As suggested by @rgacogne and verified by myself v6 actually does have TFO.

diff --git a/pdns/lwres.cc b/pdns/lwres.cc
index 1d981fede8677842e28d14c2c45fb665cb961deb..af5f6515131a403bb9eb911853d1d77b54fdb247 100644 (file)
--- a/pdns/lwres.cc
+++ b/pdns/lwres.cc
@@ -327,8 +327,7 @@ LWResult::Result asyncresolve(const ComboAddress& ip, const DNSName& domain, int
       Socket s(ip.sin4.sin_family, SOCK_STREAM);
 
       s.setNonBlocking();
-      // v6 tcp does not seem to have fast open
-      if (ip.sin4.sin_family == AF_INET && SyncRes::s_tcp_fast_open_connect) {
+      if (SyncRes::s_tcp_fast_open_connect) {
         try {
           s.setFastOpenConnect();
         }