Varnish swallowing 4xx responses from POSTs

Dridi Boukelmoune dridi at
Thu Sep 27 13:40:03 UTC 2018


On Wed, Sep 26, 2018 at 2:44 AM Tommy Becker <twbecker at> wrote:
> We have an application that we front with Varnish 4.0.5. Recently, after an application upgrade in which we migrated from Jetty 9.2 to 9.4, we began noticing a lot of 503s being returned from Varnish on POST requests. We have an endpoint that takes a payload of a potentially large amount of JSON, and validates it as it’s being read. What we have discovered is that if there is a problem with the content, we correctly return a 400 Bad Request from Jetty. Notably, this can happen before the entire content is received. When this happens, Varnish continues to send the remainder of data, despite having already seen the response. Now after our upgrade, Jetty's behavior is to send a TCP RST when this happens (since the data is unwanted by the application). Unfortunately, Varnish interprets the RST as a backend error, and goes to vcl_backend_error, having never sent the original response returned from Jetty to the client. So instead of seeing a 400 Bad Request with a helpful message, they simply get 503 Service Unavailable.

I'm pretty certain that this optimization does not comply with the
HTTP/1 specs. Even though Jetty is trying to improve the latency by
replying early, as far as Varnish is concerned it failed to send the
full request and won't bother reading the response.

> I found this issue which seems similar: Can someone help here? Is there anyway to work around this behavior?

In this case that's different because the backend is inspecting the
body as it is coming and not rejecting the request based on the size
only. So I'm afraid there's no way to work around this behavior.

As this is not a bug, we could introduce either a feature flag or a
VCL variable turned off by default to tolerate an early reset of the
request side of an HTTP/1 socket.

You could join next bugwash on Monday to bring this to the team's


More information about the varnish-misc mailing list