From 252a878a2c0e4b5961f04c765ca969224a8e0150 Mon Sep 17 00:00:00 2001
From: Sven Nebel <nebel.sven@gmail.com>
Date: Wed, 11 Sep 2019 13:36:17 +0100
Subject: [PATCH] Adding web.external-url and web.route-prefix flags (#515)

Signed-off-by: Sven Nebel <nebel.sven@gmail.com>
---
 go.mod       |   2 +-
 main.go      | 101 +++++++++++++++++++++++++++++++++++++++++++++------
 main_test.go |  53 +++++++++++++++++++++++++++
 3 files changed, 144 insertions(+), 12 deletions(-)

diff --git a/go.mod b/go.mod
index bfae6ec..b60028b 100644
--- a/go.mod
+++ b/go.mod
@@ -6,7 +6,7 @@ require (
 	github.com/go-kit/kit v0.8.0
 	github.com/kr/pretty v0.1.0 // indirect
 	github.com/miekg/dns v1.1.14
-	github.com/pkg/errors v0.8.1 // indirect
+	github.com/pkg/errors v0.8.1
 	github.com/prometheus/client_golang v1.0.0
 	github.com/prometheus/client_model v0.0.0-20190129233127-fd36f4220a90
 	github.com/prometheus/common v0.6.0
diff --git a/main.go b/main.go
index e4f9b75..3d95971 100644
--- a/main.go
+++ b/main.go
@@ -18,16 +18,21 @@ import (
 	"context"
 	"fmt"
 	"html"
+	"net"
 	"net/http"
 	_ "net/http/pprof"
+	"net/url"
 	"os"
 	"os/signal"
+	"path"
 	"strconv"
+	"strings"
 	"syscall"
 	"time"
 
 	"github.com/go-kit/kit/log"
 	"github.com/go-kit/kit/log/level"
+	"github.com/pkg/errors"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/prometheus/common/expfmt"
@@ -51,6 +56,8 @@ var (
 	timeoutOffset = kingpin.Flag("timeout-offset", "Offset to subtract from timeout in seconds.").Default("0.5").Float64()
 	configCheck   = kingpin.Flag("config.check", "If true validate the config file and then exit.").Default().Bool()
 	historyLimit  = kingpin.Flag("history.limit", "The maximum amount of items to keep in the history.").Default("100").Uint()
+	externalURL   = kingpin.Flag("web.external-url", "The URL under which Blackbox exporter is externally reachable (for example, if Blackbox exporter is served via a reverse proxy). Used for generating relative and absolute links back to Blackbox exporter itself. If the URL has a path portion, it will be used to prefix all HTTP endpoints served by Blackbox exporter. If omitted, relevant URL components will be derived automatically.").PlaceHolder("<url>").String()
+	routePrefix   = kingpin.Flag("web.route-prefix", "Prefix for the internal routes of web endpoints. Defaults to path of --web.external-url.").PlaceHolder("<path>").String()
 
 	Probers = map[string]prober.ProbeFn{
 		"http": prober.ProbeHTTP,
@@ -217,6 +224,28 @@ func run() int {
 
 	level.Info(logger).Log("msg", "Loaded config file")
 
+	// Infer or set Blackbox exporter externalURL
+	beURL, err := computeExternalURL(*externalURL, *listenAddress)
+	if err != nil {
+		level.Error(logger).Log("msg", "failed to determine external URL", "err", err)
+		return 1
+	}
+	level.Debug(logger).Log("externalURL", beURL.String())
+
+	// Default -web.route-prefix to path of -web.external-url.
+	if *routePrefix == "" {
+		*routePrefix = beURL.Path
+	}
+
+	// routePrefix must always be at least '/'.
+	*routePrefix = "/" + strings.Trim(*routePrefix, "/")
+	// routePrefix requires path to have trailing "/" in order
+	// for browsers to interpret the path-relative path correctly, instead of stripping it.
+	if *routePrefix != "/" {
+		*routePrefix = *routePrefix + "/"
+	}
+	level.Debug(logger).Log("routePrefix", *routePrefix)
+
 	hup := make(chan os.Signal, 1)
 	reloadCh := make(chan chan error)
 	signal.Notify(hup, syscall.SIGHUP)
@@ -241,7 +270,19 @@ func run() int {
 		}
 	}()
 
-	http.HandleFunc("/-/reload",
+	// Match Prometheus behaviour and redirect over externalURL for root path only
+	// if routePrefix is different than "/"
+	if *routePrefix != "/" {
+		http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+			if r.URL.Path != "/" {
+				http.NotFound(w, r)
+				return
+			}
+			http.Redirect(w, r, beURL.String(), http.StatusFound)
+		})
+	}
+
+	http.HandleFunc(path.Join(*routePrefix, "/-/reload"),
 		func(w http.ResponseWriter, r *http.Request) {
 			if r.Method != "POST" {
 				w.WriteHeader(http.StatusMethodNotAllowed)
@@ -255,23 +296,23 @@ func run() int {
 				http.Error(w, fmt.Sprintf("failed to reload config: %s", err), http.StatusInternalServerError)
 			}
 		})
-	http.Handle("/metrics", promhttp.Handler())
-	http.HandleFunc("/probe", func(w http.ResponseWriter, r *http.Request) {
+	http.Handle(path.Join(*routePrefix, "/metrics"), promhttp.Handler())
+	http.HandleFunc(path.Join(*routePrefix, "/probe"), func(w http.ResponseWriter, r *http.Request) {
 		sc.Lock()
 		conf := sc.C
 		sc.Unlock()
 		probeHandler(w, r, conf, logger, rh)
 	})
-	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+	http.HandleFunc(*routePrefix, func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "text/html")
 		w.Write([]byte(`<html>
     <head><title>Blackbox Exporter</title></head>
     <body>
     <h1>Blackbox Exporter</h1>
-    <p><a href="/probe?target=prometheus.io&module=http_2xx">Probe prometheus.io for http_2xx</a></p>
-    <p><a href="/probe?target=prometheus.io&module=http_2xx&debug=true">Debug probe prometheus.io for http_2xx</a></p>
-    <p><a href="/metrics">Metrics</a></p>
-    <p><a href="/config">Configuration</a></p>
+    <p><a href="probe?target=prometheus.io&module=http_2xx">Probe prometheus.io for http_2xx</a></p>
+    <p><a href="probe?target=prometheus.io&module=http_2xx&debug=true">Debug probe prometheus.io for http_2xx</a></p>
+    <p><a href="metrics">Metrics</a></p>
+    <p><a href="config">Configuration</a></p>
     <h2>Recent Probes</h2>
     <table border='1'><tr><th>Module</th><th>Target</th><th>Result</th><th>Debug</th>`))
 
@@ -283,7 +324,7 @@ func run() int {
 			if !r.success {
 				success = "<strong>Failure</strong>"
 			}
-			fmt.Fprintf(w, "<tr><td>%s</td><td>%s</td><td>%s</td><td><a href='logs?id=%d'>Logs</a></td></td>",
+			fmt.Fprintf(w, "<tr><td>%s</td><td>%s</td><td>%s</td><td><a href='"+path.Join(*routePrefix, "/logs")+"?id=%d'>Logs</a></td></td>",
 				html.EscapeString(r.moduleName), html.EscapeString(r.target), success, r.id)
 		}
 
@@ -291,7 +332,7 @@ func run() int {
     </html>`))
 	})
 
-	http.HandleFunc("/logs", func(w http.ResponseWriter, r *http.Request) {
+	http.HandleFunc(path.Join(*routePrefix, "/logs"), func(w http.ResponseWriter, r *http.Request) {
 		id, err := strconv.ParseInt(r.URL.Query().Get("id"), 10, 64)
 		if err != nil {
 			http.Error(w, "Invalid probe id", 500)
@@ -306,7 +347,7 @@ func run() int {
 		w.Write([]byte(result.debugOutput))
 	})
 
-	http.HandleFunc("/config", func(w http.ResponseWriter, r *http.Request) {
+	http.HandleFunc(path.Join(*routePrefix, "/config"), func(w http.ResponseWriter, r *http.Request) {
 		sc.RLock()
 		c, err := yaml.Marshal(sc.C)
 		sc.RUnlock()
@@ -366,3 +407,41 @@ func getTimeout(r *http.Request, module config.Module, offset float64) (timeoutS
 
 	return timeoutSeconds, nil
 }
+
+func startsOrEndsWithQuote(s string) bool {
+	return strings.HasPrefix(s, "\"") || strings.HasPrefix(s, "'") ||
+		strings.HasSuffix(s, "\"") || strings.HasSuffix(s, "'")
+}
+
+// computeExternalURL computes a sanitized external URL from a raw input. It infers unset
+// URL parts from the OS and the given listen address.
+func computeExternalURL(u, listenAddr string) (*url.URL, error) {
+	if u == "" {
+		hostname, err := os.Hostname()
+		if err != nil {
+			return nil, err
+		}
+		_, port, err := net.SplitHostPort(listenAddr)
+		if err != nil {
+			return nil, err
+		}
+		u = fmt.Sprintf("http://%s:%s/", hostname, port)
+	}
+
+	if startsOrEndsWithQuote(u) {
+		return nil, errors.New("URL must not begin or end with quotes")
+	}
+
+	eu, err := url.Parse(u)
+	if err != nil {
+		return nil, err
+	}
+
+	ppref := strings.TrimRight(eu.Path, "/")
+	if ppref != "" && !strings.HasPrefix(ppref, "/") {
+		ppref = "/" + ppref
+	}
+	eu.Path = ppref
+
+	return eu, nil
+}
diff --git a/main_test.go b/main_test.go
index c2341ec..f9d536b 100644
--- a/main_test.go
+++ b/main_test.go
@@ -137,3 +137,56 @@ func TestTimeoutIsSetCorrectly(t *testing.T) {
 		}
 	}
 }
+
+func TestComputeExternalURL(t *testing.T) {
+	tests := []struct {
+		input string
+		valid bool
+	}{
+		{
+			input: "",
+			valid: true,
+		},
+		{
+			input: "http://proxy.com/prometheus",
+			valid: true,
+		},
+		{
+			input: "'https://url/prometheus'",
+			valid: false,
+		},
+		{
+			input: "'relative/path/with/quotes'",
+			valid: false,
+		},
+		{
+			input: "http://alertmanager.company.com",
+			valid: true,
+		},
+		{
+			input: "https://double--dash.de",
+			valid: true,
+		},
+		{
+			input: "'http://starts/with/quote",
+			valid: false,
+		},
+		{
+			input: "ends/with/quote\"",
+			valid: false,
+		},
+	}
+
+	for _, test := range tests {
+		_, err := computeExternalURL(test.input, "0.0.0.0:9090")
+		if test.valid {
+			if err != nil {
+				t.Errorf("unexpected error %v", err)
+			}
+		} else {
+			if err == nil {
+				t.Errorf("expected error computing %s got none", test.input)
+			}
+		}
+	}
+}
-- 
2.25.1