Only use X-Forwarded-For if not behind proxy

Since you can't trust it when it comes directly from clients.

boringproxy.go

@@ -52,6 +52,7 @@ func Listen() {
 	httpsPort := flagSet.Int("https-port", 443, "HTTPS (secure) port")
 	allowHttp := flagSet.Bool("allow-http", false, "Allow unencrypted (HTTP) requests")
 	publicIp := flagSet.String("public-ip", "", "Public IP")
+	behindProxy := flagSet.Bool("behind-proxy", false, "Whether we're running behind another reverse proxy")
 	err := flagSet.Parse(os.Args[2:])
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "%s: parsing flags: %s\n", os.Args[0], err)
@@ -273,7 +274,7 @@ func Listen() {
 				return
 			}
 
-			proxyRequest(w, r, tunnel, httpClient, tunnel.TunnelPort)
+			proxyRequest(w, r, tunnel, httpClient, tunnel.TunnelPort, *behindProxy)
 		}
 	})
 

client.go

@@ -30,16 +30,18 @@ type Client struct {
 	cancelFuncs      map[string]context.CancelFunc
 	cancelFuncsMutex *sync.Mutex
 	certConfig       *certmagic.Config
+	behindProxy      bool
 }
 
 type ClientConfig struct {
-	ServerAddr string `json:"serverAddr,omitempty"`
-	Token      string `json:"token,omitempty"`
-	ClientName string `json:"clientName,omitempty"`
-	User       string `json:"user,omitempty"`
-	CertDir    string `json:"certDir,omitempty"`
-	AcmeEmail  string `json:"acmeEmail,omitempty"`
-	DnsServer  string `json:"dnsServer,omitempty"`
+	ServerAddr  string `json:"serverAddr,omitempty"`
+	Token       string `json:"token,omitempty"`
+	ClientName  string `json:"clientName,omitempty"`
+	User        string `json:"user,omitempty"`
+	CertDir     string `json:"certDir,omitempty"`
+	AcmeEmail   string `json:"acmeEmail,omitempty"`
+	DnsServer   string `json:"dnsServer,omitempty"`
+	BehindProxy bool   `json:"behindProxy,omitempty"`
 }
 
 func NewClient(config *ClientConfig) (*Client, error) {
@@ -102,6 +104,7 @@ func NewClient(config *ClientConfig) (*Client, error) {
 		cancelFuncs:      cancelFuncs,
 		cancelFuncsMutex: cancelFuncsMutex,
 		certConfig:       certConfig,
+		behindProxy:      config.BehindProxy,
 	}, nil
 }
 
@@ -294,7 +297,7 @@ func (c *Client) BoreTunnel(ctx context.Context, tunnel Tunnel) error {
 		httpMux := http.NewServeMux()
 
 		httpMux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
-			proxyRequest(w, r, tunnel, c.httpClient, tunnel.ClientPort)
+			proxyRequest(w, r, tunnel, c.httpClient, tunnel.ClientPort, c.behindProxy)
 		})
 
 		httpServer := &http.Server{

cmd/boringproxy/main.go

@@ -46,6 +46,7 @@ func main() {
 		certDir := flagSet.String("cert-dir", "", "TLS cert directory")
 		acmeEmail := flagSet.String("acme-email", "", "Email for ACME (ie Let's Encrypt)")
 		dnsServer := flagSet.String("dns-server", "", "Custom DNS server")
+		behindProxy := flagSet.Bool("behind-proxy", false, "Whether we're running behind another reverse proxy")
 
 		err := flagSet.Parse(os.Args[2:])
 		if err != nil {
@@ -65,13 +66,14 @@ func main() {
 		}
 
 		config := &boringproxy.ClientConfig{
-			ServerAddr: *server,
-			Token:      *token,
-			ClientName: *name,
-			User:       *user,
-			CertDir:    *certDir,
-			AcmeEmail:  *acmeEmail,
-			DnsServer:  *dnsServer,
+			ServerAddr:  *server,
+			Token:       *token,
+			ClientName:  *name,
+			User:        *user,
+			CertDir:     *certDir,
+			AcmeEmail:   *acmeEmail,
+			DnsServer:   *dnsServer,
+			BehindProxy: *behindProxy,
 		}
 
 		ctx := context.Background()

http_proxy.go

@@ -8,7 +8,7 @@ import (
 	"time"
 )
 
-func proxyRequest(w http.ResponseWriter, r *http.Request, tunnel Tunnel, httpClient *http.Client, port int) {
+func proxyRequest(w http.ResponseWriter, r *http.Request, tunnel Tunnel, httpClient *http.Client, port int, behindProxy bool) {
 
 	if tunnel.AuthUsername != "" || tunnel.AuthPassword != "" {
 		username, password, ok := r.BasicAuth()
@@ -54,15 +54,16 @@ func proxyRequest(w http.ResponseWriter, r *http.Request, tunnel Tunnel, httpCli
 	// TODO: Handle IPv6 addresses
 	addrParts := strings.Split(r.RemoteAddr, ":")
 	remoteIp := addrParts[0]
+	xForwardedFor := remoteIp
 
-	xForwardedFor := downstreamReqHeaders.Get("X-Forwarded-For")
-	if xForwardedFor == "" {
-		xForwardedFor = remoteIp
-	} else {
-		xForwardedFor = xForwardedFor + ", " + remoteIp
+	if behindProxy {
+		xForwardedFor := downstreamReqHeaders.Get("X-Forwarded-For")
+		if xForwardedFor != "" {
+			xForwardedFor = xForwardedFor + ", " + remoteIp
+		}
 	}
-	upstreamReq.Header.Set("X-Forwarded-For", xForwardedFor)
 
+	upstreamReq.Header.Set("X-Forwarded-For", xForwardedFor)
 	upstreamReq.Header.Set("Forwarded", fmt.Sprintf("for=%s", remoteIp))
 
 	upstreamReq.Host = fmt.Sprintf("%s:%d", tunnel.ClientAddress, tunnel.ClientPort)