From 6063b4f80f2e2dca5f2c6d35d9dbae64beee569a Mon Sep 17 00:00:00 2001
From: Otto <otto.moerbeek@open-xchange.com>
Date: Fri, 26 Nov 2021 13:28:53 +0100
Subject: [PATCH] Prepend labels starting with an underscore

---
 pdns/syncres.cc | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/pdns/syncres.cc b/pdns/syncres.cc
index 426e8bb709..26cf303a96 100644
--- a/pdns/syncres.cc
+++ b/pdns/syncres.cc
@@ -814,6 +814,15 @@ int SyncRes::doResolve(const DNSName &qname, const QType qtype, vector<DNSRecord
         child.prependRawLabel(qname.getRawLabel(qnamelen - labels - 1));
         labels++;
       }
+      // rfc9156 section-2.3, append labels if they start with an underscore
+      while (labels < qnamelen) {
+        auto prependLabel = qname.getRawLabel(qnamelen - labels - 1);
+        if (prependLabel.at(0) != '_') {
+          break;
+        }
+        child.prependRawLabel(prependLabel);
+        labels++;
+      }
 
       QLOG("Step2 New child");
 
-- 
2.47.2