1 files changed, 201 insertions, 0 deletions
diff --git a/dnsd/server.go b/dnsd/server.go
new file mode 100644
index 0000000..295431e
--- /dev/null
+++ b/dnsd/server.go
@@ -0,0 +1,201 @@
+// Package dnsd is the LAN's authoritative DNS server. It answers the configured
+// zone (e.g. penguins.lan) from user-owned records plus dynamically-synthesized
+// per-device records, and forwards anything outside the zone to upstream
+// resolvers. Modeled on the sibling hatecomputers.club hcdns server.
+package dnsd
+
+import (
+	"database/sql"
+	"fmt"
+	"log"
+	"strings"
+
+	"github.com/miekg/dns"
+
+	"penguins.lan/args"
+	"penguins.lan/database"
+	"penguins.lan/utils"
+)
+
+const maxRecursion = 10
+
+type Handler struct {
+	DB        *sql.DB
+	Zone      string // fqdn, e.g. "penguins.lan."
+	ServerIP  string // apex A target; "" leaves the apex unanswered
+	Resolvers []string
+}
+
+// resolveExternal forwards a query to the first upstream resolver that answers.
+// With no resolvers configured (the LAN has no outside internet) it returns
+// nothing.
+func (h *Handler) resolveExternal(name string, qtype uint16) ([]dns.RR, error) {
+	if len(h.Resolvers) == 0 {
+		return nil, nil
+	}
+	client := &dns.Client{}
+	msg := &dns.Msg{}
+	msg.SetQuestion(name, qtype)
+	msg.RecursionDesired = true
+
+	var lastErr error
+	for _, resolver := range h.Resolvers {
+		in, _, err := client.Exchange(msg, resolver)
+		if err != nil {
+			lastErr = err
+			continue
+		}
+		return in.Answer, nil
+	}
+	return nil, lastErr
+}
+
+// synthesizeDevice answers <compact-mac>.<username>.<zone> with the device's
+// current LAN IP, if the label is a MAC owned by that user and the device is
+// live. These records aren't stored — they track DHCP in real time.
+func (h *Handler) synthesizeDevice(name string, qtype uint16) ([]dns.RR, error) {
+	if qtype != dns.TypeA {
+		return nil, nil
+	}
+	rest, ok := strings.CutSuffix(name, "."+h.Zone)
+	if !ok {
+		return nil, nil
+	}
+	labels := strings.Split(rest, ".")
+	if len(labels) != 2 {
+		return nil, nil // only <mac>.<username> lives directly under the zone
+	}
+	macLabel, username := labels[0], labels[1]
+
+	mac, err := utils.ParseMacCompact(macLabel)
+	if err != nil {
+		return nil, nil // not a MAC label
+	}
+	owned, err := database.MACOwnedByUsername(h.DB, mac.Format(), username)
+	if err != nil {
+		return nil, err
+	}
+	if !owned {
+		return nil, nil
+	}
+	ip, err := database.IPForMac(h.DB, mac.Format())
+	if err != nil || ip == "" {
+		return nil, err
+	}
+	rr, err := dns.NewRR(fmt.Sprintf("%s 30 IN A %s", name, ip))
+	if err != nil {
+		return nil, err
+	}
+	return []dns.RR{rr}, nil
+}
+
+// resolveLocal answers an in-zone name: stored CNAMEs (followed recursively),
+// then stored records of the requested type, then the apex and dynamic device
+// records as fallbacks.
+func (h *Handler) resolveLocal(name string, qtype uint16, depth int) ([]dns.RR, error) {
+	if depth == 0 {
+		return nil, fmt.Errorf("dns: too much recursion resolving %s", name)
+	}
+
+	var answers []dns.RR
+
+	cnames, err := database.FindDNSRecords(h.DB, name, "CNAME")
+	if err != nil {
+		return nil, err
+	}
+	for _, rec := range cnames {
+		rr, err := dns.NewRR(fmt.Sprintf("%s %d IN CNAME %s", rec.Name, rec.TTL, dns.Fqdn(rec.Content)))
+		if err != nil {
+			return nil, err
+		}
+		answers = append(answers, rr)
+
+		if qtype != dns.TypeCNAME {
+			followed, err := h.resolveLocal(dns.Fqdn(rec.Content), qtype, depth-1)
+			if err != nil {
+				return nil, err
+			}
+			answers = append(answers, followed...)
+		}
+	}
+
+	qtypeName := dns.TypeToString[qtype]
+	records, err := database.FindDNSRecords(h.DB, name, qtypeName)
+	if err != nil {
+		return nil, err
+	}
+	for _, rec := range records {
+		rr, err := dns.NewRR(fmt.Sprintf("%s %d IN %s %s", rec.Name, rec.TTL, qtypeName, rec.Content))
+		if err != nil {
+			return nil, err
+		}
+		answers = append(answers, rr)
+	}
+
+	if len(answers) > 0 {
+		return answers, nil
+	}
+
+	if name == h.Zone && qtype == dns.TypeA && h.ServerIP != "" {
+		rr, err := dns.NewRR(fmt.Sprintf("%s 300 IN A %s", name, h.ServerIP))
+		if err != nil {
+			return nil, err
+		}
+		return []dns.RR{rr}, nil
+	}
+
+	return h.synthesizeDevice(name, qtype)
+}
+
+func (h *Handler) inZone(name string) bool {
+	return name == h.Zone || strings.HasSuffix(name, "."+h.Zone)
+}
+
+func (h *Handler) ServeDNS(w dns.ResponseWriter, r *dns.Msg) {
+	msg := &dns.Msg{}
+	msg.SetReply(r)
+
+	for _, q := range r.Question {
+		name := dns.Fqdn(strings.ToLower(q.Name))
+		if h.inZone(name) {
+			msg.Authoritative = true
+			answers, err := h.resolveLocal(name, q.Qtype, maxRecursion)
+			if err != nil {
+				log.Println("dnsd: resolve failed:", err)
+				msg.SetRcode(r, dns.RcodeServerFailure)
+				w.WriteMsg(msg)
+				return
+			}
+			msg.Answer = append(msg.Answer, answers...)
+		} else {
+			answers, err := h.resolveExternal(name, q.Qtype)
+			if err != nil {
+				log.Println("dnsd: forward failed:", err)
+				msg.SetRcode(r, dns.RcodeServerFailure)
+				w.WriteMsg(msg)
+				return
+			}
+			msg.Answer = append(msg.Answer, answers...)
+		}
+	}
+
+	if len(msg.Answer) == 0 && msg.Authoritative {
+		msg.SetRcode(r, dns.RcodeNameError)
+	}
+	w.WriteMsg(msg)
+}
+
+func MakeServer(argv *args.Arguments, db *sql.DB) *dns.Server {
+	handler := &Handler{
+		DB:        db,
+		Zone:      dns.Fqdn(argv.DnsZone),
+		ServerIP:  argv.ServerIP,
+		Resolvers: argv.DnsResolvers,
+	}
+	return &dns.Server{
+		Addr:    fmt.Sprintf(":%d", argv.DnsPort),
+		Net:     "udp",
+		Handler: handler,
+		UDPSize: 65535,
+	}
+}