Generate MagicDNS search domains for any tailnet range

dns.go

@@ -2,12 +2,14 @@ package headscale
 
 import (
 	"fmt"
+	"strings"
 
+	"inet.af/netaddr"
 	"tailscale.com/util/dnsname"
 )
 
-func (h *Headscale) generateMagicDNSRootDomains() (*[]dnsname.FQDN, error) {
-	base, err := dnsname.ToFQDN(h.cfg.BaseDomain)
+func generateMagicDNSRootDomains(ipPrefix netaddr.IPPrefix, baseDomain string) (*[]dnsname.FQDN, error) {
+	base, err := dnsname.ToFQDN(baseDomain)
 	if err != nil {
 		return nil, err
 	}
@@ -17,14 +19,27 @@ func (h *Headscale) generateMagicDNSRootDomains() (*[]dnsname.FQDN, error) {
 	ipv6base := dnsname.FQDN("0.e.1.a.c.5.1.1.a.7.d.f.ip6.arpa.")
 	fqdns := []dnsname.FQDN{base, ipv6base}
 
-	for i := 64; i <= 127; i++ {
-		fqdn, err := dnsname.ToFQDN(fmt.Sprintf("%d.100.in-addr.arpa.", i))
+	netRange := ipPrefix.IPNet()
+	maskBits, _ := netRange.Mask.Size()
+
+	lastByte := maskBits / 8
+	unmaskedBits := 8 - maskBits%8
+	min := uint(netRange.IP[lastByte])
+	max := uint((min + 1<<uint(unmaskedBits)) - 1)
+
+	rdnsSlice := []string{}
+	for i := lastByte - 1; i >= 0; i-- {
+		rdnsSlice = append(rdnsSlice, fmt.Sprintf("%d", netRange.IP[i]))
+	}
+	rdnsSlice = append(rdnsSlice, "in-addr.arpa.")
+	rdnsBase := strings.Join(rdnsSlice, ".")
+
+	for i := min; i <= max; i++ {
+		fqdn, err := dnsname.ToFQDN(fmt.Sprintf("%d.%s", i, rdnsBase))
 		if err != nil {
-			// TODO: propagate error
 			continue
 		}
 		fqdns = append(fqdns, fqdn)
 	}
-
 	return &fqdns, nil
 }