From fb9d9ba86e42680cde20c890de8857cbfe40f2c3 Mon Sep 17 00:00:00 2001
From: Jordan Whited <jordan@tailscale.com>
Date: Wed, 10 Sep 2025 16:48:40 -0700
Subject: [PATCH] wgengine/magicsock: add TS_DEBUG_NEVER_DIRECT_UDP debug knob
 (#17094)

Updates tailscale/corp#30903

Signed-off-by: Jordan Whited <jordan@tailscale.com>
---
 wgengine/magicsock/debugknobs.go       | 3 +++
 wgengine/magicsock/debugknobs_stubs.go | 1 +
 wgengine/magicsock/endpoint.go         | 3 +++
 3 files changed, 7 insertions(+)

diff --git a/wgengine/magicsock/debugknobs.go b/wgengine/magicsock/debugknobs.go
index f8fd9f040..b0a47ff87 100644
--- a/wgengine/magicsock/debugknobs.go
+++ b/wgengine/magicsock/debugknobs.go
@@ -62,6 +62,9 @@ var (
 	//
 	//lint:ignore U1000 used on Linux/Darwin only
 	debugPMTUD = envknob.RegisterBool("TS_DEBUG_PMTUD")
+	// debugNeverDirectUDP disables the use of direct UDP connections, forcing
+	// all peer communication over DERP or peer relay.
+	debugNeverDirectUDP = envknob.RegisterBool("TS_DEBUG_NEVER_DIRECT_UDP")
 	// Hey you! Adding a new debugknob? Make sure to stub it out in the
 	// debugknobs_stubs.go file too.
 )
diff --git a/wgengine/magicsock/debugknobs_stubs.go b/wgengine/magicsock/debugknobs_stubs.go
index 336d7baa1..7dee1d6b0 100644
--- a/wgengine/magicsock/debugknobs_stubs.go
+++ b/wgengine/magicsock/debugknobs_stubs.go
@@ -31,3 +31,4 @@ func debugRingBufferMaxSizeBytes() int { return 0 }
 func inTest() bool                     { return false }
 func debugPeerMap() bool               { return false }
 func pretendpoints() []netip.AddrPort  { return []netip.AddrPort{} }
+func debugNeverDirectUDP() bool        { return false }
diff --git a/wgengine/magicsock/endpoint.go b/wgengine/magicsock/endpoint.go
index b8778b8d8..1f36aabd3 100644
--- a/wgengine/magicsock/endpoint.go
+++ b/wgengine/magicsock/endpoint.go
@@ -1286,6 +1286,9 @@ func (de *endpoint) startDiscoPingLocked(ep epAddr, now mono.Time, purpose disco
 	if runtime.GOOS == "js" {
 		return
 	}
+	if debugNeverDirectUDP() && !ep.vni.IsSet() && ep.ap.Addr() != tailcfg.DerpMagicIPAddr {
+		return
+	}
 	epDisco := de.disco.Load()
 	if epDisco == nil {
 		return