -H "If-None-Match: \"abc123\"" https://example.com/style.css

The first shows the ETag; the second, with that ETag supplied, should return 304 Not Modified. Timestamps for If-Modified-Since must be in HTTP date format; the Unix timestamp converter converts epoch values from logs.

Frequently asked questions

Is 304 an error?

No. Browser developer tools show many 304s on a reload; each one is a resource the browser already had and did not need to download again.

Why does my site never return 304?

The server is not sending validators (ETag or Last-Modified), or a proxy strips them, or the client is not sending conditional headers. Check the response headers of a 200 first.

Should I use ETag or Last-Modified?

Both if you can. ETag is precise (content hash or version); Last-Modified has one-second granularity. When both are present, clients send both and the server checks ETag first.

Sources

Command records

curl Transfer data and inspect HTTP from the command linecurl transfers data to and from a server and is built into Windows 10 and 11. For diagnostics…

HTTP records

HTTP 200 OKHTTP 200 OK means the server received the request, understood it and is returning the result.…HTTP 301 Moved PermanentlyHTTP 301 Moved Permanently tells the client that the resource now lives at the URL in the…HTTP 404 Not FoundHTTP 404 Not Found means the server is reachable but has nothing at the requested URL. The…HTTP 302 FoundHTTP 302 Found tells the client that the resource is temporarily at the URL in the Location…

Utilities

URL ParserParse a URL into its scheme, host, port, path, query parameters and fragment, and decode the…Unix Timestamp ConverterConvert Unix epoch timestamps (seconds or milliseconds) to human-readable UTC and local dates,…