feat: add server flag to force DERP to use always websockets (#9238)

cli/netcheck_test.go

@@ -31,7 +31,7 @@ func TestNetcheck(t *testing.T) {
 	require.NoError(t, json.Unmarshal(b, &report))
 
 	assert.True(t, report.Healthy)
-	require.Len(t, report.Regions, 1+5) // 1 built-in region + 5 STUN regions by default
+	require.Len(t, report.Regions, 1+1) // 1 built-in region + 1 test-managed STUN region
 	for _, v := range report.Regions {
 		require.Len(t, v.NodeReports, len(v.Region.Nodes))
 	}

cli/testdata/coder_server_--help.golden

@@ -172,6 +172,13 @@ backed by Tailscale and WireGuard.
           URL to fetch a DERP mapping on startup. See:
           https://tailscale.com/kb/1118/custom-derp-servers/.
 
+      --derp-force-websockets bool, $CODER_DERP_FORCE_WEBSOCKETS
+          Force clients and agents to always use WebSocket to connect to DERP
+          relay servers. By default, DERP uses `Upgrade: derp`, which may cause
+          issues with some reverse proxies. Clients may automatically fallback
+          to WebSocket if they detect an issue with `Upgrade: derp`, but this
+          does not work in all situations.
+
       --derp-server-enable bool, $CODER_DERP_SERVER_ENABLE (default: true)
           Whether to enable or disable the embedded DERP relay server.
 

cli/testdata/server-config.yaml.golden

@@ -136,6 +136,12 @@ networking:
     # this change has been made, but new connections will still be proxied regardless.
     # (default: <unset>, type: bool)
     blockDirect: false
+    # Force clients and agents to always use WebSocket to connect to DERP relay
+    # servers. By default, DERP uses `Upgrade: derp`, which may cause issues with some
+    # reverse proxies. Clients may automatically fallback to WebSocket if they detect
+    # an issue with `Upgrade: derp`, but this does not work in all situations.
+    # (default: <unset>, type: bool)
+    forceWebSockets: false
     # URL to fetch a DERP mapping on startup. See:
     # https://tailscale.com/kb/1118/custom-derp-servers/.
     # (default: <unset>, type: string)