From faa891c1f2ff759f4e5cbc158f310b3201d91b59 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Thu, 19 Aug 2021 08:36:13 -0700
Subject: [PATCH] client/tailscale,ipn/localapi: warn on tailscale/tailscaled
 version skew

Fixes #1905

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
---
 client/tailscale/tailscale.go | 5 +++++
 ipn/localapi/localapi.go      | 2 ++
 2 files changed, 7 insertions(+)

diff --git a/client/tailscale/tailscale.go b/client/tailscale/tailscale.go
index fb4f7153b..e5e22020a 100644
--- a/client/tailscale/tailscale.go
+++ b/client/tailscale/tailscale.go
@@ -17,6 +17,7 @@ import (
 	"net"
 	"net/http"
 	"net/url"
+	"os"
 	"strconv"
 	"strings"
 	"time"
@@ -28,6 +29,7 @@ import (
 	"tailscale.com/paths"
 	"tailscale.com/safesocket"
 	"tailscale.com/tailcfg"
+	"tailscale.com/version"
 )
 
 // TailscaledSocket is the tailscaled Unix socket.
@@ -94,6 +96,9 @@ func send(ctx context.Context, method, path string, wantStatus int, body io.Read
 		return nil, err
 	}
 	defer res.Body.Close()
+	if server := res.Header.Get("Tailscale-Version"); server != version.Long {
+		fmt.Fprintf(os.Stderr, "Warning: client version %q != tailscaled server version %q\n", version.Long, server)
+	}
 	slurp, err := ioutil.ReadAll(res.Body)
 	if err != nil {
 		return nil, err
diff --git a/ipn/localapi/localapi.go b/ipn/localapi/localapi.go
index 6a5116d90..e424dc5d7 100644
--- a/ipn/localapi/localapi.go
+++ b/ipn/localapi/localapi.go
@@ -30,6 +30,7 @@ import (
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
+	"tailscale.com/version"
 )
 
 func randHex(n int) string {
@@ -64,6 +65,7 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "server has no local backend", http.StatusInternalServerError)
 		return
 	}
+	w.Header().Set("Tailscale-Version", version.Long)
 	if h.RequiredPassword != "" {
 		_, pass, ok := r.BasicAuth()
 		if !ok {