cmd/proxy-to-grafana: use grafana's authproxy to log in tailnet users (#4208)

Signed-off-by: Nick O'Neill <nick@tailscale.com>

cmd/proxy-to-grafana/proxy-to-grafana.go

@@ -0,0 +1,154 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// proxy-to-grafana is a reverse proxy which identifies users based on their
+// originating Tailscale identity and maps them to corresponding Grafana
+// users, creating them if needed.
+//
+// It uses Grafana's AuthProxy feature:
+// https://grafana.com/docs/grafana/latest/auth/auth-proxy/
+//
+// Set the TS_AUTHKEY environment variable to have this server automatically
+// join your tailnet, or look for the logged auth link on first start.
+//
+// Use this Grafana configuration to enable the auth proxy:
+//
+// ```
+// [auth.proxy]
+// enabled = true
+// header_name = X-WEBAUTH-USER
+// header_property = username
+// auto_sign_up = true
+// whitelist = 127.0.0.1
+// headers = Name:X-WEBAUTH-NAME
+// enable_login_token = true
+// ```
+package main
+
+import (
+	"context"
+	"crypto/tls"
+	"flag"
+	"fmt"
+	"log"
+	"net"
+	"net/http"
+	"net/http/httputil"
+	"net/url"
+	"strings"
+	"time"
+
+	"tailscale.com/client/tailscale"
+	"tailscale.com/tailcfg"
+	"tailscale.com/tsnet"
+)
+
+var (
+	hostname     = flag.String("hostname", "", "Tailscale hostname to serve on, used as the base name for MagicDNS or subdomain in your domain alias for HTTPS.")
+	backendAddr  = flag.String("backend-addr", "", "Address of the Grafana server served over HTTP, in host:port format. Typically localhost:nnnn.")
+	tailscaleDir = flag.String("state-dir", "./", "Alternate directory to use for Tailscale state storage. If empty, a default is used.")
+	useHTTPS     = flag.Bool("use-https", false, "Serve over HTTPS via your *.ts.net subdomain if enabled in Tailscale admin.")
+)
+
+func main() {
+	flag.Parse()
+	if *hostname == "" || strings.Contains(*hostname, ".") {
+		log.Fatal("missing or invalid --hostname")
+	}
+	if *backendAddr == "" {
+		log.Fatal("missing --backend-addr")
+	}
+	ts := &tsnet.Server{
+		Dir:      *tailscaleDir,
+		Hostname: *hostname,
+	}
+
+	url, err := url.Parse(fmt.Sprintf("http://%s", *backendAddr))
+	if err != nil {
+		log.Fatalf("couldn't parse backend address: %v", err)
+	}
+
+	proxy := httputil.NewSingleHostReverseProxy(url)
+	originalDirector := proxy.Director
+	proxy.Director = func(req *http.Request) {
+		originalDirector(req)
+		modifyRequest(req)
+	}
+
+	var ln net.Listener
+	if *useHTTPS {
+		ln, err = ts.Listen("tcp", ":443")
+		ln = tls.NewListener(ln, &tls.Config{
+			GetCertificate: tailscale.GetCertificate,
+		})
+
+		go func() {
+			// wait for tailscale to start before trying to fetch cert names
+			for i := 0; i < 60; i++ {
+				st, err := tailscale.Status(context.Background())
+				if err != nil {
+					log.Fatal(err)
+				}
+				log.Printf("tailscale status: %v", st.BackendState)
+				if st.BackendState == "Running" {
+					break
+				}
+				time.Sleep(time.Second)
+			}
+
+			l80, err := ts.Listen("tcp", ":80")
+			if err != nil {
+				log.Fatal(err)
+			}
+			name, ok := tailscale.ExpandSNIName(context.Background(), *hostname)
+			if !ok {
+				log.Fatalf("can't get hostname for https redirect")
+			}
+			if err := http.Serve(l80, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				http.Redirect(w, r, fmt.Sprintf("https://%s", name), http.StatusMovedPermanently)
+			})); err != nil {
+				log.Fatal(err)
+			}
+		}()
+	} else {
+		ln, err = ts.Listen("tcp", ":80")
+	}
+	if err != nil {
+		log.Fatal(err)
+	}
+	log.Printf("proxy-to-grafana running at %v, proxying to %v", ln.Addr(), *backendAddr)
+	log.Fatal(http.Serve(ln, proxy))
+}
+
+func modifyRequest(req *http.Request) {
+	// with enable_login_token set to true, we get a cookie that handles
+	// auth for paths that are not /login
+	if req.URL.Path != "/login" {
+		return
+	}
+
+	user, err := getTailscaleUser(req.Context(), req.RemoteAddr)
+	if err != nil {
+		log.Printf("error getting Tailscale user: %v", err)
+		return
+	}
+
+	req.Header.Set("X-Webauth-User", user.LoginName)
+	req.Header.Set("X-Webauth-Name", user.DisplayName)
+}
+
+func getTailscaleUser(ctx context.Context, ipPort string) (*tailcfg.UserProfile, error) {
+	whois, err := tailscale.WhoIs(ctx, ipPort)
+	if err != nil {
+		return nil, fmt.Errorf("failed to identify remote host: %w", err)
+	}
+	if len(whois.Node.Tags) != 0 {
+		return nil, fmt.Errorf("tagged nodes are not users")
+	}
+	if whois.UserProfile == nil || whois.UserProfile.LoginName == "" {
+		return nil, fmt.Errorf("failed to identify remote user")
+	}
+
+	return whois.UserProfile, nil
+}