Behind a reverse proxy a requests may fail with this error

WARNING	o.e.jetty.util.log.JavaUtilLog#warn: badMessage: 400 Bad Host header for HttpChannelOverHttp@123456{r=0,c=false,a=IDLE,uri=/foo/bar/}

In Jenkins 2.0 Jetty was upgraded from 8.1 to 9.2 which removes AJP functionality, and also requires that if the HOST: field is used it can not be empty.

KO:

GET /foo/bar HTTP/1.1\r\nHOST: \r\nConnection: Close\r\n\r\n

OK:

GET /foo/bar HTTP/1.1\r\nHOST:null \r\nConnection: Close\r\n\r\n

Current winstone version includes Jetty server 9.2.15.v20160210.

The fix is in 9.3.... See:
https://github.com/eclipse/jetty.project/issues/592. https://github.com/eclipse/jetty.project/commit/7e16731b9dac0d5b6ae5424a13c83fd4ef46dc4f

The Jetty version embedded in winstone must be upgraded to have a fix
The workaround is to take care to never send an empty HOST