From 5234e58e2380f8deb0710b81b5cb1cdf277fe0d1 Mon Sep 17 00:00:00 2001
From: Jonni Liljamo <jonni@liljamo.com>
Date: Thu, 24 Oct 2024 12:13:15 +0300
Subject: [PATCH] feat: answer TXT queries for _acme-challenge

---
 internal/dns/query.go | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/internal/dns/query.go b/internal/dns/query.go
index 889b9a0..433985a 100644
--- a/internal/dns/query.go
+++ b/internal/dns/query.go
@@ -35,6 +35,8 @@ func parseQuery(m *dns.Msg, r *dns.Msg) {
 			handleNSRecord(&q, m, r)
 		case dns.TypeSOA:
 			handleSOARecord(&q, m, r)
+		case dns.TypeTXT:
+			handleTXTRecord(&q, m, r)
 		default:
 			m.SetRcode(r, dns.RcodeNotImplemented)
 		}
@@ -117,3 +119,37 @@ func handleSOARecord(q *dns.Question, m *dns.Msg, r *dns.Msg) {
 		m.SetRcode(r, dns.RcodeNameError)
 	}
 }
+
+func handleTXTRecord(q *dns.Question, m *dns.Msg, r *dns.Msg) {
+	qName := strings.ToLower(q.Name)
+
+	// NOTE: This handles only _acme-challenge queries
+	expectedPrefix := "_acme-challenge."
+	if !strings.HasPrefix(qName, expectedPrefix) {
+		m.SetRcode(r, dns.RcodeNameError)
+		return
+	}
+	qNameWithoutPrefix, ok := strings.CutPrefix(qName, expectedPrefix)
+	if !ok {
+		m.SetRcode(r, dns.RcodeFormatError)
+		return
+	}
+
+	var ddnsDomain string
+	if index := strings.IndexByte(qNameWithoutPrefix, '.'); index >= 0 {
+		ddnsDomain = qNameWithoutPrefix[:index]
+	} else {
+		m.SetRcode(r, dns.RcodeNameError)
+		return
+	}
+
+	txtRecord, err := db.FetchDomainAcmeChallenge(ddnsDomain)
+	if err != nil {
+		m.SetRcode(r, dns.RcodeNameError)
+	} else {
+		m.Answer = append(m.Answer, &dns.TXT{
+			Hdr: dns.RR_Header{Name: q.Name, Rrtype: dns.TypeTXT, Class: dns.ClassINET, Ttl: 0},
+			Txt: []string{txtRecord},
+		})
+	}
+}
-- 
2.44.1