From a3f134e3fb1df7df11eebd6c5e386f6f5c3f4eef Mon Sep 17 00:00:00 2001
From: xenolf <xenolf@users.noreply.github.com>
Date: Fri, 22 Jan 2016 01:38:15 +0100
Subject: [PATCH] Check DNS entry for validity before hitting boulder

---
 acme/dns_challenge.go      | 47 ++++++++++++++++++++++++++++++++++++++
 acme/dns_challenge_test.go |  3 +++
 2 files changed, 50 insertions(+)

diff --git a/acme/dns_challenge.go b/acme/dns_challenge.go
index 61d0df5a..198a1bcb 100644
--- a/acme/dns_challenge.go
+++ b/acme/dns_challenge.go
@@ -9,8 +9,18 @@ import (
 	"net/http"
 	"strings"
 	"time"
+
+	"github.com/miekg/dns"
 )
 
+type preCheckDNSFunc func() bool
+
+var preCheckDNS = func() bool {
+	return true
+}
+
+var preCheckDNSFallbackCount = 5
+
 // DNSProvider represents a service for creating dns records.
 type DNSProvider interface {
 	// CreateTXT creates a TXT record
@@ -44,6 +54,43 @@ func (s *dnsChallenge) Solve(chlng challenge, domain string) error {
 		return err
 	}
 
+	if preCheckDNS() {
+		// check if the expected DNS entry was created. If not wait for some time and try again.
+		m := new(dns.Msg)
+		m.SetQuestion(domain+".", dns.TypeSOA)
+		c := new(dns.Client)
+		in, _, err := c.Exchange(m, "8.8.8.8:53")
+		if err != nil {
+			return err
+		}
+
+		var authorativeNS string
+		for _, answ := range in.Answer {
+			soa := answ.(*dns.SOA)
+			authorativeNS = soa.Ns
+		}
+
+		fallbackCnt := 0
+		for fallbackCnt < preCheckDNSFallbackCount {
+			m.SetQuestion(fqdn, dns.TypeTXT)
+			in, _, err = c.Exchange(m, authorativeNS+":53")
+			if err != nil {
+				return err
+			}
+
+			if len(in.Answer) > 0 {
+				break
+			}
+
+			fallbackCnt++
+			if fallbackCnt >= preCheckDNSFallbackCount {
+				return errors.New("Could not retrieve the value from DNS in a timely manner. Aborting.")
+			}
+
+			time.Sleep(time.Second * time.Duration(fallbackCnt))
+		}
+	}
+
 	jsonBytes, err := json.Marshal(challenge{Resource: "challenge", Type: chlng.Type, Token: chlng.Token, KeyAuthorization: keyAuth})
 	if err != nil {
 		return errors.New("Failed to marshal network message...")
diff --git a/acme/dns_challenge_test.go b/acme/dns_challenge_test.go
index 567391e6..e69d7c4b 100644
--- a/acme/dns_challenge_test.go
+++ b/acme/dns_challenge_test.go
@@ -11,6 +11,9 @@ import (
 )
 
 func TestDNSValidServerResponse(t *testing.T) {
+	preCheckDNS = func() bool {
+		return false
+	}
 	privKey, _ := generatePrivateKey(rsakey, 512)
 
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {