From 785adeb6c18f6bae35c6af6397f91e6d35e9d209 Mon Sep 17 00:00:00 2001
From: Douglas Mayle <dmayle@dmayle.com>
Date: Wed, 20 May 2020 16:09:37 +0200
Subject: [PATCH] Fix issue with too large DNS messages

This is an import of the fix for issue 10135 in the rancher fork of this
project ( rancher/rancher#10135 ).
---
 provider/rfc2136/rfc2136.go | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/provider/rfc2136/rfc2136.go b/provider/rfc2136/rfc2136.go
index 77c92464c..7cb9f48c2 100644
--- a/provider/rfc2136/rfc2136.go
+++ b/provider/rfc2136/rfc2136.go
@@ -33,6 +33,11 @@ import (
 	"sigs.k8s.io/external-dns/provider"
 )
 
+const (
+	// maximum size of a UDP transport message in DNS protocol
+	udpMaxMsgSize = 512
+)
+
 // rfc2136 provider type
 type rfc2136Provider struct {
 	nameserver    string
@@ -309,6 +314,10 @@ func (r rfc2136Provider) SendMessage(msg *dns.Msg) error {
 		msg.SetTsig(r.tsigKeyName, r.tsigSecretAlg, 300, time.Now().Unix())
 	}
 
+	if msg.Len() > udpMaxMsgSize {
+		c.Net = "tcp"
+	}
+
 	resp, _, err := c.Exchange(msg, r.nameserver)
 	if err != nil {
 		log.Infof("error in dns.Client.Exchange: %s", err)