Sign responses to TSIG-signed UPDATEs (RFC 8945 §5.4.2)

When a request arrives with TSIG, attach a TSIG record to the response
so dns.ResponseWriter computes the MAC at write time using the secret
in TsigSecret. Without this, BIND nsupdate complains "expected a TSIG
or SIG(0)" on every UPDATE, even when the update applies successfully.

Two response paths fixed:
  - handleUpdate success/per-rcode replies (update.go)
  - ServeDNS rejection when TSIG verification fails (plugin.go)

The new helper in tsig.go is a no-op for unsigned requests. Unknown
keys still silently skip signing — we can't authenticate to a peer we
don't share a key with.

Tests verify both branches: signed request → response carries matching
TSIG (key name + algorithm); unsigned request → response stays plain.

tsig.go

@@ -3,10 +3,37 @@ package rfc2136
 import (
 	"fmt"
 	"strings"
+	"time"
 
 	"github.com/miekg/dns"
 )
 
+// tsigResponseFudge is the time tolerance (seconds) embedded in
+// responses we sign. RFC 8945 §10 suggests 300s; we mirror that.
+const tsigResponseFudge = 300
+
+// signResponseIfSigned attaches a TSIG record to resp using the
+// request's key name and algorithm. This causes the downstream
+// dns.ResponseWriter to compute and serialize the MAC at WriteMsg
+// time (using the secret from the server's TsigSecret map, which
+// setup.go populated). Per RFC 8945 §5.4.2, the response to a
+// TSIG-signed message MUST itself be signed if the server knows the
+// key — otherwise the client cannot authenticate the answer and
+// rejects it with "expected a TSIG or SIG(0)" (BIND nsupdate's exact
+// complaint).
+//
+// If the request was not TSIG-signed, this is a no-op. If the key is
+// not in the server's TsigSecret map (e.g. unknown key), miekg/dns
+// will skip signing at write time and the response goes back
+// unsigned — that's the correct shape for "I don't have your key."
+func signResponseIfSigned(resp, req *dns.Msg) {
+	tsig := req.IsTsig()
+	if tsig == nil {
+		return
+	}
+	resp.SetTsig(tsig.Hdr.Name, tsig.Algorithm, tsigResponseFudge, time.Now().Unix())
+}
+
 // checkTSIG verifies that the incoming UPDATE message is properly signed
 // with a TSIG key we know about. The actual signature math has already
 // been done by the underlying dns.Server (because setup.go registered