[client] Skip dns upstream servers pointing to our dns server IP to prevent loops (#4330)

client/internal/dns/server.go

@@ -695,6 +695,12 @@ func (s *DefaultServer) createHandlersForDomainGroup(domainGroup nsGroupsByDomai
 					ns.IP.String(), ns.NSType.String(), nbdns.UDPNameServerType.String())
 				continue
 			}
+
+			if ns.IP == s.service.RuntimeIP() {
+				log.Warnf("skipping nameserver %s as it matches our DNS server IP, preventing potential loop", ns.IP)
+				continue
+			}
+
 			handler.upstreamServers = append(handler.upstreamServers, ns.AddrPort())
 		}