HTTP 404 on Vaadin 7 when accessed through a Juniper Secure Gateway (reverse proxy VPN)

Originally by cmcbrien

We are looking to deploy an internal application so that it can be made accessible to the outside world through our secure gateway. An example would be http://www.company.com/app/ (click on a button). A communication error will appear.

Appended is a network trace showing that some requests from the client succeed through the gateway while others cause a HTTP 404 response.

Here’s a snipit of a failed request:

URL
/dashboard/UIDL/?v-sh=1080&v-sw=1920&v-cw=1920&v-ch=977&v-curdate=1365026143941&v-tzo=360&v-dstd=60&v-rtzo=420&v-dston=true&v-vw=1920&v-vh=977&v-loc=http%3A%2F%2Fdemo.vaadin.com%2Fdashboard%2F%23!%2Fdashboard&v-wn=dashboard-1047860588-0.031643264512902&v-wsver=7.0.1&v-uiId=1

Method: POST
Result: 404
Type: text/html
Received: 2.19 KB
Taken: 31 ms

Imported from https://dev.vaadin.com/ issue #11482

Start by reviewing the appended network trace and the reported POST request to /dashboard/UIDL/ through the Juniper Secure Gateway. Reproduce the Vaadin 7 application behind that reverse proxy; done means the request no longer returns HTTP 404 and the application communication succeeds.