Reverse proxy - Issues

Symptoms

An error message is displayed in the "Manage Jenkins" page

It appears that your reverse proxy setup is broken

This message can also appear if you don’t access Jenkins through a reverse proxy: Make sure the Jenkins URL configured in the System Configuration matches the URL you’re using to access Jenkins.

Background

For a reverse proxy to work correctly, it needs to rewrite both the request and the response. Request rewriting involves receiving an inbound HTTP call and then making a forwarding request to Jenkins (sometimes with some HTTP headers modified, sometimes not). Failing to configure the request rewriting is easy to catch, because you just won’t see any pages at all.

But correct reverse proxying also involves one of two options, EITHER

  • rewrite the response with a "Location" header in the response, which is used during redirects. Jenkins sends Location: http://actual.server:8080/jenkins/foobar and the reverse proxy must rewrite it to Location: http://nice.name/jenkins/foobar. Unfortunately, failing to configure this correctly is harder to catch; OR

  • set the headers X-Forwarded-Host (and perhaps X-Forwarded-Port) on the forwarded request. Jenkins will parse those headers and generate all the redirects and other links on the basis of those headers. Depending on your reverse proxy it may be easier to set X-Forwarded-Host and X-Forwarded-Port to the hostname and port in the original Host header respectively or it may be easier to just pass the original Host header through as X-Forwarded-Host and delete the X-Forwarded-Port # header from the request. You will also need to set the X-Forwarded-Proto header if your reverse proxy is changing from https to http or vice-versa.

Jenkins has proactive monitoring to make sure this is configured correctly. It uses XmlHttpRequest to request a specific URL in Jenkins (via relative path, so this will always get through provided the request is properly rewritten), which will then redirect the user to another page in Jenkins (this only works correctly if you configured the response rewriting correctly), which then returns 200.

This error message indicates that this test is failing - and the most likely cause is that the response rewriting is misconfigured. See the configuration examples for additional tips about configuring a reverse proxy.

Be sure to set the X-Forwarded-Proto header if your reverse proxy is accessed via HTTPS and then Jenkins itself is accessed via HTTP i.e. proxying HTTPS to HTTP.

Changing the context path of Jenkins with a reverse proxy is fraught with danger. There are lots of URLs that you need to rewrite correctly, and even if you get the ones in HTML files you may miss some in javascript, CSS or XML resources.

The recommendation is to ensure that Jenkins is running at the context path that your reverse proxy is serving Jenkins at. You will have the least pain if you keep to this principle.

While it is technically possible to use rewrite rules to change the context path, you should be aware that it would be a lot of work to find and fix everything in your rewrite rules and the reverse proxy will spend most of its time rewriting responses from Jenkins. Much easier to change Jenkins to run at the context path your reverse proxy is expecting, e.g. if your reverse proxy is forwarding requests at https://manchu.example.org/foobar/ to Jenkins then you could just use java -jar jenkins.war --prefix=/foobar to start jenkins using /foobar as the context path

Further Diagnosis

For further diagnosis, try using cURL:

BASE=administrativeMonitor/hudson.diagnosis.ReverseProxySetupMonitor
curl -iL -e http://your.reverse.proxy/jenkins/manage \
            http://your.reverse.proxy/jenkins/${BASE}/test

(assuming your Jenkins is located at http://your.reverse.proxy/jenkins/ - and is open to anonymous read access)