Custom DNS Server returns only TXT records

This small DNS server only returns one type of record, a TXT record,
meant to be a token assigned by a certificate authority (e.g. Let's
Encrypt) to verify domain ownership.

The TXT record will be updateable by an API endpoint on the webserver
(same executable as the DNS server), but I haven't yet written that
portion.

Drive-by: in our _other_ (main) sslip.io DNS server, I changed `break` →
`continue` in the main loop. Had we gotten a malformed UDP packet, we
would have exited, but now we continue to the next packet. Exiting is
not that big a deal—`monit` would have restarted the server—but moving
on to the next packet is a more robust approach.

[#6]

bosh-release/src/wildcard-dns-http-server/main.go

@@ -0,0 +1,90 @@
+package main
+
+import (
+	"log"
+	"net"
+	"sync"
+
+	"golang.org/x/net/dns/dnsmessage"
+)
+
+var txt = `Set this TXT record: curl -X POST http://localhost/update -d  '{"txt":"Certificate Authority's validation token"}'`
+
+func main() {
+	conn, err := net.ListenUDP("udp", &net.UDPAddr{Port: 53})
+	if err != nil {
+		log.Fatal(err.Error())
+	}
+
+	var group sync.WaitGroup
+	group.Add(1)
+	go dnsServer(conn, &group)
+	group.Add(1)
+	go httpServer(&group)
+	group.Wait()
+}
+
+func dnsServer(conn *net.UDPConn, group *sync.WaitGroup) {
+	var query dnsmessage.Message
+
+	defer group.Done()
+	log.Println("I'm firing up the DNS server.")
+	queryRaw := make([]byte, 512)
+	for {
+		_, addr, err := conn.ReadFromUDP(queryRaw)
+		if err != nil {
+			log.Println(err.Error())
+			continue
+		}
+		err = query.Unpack(queryRaw)
+		if err != nil {
+			log.Println(err.Error())
+			continue
+		}
+		// Technically, there can be multiple questions in a DNS message; practically, there's only one
+		if len(query.Questions) != 1 {
+			log.Printf("I expected one question but got %d.\n", len(query.Questions))
+			continue
+		}
+		// We only return answers to TXT records, nothing else
+		if query.Questions[0].Type != dnsmessage.TypeTXT {
+			log.Println("I expected a question for a TypeTXT record but got a question for a " + query.Questions[0].Type.String() + " record.")
+			continue
+		}
+		reply := dnsmessage.Message{
+			Header: dnsmessage.Header{
+				ID:               query.ID,
+				Response:         true,
+				Authoritative:    true,
+				RecursionDesired: query.RecursionDesired,
+			},
+			Questions: query.Questions,
+			Answers: []dnsmessage.Resource{
+				{
+					Header: dnsmessage.ResourceHeader{
+						Name:  query.Questions[0].Name,
+						Type:  dnsmessage.TypeTXT,
+						Class: dnsmessage.ClassINET,
+					},
+					Body: &dnsmessage.TXTResource{TXT: []string{txt}},
+				},
+			},
+		}
+		replyRaw, err := reply.Pack()
+		if err != nil {
+			log.Println(err.Error())
+			continue
+		}
+		_, err = conn.WriteToUDP(replyRaw, addr)
+		if err != nil {
+			log.Println(err.Error())
+			continue
+		}
+		log.Printf("%v.%d %s → \"%s\"\n", addr.IP, addr.Port, query.Questions[0].Type.String(), txt)
+	}
+}
+
+func httpServer(group *sync.WaitGroup) {
+	defer group.Done()
+	log.Println("I'm firing up the HTTP server.")
+}