Define an "anchor" as a statically defined device that serves as a network lookup point.

include/ZeroTierOne.h

@@ -164,6 +164,11 @@ extern "C" {
  */
 #define ZT_NETWORK_STATIC_DEVICE_IS_RELAY 0x0001
 
+/**
+ * This device is a network "anchor," meaning it is used for things like multicast GATHER
+ */
+#define ZT_NETWORK_STATIC_DEVICE_IS_ANCHOR 0x0002
+
 /**
  * A null/empty sockaddr (all zero) to signify an unspecified socket address
  */