diff --git a/src/trunnel/ed25519_cert.trunnel b/src/trunnel/ed25519_cert.trunnel
index e424ce5464..8d6483d558 100644
--- a/src/trunnel/ed25519_cert.trunnel
+++ b/src/trunnel/ed25519_cert.trunnel
@@ -28,6 +28,12 @@ const LS_IPV6 = 0x01;
 const LS_LEGACY_ID = 0x02;
 const LS_ED25519_ID = 0x03;
 
+// XXX hs_link_specifier_dup() violates the opaqueness of link_specifier_t by
+//  taking its sizeof(). If we ever want to turn on TRUNNEL_OPAQUE, or
+//  if we ever make link_specifier contain other types, we will
+//  need to refactor that function to do the copy by encoding and decoding the
+//  object.
+
 // amended from tor.trunnel
 struct link_specifier {
   u8 ls_type;