Upgrading to Jenkins 2.0 causes ha health check to fail

Issue

Upgrading to Jenkins 2.0 with HA (active/passive) caused the following error:

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

Environment

CloudBees Jenkins Enterprise
Jenkins LTS
High Availability Plugin

Resolution

In Jenkins 2.7.19, 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.

Example of invalid string for health check:

GET /ha/health-check HTTP/1.1\r\nHOST: \r\nConnection: Close\r\n\r\n

Example of good string for health check:

GET /ha/health-check HTTP/1.1\r\nHOST:null \r\nConnection: Close\r\n\r\n

The issue is tracked as JENKINS-40693

Tested product/plugin versions

CloudBees Jenkins Platform v/2.7.19.1

References

JENKINS-40693