UI ignores route prefix on redirect to home page.

[scott-david-walker]

What did you do?

When you make use of the route prefix it gets ignored by the UI and redirects immediately to /graph page.

We have set external url to our hostname E.G. https://myurl.com and route prefix to /sys/metrics but when going to https://myurl.com/sys/metrics it redirects to /graph where nothing is available. Whereas if we go directly to https://myurl.com/sys/metrics/graph then everything works as expected.

We think it's an issue with the code below where the get route for the prefix might be overwritten by the next get route which is always applied

prometheus/web/web.go

Lines 354 to 374 in 94edd08

	if o.RoutePrefix != "/" {
	// If the prefix is missing for the root path, prepend it.
	router.Get("/", func(w http.ResponseWriter, r *http.Request) {
	http.Redirect(w, r, o.RoutePrefix, http.StatusFound)
	})
	router = router.WithPrefix(o.RoutePrefix)
	}
	homePage := "/graph"
	if o.IsAgent {
	homePage = "/agent"
	}
	readyf := h.testReady
	router.Get("/", func(w http.ResponseWriter, r *http.Request) {
	http.Redirect(w, r, path.Join(o.ExternalURL.Path, homePage), http.StatusFound)
	})
	// The console library examples at 'console_libraries/prom.lib' still depend on old asset files being served under `classic`.
	router.Get("/classic/static/*filepath", func(w http.ResponseWriter, r *http.Request) {

What did you expect to see?

Going to a route with a prefix should redirect to {prefix}/graph

What did you see instead? Under which circumstances?

Going to a route with a prefix redirects to /graph without prefix applied and therefore redirects to an invalid route

System information

No response

Prometheus version

version	2.41.0
revision	c0d8a56c69014279464c0e15d8bfb0e153af0dab
branch	HEAD
buildUser	root@d20a03e77067
buildDate	20221220-10:40:45
goVersion	go1.19.4

Prometheus configuration file

--web.route-prefix=/sys/metrics
--web.external-url=https://myurl.com

Alertmanager version

No response

Alertmanager configuration file

No response

Logs

No response

[roidelapluie]

It looks like you should use --web.external-url=https://myurl.com/sys/metrics

[scott-david-walker]

I still don't feel that this is a valid response. We seem to be using the parameters in the correct way and the only thing not working is the redirect. It feels strange to have the option of using the prefix only for the response to be 'don't use the prefix'.

I do see this as a bug so prefer to have the issue reopened.

We're using this helm chart and doing the proposed workaround causes issues finding the readiness and liveness endpoint.