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// 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,
}
}
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