From 42dac7c5c2598c27918528daa4405fbb79e5f0f5 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Mon, 8 Jul 2024 07:21:21 -0700
Subject: [PATCH] wgengine/magicsock: add debug envknob for injecting an
 endpoint

For testing. Lee wants to play with 'AWS Global Accelerator Custom
Routing with Amazon Elastic Kubernetes Service'. If this works well
enough, we can promote it.

Updates #12578

Change-Id: I5018347ed46c15c9709910717d27305d0aedf8f4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
---
 tailcfg/tailcfg.go                     |  3 +++
 wgengine/magicsock/debugknobs.go       | 19 +++++++++++++++++++
 wgengine/magicsock/debugknobs_stubs.go |  7 ++++++-
 wgengine/magicsock/magicsock.go        |  6 ++++++
 4 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/tailcfg/tailcfg.go b/tailcfg/tailcfg.go
index 9276cebc2..24af5137c 100644
--- a/tailcfg/tailcfg.go
+++ b/tailcfg/tailcfg.go
@@ -1184,6 +1184,7 @@ const (
 	EndpointSTUN           = EndpointType(2)
 	EndpointPortmapped     = EndpointType(3)
 	EndpointSTUN4LocalPort = EndpointType(4) // hard NAT: STUN'ed IPv4 address + local fixed port
+	EndpointExplicitConf   = EndpointType(5) // explicitly configured (routing to be done by client)
 )
 
 func (et EndpointType) String() string {
@@ -1198,6 +1199,8 @@ func (et EndpointType) String() string {
 		return "portmap"
 	case EndpointSTUN4LocalPort:
 		return "stun4localport"
+	case EndpointExplicitConf:
+		return "explicitconf"
 	}
 	return "other"
 }
diff --git a/wgengine/magicsock/debugknobs.go b/wgengine/magicsock/debugknobs.go
index 867109a42..90b49d86f 100644
--- a/wgengine/magicsock/debugknobs.go
+++ b/wgengine/magicsock/debugknobs.go
@@ -6,6 +6,10 @@
 package magicsock
 
 import (
+	"log"
+	"net/netip"
+	"sync"
+
 	"tailscale.com/envknob"
 )
 
@@ -68,3 +72,18 @@ var (
 // checked every time at runtime, because tests set this after program
 // startup.
 func inTest() bool { return envknob.Bool("IN_TS_TEST") }
+
+// pretendpoint returns TS_DEBUG_PRETENDPOINT as an AddrPort, if set.
+// See https://github.com/tailscale/tailscale/issues/12578 and
+// https://github.com/tailscale/tailscale/pull/12735.
+var pretendpoint = sync.OnceValue(func() (ap netip.AddrPort) {
+	s := envknob.String("TS_DEBUG_PRETENDPOINT")
+	if s == "" {
+		return
+	}
+	ap, err := netip.ParseAddrPort(s)
+	if err != nil {
+		log.Printf("ignoring invalid TS_DEBUG_PRETENDPOINT %q: %v", s, err)
+	}
+	return ap
+})
diff --git a/wgengine/magicsock/debugknobs_stubs.go b/wgengine/magicsock/debugknobs_stubs.go
index 7ee316e0f..7a2f60f6b 100644
--- a/wgengine/magicsock/debugknobs_stubs.go
+++ b/wgengine/magicsock/debugknobs_stubs.go
@@ -5,7 +5,11 @@
 
 package magicsock
 
-import "tailscale.com/types/opt"
+import (
+	"net/netip"
+
+	"tailscale.com/types/opt"
+)
 
 // All knobs are disabled on iOS and Wasm.
 //
@@ -26,3 +30,4 @@ func debugEnablePMTUD() opt.Bool       { return "" }
 func debugRingBufferMaxSizeBytes() int { return 0 }
 func inTest() bool                     { return false }
 func debugPeerMap() bool               { return false }
+func pretendpoint() netip.AddrPort     { return netip.AddrPort{} }
diff --git a/wgengine/magicsock/magicsock.go b/wgengine/magicsock/magicsock.go
index 30181f8e1..3ccf30c5e 100644
--- a/wgengine/magicsock/magicsock.go
+++ b/wgengine/magicsock/magicsock.go
@@ -919,6 +919,12 @@ func (c *Conn) determineEndpoints(ctx context.Context) ([]tailcfg.Endpoint, erro
 		}
 	}
 
+	// Temporarily (2024-07-08) during investigations, allow setting
+	// a pretend endpoint for testing NAT traversal scenarios.
+	// TODO(bradfitz): probably promote this to the config file.
+	// https://github.com/tailscale/tailscale/issues/12578
+	addAddr(pretendpoint(), tailcfg.EndpointExplicitConf)
+
 	// Update our set of endpoints by adding any endpoints that we
 	// previously found but haven't expired yet. This also updates the
 	// cache with the set of endpoints discovered in this function.