feat: implement DNS server

stub.go

@@ -1,14 +1,23 @@
 package stub
 
 import (
+	"context"
+
+	"github.com/miekg/dns"
+	"github.com/mholt/acmez"
+	"github.com/mholt/acmez/acme"
 	"github.com/caddyserver/caddy/v2"
 	"github.com/caddyserver/caddy/v2/caddyconfig/caddyfile"
 )
 
+// TTL of the challenge TXT record to serve
+const challenge_ttl = 600 // (anything is probably fine here)
+
 type StubDNS struct {
 	// the address & port on which to serve DNS for the challenge
 	Address string `json:"address,omitempty"`
 
+	server *dns.Server // set in Present()
 }
 
 
@@ -69,8 +78,73 @@ func (s *StubDNS) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 }
 
 
+func (s *StubDNS) Present(ctx context.Context, challenge acme.Challenge) error {
+	// get challenge parameters
+	fqdn := dns.Fqdn(challenge.DNS01TXTRecordName())
+	content := challenge.DNS01KeyAuthorization()
+	// spawn the server
+	handler := s.make_handler(fqdn, content)
+	dns.HandleFunc(".", handler)
+	server := &dns.Server{Addr: s.Address, Net: "udp", TsigSecret: nil,}
+	go server.ListenAndServe()
+
+	// store the server for shutdown later
+	s.server = server
+	return nil
+}
+
+func (p *StubDNS) CleanUp(ctx context.Context, _ acme.Challenge) error {
+	if p.server == nil {
+		return nil
+	} else {
+		return p.server.ShutdownContext(ctx)
+	}
+}
+
+func (s *StubDNS) make_handler(fqdn string, txt string) dns.HandlerFunc {
+	handler := func(w dns.ResponseWriter, r *dns.Msg) {
+		m := new(dns.Msg)
+		m.SetReply(r)
+		if len(r.Question) != 1 {
+			m.Rcode = dns.RcodeRefused
+			m.Answer = []dns.RR{}
+			w.WriteMsg(m)
+			return
+		}
+
+		q := r.Question[0]
+		domain := q.Name
+
+		valid := r.Response == false &&
+			(q.Qclass == dns.ClassINET || q.Qclass == dns.ClassANY) &&
+			q.Qtype == dns.TypeTXT
+		if !valid {
+			m.Rcode = dns.RcodeNotImplemented
+			m.Answer = []dns.RR{}
+		} else if domain != fqdn {
+			m.Rcode = dns.RcodeNameError
+			m.Answer = []dns.RR{}
+		} else {
+			m.Authoritative = true
+			rr := new(dns.TXT)
+			rr.Hdr = dns.RR_Header{
+				Name: domain,
+				Rrtype: dns.TypeTXT,
+				Class: dns.ClassINET,
+				Ttl: uint32(challenge_ttl),
+			}
+			rr.Txt = []string{txt}
+			m.Answer = []dns.RR{rr}
+		}
+		w.WriteMsg(m)
+	}
+
+	return handler
+}
+
 // Interface guards
 var (
+	_ acmez.Solver = (*StubDNS)(nil)
 	_ caddy.Provisioner = (*StubDNS)(nil)
 	_ caddyfile.Unmarshaler = (*StubDNS)(nil)
 )