diff --git a/wgengine/magicsock/debugknobs.go b/wgengine/magicsock/debugknobs.go
index 824c677f6..4f0d234a8 100644
--- a/wgengine/magicsock/debugknobs.go
+++ b/wgengine/magicsock/debugknobs.go
@@ -55,7 +55,8 @@
 	// sockets.
 	debugEnablePMTUD = envknob.RegisterOptBool("TS_DEBUG_ENABLE_PMTUD")
 	// debugPMTUD prints extra debugging about peer MTU path discovery.
-	debugPMTUD = envknob.RegisterBool("TS_DEBUG_PMTUD")
+	debugPMTUD            = envknob.RegisterBool("TS_DEBUG_PMTUD")
+	debugPreferDERPRegion = envknob.RegisterInt("TS_DEBUG_PREFER_DERP_REGION")
 	// 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 de49865bf..de4999d4e 100644
--- a/wgengine/magicsock/debugknobs_stubs.go
+++ b/wgengine/magicsock/debugknobs_stubs.go
@@ -28,3 +28,4 @@ func debugEnablePMTUD() opt.Bool       { return "" }
 func debugRingBufferMaxSizeBytes() int { return 0 }
 func inTest() bool                     { return false }
 func debugPeerMap() bool               { return false }
+func debugPreferDERPRegion() int       { return 0 }
diff --git a/wgengine/magicsock/magicsock.go b/wgengine/magicsock/magicsock.go
index 35755e03d..9fc80c2d4 100644
--- a/wgengine/magicsock/magicsock.go
+++ b/wgengine/magicsock/magicsock.go
@@ -649,6 +649,10 @@ func (c *Conn) updateNetInfo(ctx context.Context) (*netcheck.Report, error) {
 	ni.WorkingUDP.Set(report.UDP)
 	ni.WorkingICMPv4.Set(report.ICMPv4)
 	ni.PreferredDERP = report.PreferredDERP
+	preferDERP := debugPreferDERPRegion()
+	if preferDERP > 0 {
+		ni.PreferredDERP = preferDERP
+	}
 
 	if ni.PreferredDERP == 0 {
 		// Perhaps UDP is blocked. Pick a deterministic but arbitrary