From 8cfc895b24f94e0af0f3508a23df630475176381 Mon Sep 17 00:00:00 2001
From: Viktor Liu <viktor@netbird.io>
Date: Sun, 19 Jan 2025 19:59:30 +0100
Subject: [PATCH] Add reverse dns zone

---
 client/internal/dns.go    | 85 +++++++++++++++++++++++++++++++++++++++
 client/internal/engine.go |  9 +++--
 2 files changed, 91 insertions(+), 3 deletions(-)
 create mode 100644 client/internal/dns.go

diff --git a/client/internal/dns.go b/client/internal/dns.go
new file mode 100644
index 00000000000..b9b8a1ca93e
--- /dev/null
+++ b/client/internal/dns.go
@@ -0,0 +1,85 @@
+package internal
+
+import (
+	"net"
+	"slices"
+	"strings"
+
+	"github.com/miekg/dns"
+	log "github.com/sirupsen/logrus"
+
+	nbdns "github.com/netbirdio/netbird/dns"
+)
+
+func createPTRRecord(aRecord nbdns.SimpleRecord, ipNet *net.IPNet) (nbdns.SimpleRecord, bool) {
+	ip := net.ParseIP(aRecord.RData)
+	if ip == nil || ip.To4() == nil {
+		return nbdns.SimpleRecord{}, false
+	}
+
+	if !ipNet.Contains(ip) {
+		return nbdns.SimpleRecord{}, false
+	}
+
+	ipOctets := strings.Split(ip.String(), ".")
+	slices.Reverse(ipOctets)
+	rdnsName := dns.Fqdn(strings.Join(ipOctets, ".") + ".in-addr.arpa")
+
+	return nbdns.SimpleRecord{
+		Name:  rdnsName,
+		Type:  int(dns.TypePTR),
+		Class: aRecord.Class,
+		TTL:   aRecord.TTL,
+		RData: dns.Fqdn(aRecord.Name),
+	}, true
+}
+
+func addReverseZone(config *nbdns.Config, ipNet *net.IPNet) {
+	networkIP := ipNet.IP.Mask(ipNet.Mask)
+
+	maskOnes, _ := ipNet.Mask.Size()
+	// round up to nearest byte
+	octetsToUse := (maskOnes + 7) / 8
+
+	octets := strings.Split(networkIP.String(), ".")
+	if octetsToUse > len(octets) {
+		log.Warnf("invalid network mask size for reverse DNS: %d", maskOnes)
+		return
+	}
+
+	reverseOctets := make([]string, octetsToUse)
+	for i := 0; i < octetsToUse; i++ {
+		reverseOctets[octetsToUse-1-i] = octets[i]
+	}
+
+	zoneName := dns.Fqdn(strings.Join(reverseOctets, ".") + ".in-addr.arpa")
+
+	for _, zone := range config.CustomZones {
+		if zone.Domain == zoneName {
+			log.Debugf("reverse DNS zone %s already exists", zoneName)
+			return
+		}
+	}
+
+	var records []nbdns.SimpleRecord
+
+	for _, zone := range config.CustomZones {
+		for _, record := range zone.Records {
+			if record.Type != int(dns.TypeA) {
+				continue
+			}
+
+			if ptrRecord, ok := createPTRRecord(record, ipNet); ok {
+				records = append(records, ptrRecord)
+			}
+		}
+	}
+
+	reverseZone := nbdns.CustomZone{
+		Domain:  zoneName,
+		Records: records,
+	}
+
+	config.CustomZones = append(config.CustomZones, reverseZone)
+	log.Debugf("added reverse DNS zone: %s with %d records", zoneName, len(records))
+}
diff --git a/client/internal/engine.go b/client/internal/engine.go
index b3689c91153..dceb08ca583 100644
--- a/client/internal/engine.go
+++ b/client/internal/engine.go
@@ -972,7 +972,7 @@ func (e *Engine) updateNetworkMap(networkMap *mgmProto.NetworkMap) error {
 		protoDNSConfig = &mgmProto.DNSConfig{}
 	}
 
-	if err := e.dnsServer.UpdateDNSServer(serial, toDNSConfig(protoDNSConfig)); err != nil {
+	if err := e.dnsServer.UpdateDNSServer(serial, toDNSConfig(protoDNSConfig, e.wgInterface.Address().Network)); err != nil {
 		log.Errorf("failed to update dns server, err: %v", err)
 	}
 
@@ -1041,7 +1041,7 @@ func toRouteDomains(myPubKey string, protoRoutes []*mgmProto.Route) []string {
 	return dnsRoutes
 }
 
-func toDNSConfig(protoDNSConfig *mgmProto.DNSConfig) nbdns.Config {
+func toDNSConfig(protoDNSConfig *mgmProto.DNSConfig, network *net.IPNet) nbdns.Config {
 	dnsUpdate := nbdns.Config{
 		ServiceEnable:    protoDNSConfig.GetServiceEnable(),
 		CustomZones:      make([]nbdns.CustomZone, 0),
@@ -1081,6 +1081,9 @@ func toDNSConfig(protoDNSConfig *mgmProto.DNSConfig) nbdns.Config {
 		}
 		dnsUpdate.NameServerGroups = append(dnsUpdate.NameServerGroups, dnsNSGroup)
 	}
+
+	addReverseZone(&dnsUpdate, network)
+
 	return dnsUpdate
 }
 
@@ -1387,7 +1390,7 @@ func (e *Engine) readInitialSettings() ([]*route.Route, *nbdns.Config, error) {
 		return nil, nil, err
 	}
 	routes := toRoutes(netMap.GetRoutes())
-	dnsCfg := toDNSConfig(netMap.GetDNSConfig())
+	dnsCfg := toDNSConfig(netMap.GetDNSConfig(), e.wgInterface.Address().Network)
 	return routes, &dnsCfg, nil
 }