From b2f9c977719626bebd9c9f9f7e52e50f2514ace7 Mon Sep 17 00:00:00 2001
From: Tero Marttila <terom@fixme.fi>
Date: Sun, 19 Jun 2016 20:05:55 +0300
Subject: autodiscover server from zone SOA

---
 dns.go | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

(limited to 'dns.go')

diff --git a/dns.go b/dns.go
index 4822012..ff3a109 100644
--- a/dns.go
+++ b/dns.go
@@ -3,6 +3,9 @@ package main
 import (
 	"github.com/miekg/dns"
 	"fmt"
+	"time"
+	"log"
+	"net"
 )
 
 const TSIG_FUDGE_SECONDS = 300
@@ -24,3 +27,51 @@ func (t *TSIGAlgorithm) UnmarshalFlag(value string) error {
 
 	return nil
 }
+
+func query(query *dns.Msg) (*dns.Msg, error) {
+	clientConfig, err := dns.ClientConfigFromFile("/etc/resolv.conf")
+	if err != nil {
+		return nil, err
+	}
+
+	timeout := time.Duration(clientConfig.Timeout * int(time.Second))
+
+	var client = new(dns.Client)
+
+	client.DialTimeout = timeout
+	client.ReadTimeout = timeout
+	client.WriteTimeout = timeout
+
+	for _, server := range clientConfig.Servers {
+		addr := net.JoinHostPort(server, "53")
+
+		if answer, _, err := client.Exchange(query, addr); err != nil {
+			log.Printf("query %v: %v", server, err)
+			continue
+		} else {
+			return answer, nil
+		}
+	}
+
+	return nil, fmt.Errorf("DNS query failed")
+}
+
+// Discover likely master NS for zone
+func discoverZoneServer(zone string) (string, error) {
+	var q = new(dns.Msg)
+
+	q.SetQuestion(zone, dns.TypeSOA)
+
+	r, err := query(q)
+	if err != nil {
+		return "", err
+	}
+
+	for _, rr := range r.Answer {
+		if soa, ok := rr.(*dns.SOA); ok {
+			return soa.Ns, nil
+		}
+	}
+
+	return "", fmt.Errorf("No SOA response")
+}
-- 
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