From 800959227b23482312b73d0c4206e9f4b9bc566d Mon Sep 17 00:00:00 2001
From: Otto Moerbeek <otto.moerbeek@open-xchange.com>
Date: Mon, 24 Oct 2022 10:59:49 +0200
Subject: [PATCH] If we encounter a loop in QM, continue with the next
 iteration.

We would not make progres anyway, and end up repeatedly asking the full name
from the roots as the "Trying less specific NS" code path is hit.

Noted by and fix from  @phonedph1

Fixes #12090

As this is an optimzation for broken domains, this is not an immediate candidate
for backporting, though it should be considered for that after more testing.
---
 pdns/syncres.cc | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/pdns/syncres.cc b/pdns/syncres.cc
index 443b94fd63..5dc97741ce 100644
--- a/pdns/syncres.cc
+++ b/pdns/syncres.cc
@@ -1737,6 +1737,19 @@ int SyncRes::doResolve(const DNSName &qname, const QType qtype, vector<DNSRecord
         return res;
       }
 
+      // If we have seen this child during resolution already; just skip it. We tried to QM it already or otherwise broken.
+      bool skipStep4 = false;
+      for (const auto& visitedNS : beenthere) {
+        if (visitedNS.qname == child) {
+          skipStep4 = true;
+          break;
+        }
+      }
+      if (skipStep4) {
+        QLOG("Step4 Being skipped as visited this child name already");
+        continue;
+      }
+
       // Step 4
       QLOG("Step4 Resolve A for child");
       bool oldFollowCNAME = d_followCNAME;
-- 
2.47.2