all: spoof loopback ranges in client info

Sending them are useless, so using RFC1918 address instead.

cmd/cli/dns_proxy.go

@@ -573,9 +573,23 @@ func (p *prog) getClientInfo(remoteIP string, msg *dns.Msg) *ctrld.ClientInfo {
 		ci.Hostname = p.ciTable.LookupHostname(ci.IP, ci.Mac)
 	}
 	ci.Self = queryFromSelf(ci.IP)
+	p.spoofLoopbackIpInClientInfo(ci)
 	return ci
 }
 
+// spoofLoopbackIpInClientInfo replaces loopback IPs in client info.
+//
+// - Preference IPv4.
+// - Preference RFC1918.
+func (p *prog) spoofLoopbackIpInClientInfo(ci *ctrld.ClientInfo) {
+	if ip := net.ParseIP(ci.IP); ip == nil || !ip.IsLoopback() {
+		return
+	}
+	if ip := p.ciTable.LookupRFC1918IPv4(ci.Mac); ip != "" {
+		ci.IP = ip
+	}
+}
+
 // queryFromSelf reports whether the input IP is from device running ctrld.
 func queryFromSelf(ip string) bool {
 	netIP := netip.MustParseAddr(ip)

internal/clientinfo/client_info.go

@@ -241,6 +241,21 @@ func (t *Table) LookupHostname(ip, mac string) string {
 	return ""
 }
 
+// LookupRFC1918IPv4 returns the RFC1918 IPv4 address for the given MAC address, if any.
+func (t *Table) LookupRFC1918IPv4(mac string) string {
+	t.initOnce.Do(t.init)
+	for _, r := range t.ipResolvers {
+		ip, err := netip.ParseAddr(r.LookupIP(mac))
+		if err != nil || ip.Is6() {
+			continue
+		}
+		if ip.IsPrivate() {
+			return ip.String()
+		}
+	}
+	return ""
+}
+
 type macEntry struct {
 	mac string
 	src string

internal/clientinfo/client_info_test.go

@@ -25,3 +25,22 @@ func Test_normalizeIP(t *testing.T) {
 		})
 	}
 }
+
+func TestTable_LookupRFC1918IPv4(t *testing.T) {
+	table := &Table{
+		dhcp: &dhcp{},
+		arp:  &arpDiscover{},
+	}
+
+	table.ipResolvers = append(table.ipResolvers, table.dhcp)
+	table.ipResolvers = append(table.ipResolvers, table.arp)
+
+	macAddress := "cc:19:f9:8a:49:e6"
+	rfc1918IPv4 := "10.0.10.245"
+	table.dhcp.ip.Store(macAddress, "127.0.0.1")
+	table.arp.ip.Store(macAddress, rfc1918IPv4)
+
+	if got := table.LookupRFC1918IPv4(macAddress); got != rfc1918IPv4 {
+		t.Fatalf("unexpected result, want: %s, got: %s", rfc1918IPv4, got)
+	}
+}