autodiscover server from zone SOA

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")
+}

main.go

@@ -62,5 +62,7 @@ func main() {
 	// update
 	if err := update.Update(addrs, options.Verbose); err != nil {
 		log.Fatalf("update: %v", err)
+	} else {
+		log.Printf("update: ok")
 	}
 }

update.go

@@ -20,13 +20,37 @@ type Update struct {
 }
 
 func (u *Update) Init(name string, zone string, server string) error {
-	u.name = dns.Fqdn(name)
-	u.zone = dns.Fqdn(zone)
-
-	if _, _, err := net.SplitHostPort(server); err == nil {
-		u.server = server
+	if name == "" {
+		return fmt.Errorf("Missing name")
 	} else {
-		u.server = net.JoinHostPort(server, "53")
+		u.name = dns.Fqdn(name)
+	}
+
+	if zone == "" {
+		// guess
+		if labels := dns.Split(u.name); len(labels) > 1 {
+			u.zone = u.name[labels[1]:]
+		} else {
+			return fmt.Errorf("Missing zone")
+		}
+	} else {
+		u.zone = dns.Fqdn(zone)
+	}
+
+	if server == "" {
+		if server, err := discoverZoneServer(u.zone); err != nil {
+			return fmt.Errorf("Failed to discver server")
+		} else {
+			log.Printf("discover server=%v", server)
+
+			u.server = net.JoinHostPort(server, "53")
+		}
+	} else {
+		if _, _, err := net.SplitHostPort(server); err == nil {
+			u.server = server
+		} else {
+			u.server = net.JoinHostPort(server, "53")
+		}
 	}
 
 	return nil