Add mdns package and Client implementation (#55)

* add mdns

* define mdns.Client

* fix some mdns stuff

* refine dns package for use with mdns

* remove Querier and Responder and replace with Client

* add record setting methods on dns.record to reuse record buffer

* protect against unbounded client answer growth

* round off sharp mdns edges; reduce allocs

* add mutlicast to stacks; add xnet tests for mdns

* fix documentation on acceptmulticast field

* mdns working

internet/pcap/capture.go

@@ -19,6 +19,7 @@ import (
 	"github.com/soypat/lneto/ipv4"
 	"github.com/soypat/lneto/ipv4/icmpv4"
 	"github.com/soypat/lneto/ipv6"
+	"github.com/soypat/lneto/mdns"
 	"github.com/soypat/lneto/ntp"
 	"github.com/soypat/lneto/tcp"
 	"github.com/soypat/lneto/udp"
@@ -359,7 +360,7 @@ func (pc *PacketBreakdown) CaptureUDP(dst []Frame, pkt []byte, bitOffset int) ([
 	srcport := ufrm.SourcePort()
 	if dhcpv4.PayloadIsDHCPv4(payload) {
 		dst, err = pc.CaptureDHCPv4(dst, pkt, end)
-	} else if dstport == dns.ServerPort || srcport == dns.ServerPort {
+	} else if dstport == dns.ServerPort || srcport == dns.ServerPort || dstport == mdns.Port || srcport == mdns.Port {
 		dst, err = pc.CaptureDNS(dst, pkt, end)
 	} else if dstport == ntp.ServerPort || srcport == ntp.ServerPort {
 		dst, err = pc.CaptureNTP(dst, pkt, end)
@@ -437,7 +438,7 @@ func (pc *PacketBreakdown) CaptureDNS(dst []Frame, pkt []byte, bitOffset int) ([
 		return dst, errNotByteAligned
 	}
 	dnsData := pkt[bitOffset/8:]
-	pc.dmsg.LimitResourceDecoding(20, 20, 20, 20)
+	pc.dmsg.LimitResourceDecoding(4, 4, 4, 4)
 	off, incomplete, err := pc.dmsg.Decode(dnsData)
 	if err != nil && !incomplete {
 		return dst, err
@@ -1265,6 +1266,43 @@ var baseDHCPv4Fields = [...]FrameField{
 	},
 }
 
+var baseDNSFields = [...]FrameField{
+	{
+		Class:          FieldClassID,
+		FrameBitOffset: 0,
+		BitLength:      2 * octet,
+	},
+	{
+		Class:          FieldClassFlags,
+		FrameBitOffset: 2 * octet,
+		BitLength:      2 * octet,
+	},
+	{
+		Name:           "Questions",
+		Class:          FieldClassSize,
+		FrameBitOffset: 4 * octet,
+		BitLength:      2 * octet,
+	},
+	{
+		Name:           "Answers",
+		Class:          FieldClassSize,
+		FrameBitOffset: 6 * octet,
+		BitLength:      2 * octet,
+	},
+	{
+		Name:           "Authorities",
+		Class:          FieldClassSize,
+		FrameBitOffset: 8 * octet,
+		BitLength:      2 * octet,
+	},
+	{
+		Name:           "Additionals",
+		Class:          FieldClassSize,
+		FrameBitOffset: 10 * octet,
+		BitLength:      2 * octet,
+	},
+}
+
 var baseNTPFields = [...]FrameField{
 	{
 		Name:           "Mode",

internet/stack-ethernet.go

@@ -33,11 +33,12 @@ type StackEthernetConfig struct {
 }
 
 type StackEthernet struct {
-	connID   uint64
-	handlers handlers
-	mac      [6]byte
-	gwmac    [6]byte
-	mtu      uint16
+	connID          uint64
+	handlers        handlers
+	mac             [6]byte
+	gwmac           [6]byte
+	mtu             uint16
+	acceptMulticast bool
 	// crcupdate set when crc32 has been configured to be appended.
 	crcupdate func(crc uint32, p []byte) uint32
 }
@@ -50,6 +51,10 @@ func (ls *StackEthernet) Gateway6() (gw [6]byte) {
 	return ls.gwmac
 }
 
+func (ls *StackEthernet) SetAcceptMulticast(accept bool) {
+	ls.acceptMulticast = accept
+}
+
 func (ls *StackEthernet) SetHardwareAddr6(mac [6]byte) {
 	ls.mac = mac
 }
@@ -83,11 +88,12 @@ func (ls *StackEthernet) Configure(cfg StackEthernetConfig) error {
 	}
 	ls.handlers.reset("StackEthernet", cfg.MaxNodes)
 	*ls = StackEthernet{
-		connID:   ls.connID + 1,
-		handlers: ls.handlers,
-		mac:      cfg.MAC,
-		gwmac:    cfg.Gateway,
-		mtu:      uint16(cfg.MTU),
+		connID:          ls.connID + 1,
+		handlers:        ls.handlers,
+		mac:             cfg.MAC,
+		gwmac:           cfg.Gateway,
+		mtu:             uint16(cfg.MTU),
+		acceptMulticast: ls.acceptMulticast,
 	}
 	if cfg.AppendCRC32 {
 		ls.crcupdate = cfg.CRC32Update
@@ -121,7 +127,9 @@ func (ls *StackEthernet) Demux(carrierData []byte, frameOffset int) (err error)
 	dstaddr := efrm.DestinationHardwareAddr()
 	var vld lneto.Validator
 	if !efrm.IsBroadcast() && ls.mac != *dstaddr {
-		goto DROP
+		if !ls.acceptMulticast || dstaddr[0]&1 == 0 {
+			goto DROP
+		}
 	}
 	efrm.ValidateSize(&vld)
 	if vld.HasError() {

internet/stack-ip.go

@@ -16,11 +16,12 @@ import (
 var _ StackNode = (*StackIP)(nil)
 
 type StackIP struct {
-	connID    uint64
-	ipID      uint16
-	ip        [4]byte
-	validator lneto.Validator
-	handlers  handlers
+	connID          uint64
+	ipID            uint16
+	ip              [4]byte
+	acceptMulticast bool
+	validator       lneto.Validator
+	handlers        handlers
 }
 
 func (sb *StackIP) Reset(addr netip.Addr, maxNodes int) error {
@@ -33,10 +34,11 @@ func (sb *StackIP) Reset(addr netip.Addr, maxNodes int) error {
 	}
 	sb.handlers.reset("StackIP", maxNodes)
 	*sb = StackIP{
-		connID:    sb.connID + 1,
-		validator: sb.validator,
-		handlers:  sb.handlers,
-		ip:        sb.ip,
+		connID:          sb.connID + 1,
+		validator:       sb.validator,
+		handlers:        sb.handlers,
+		ip:              sb.ip,
+		acceptMulticast: sb.acceptMulticast,
 	}
 	return nil
 }
@@ -65,6 +67,10 @@ func (sb *StackIP) Addr() netip.Addr {
 	return netip.AddrFrom4(sb.ip)
 }
 
+func (sb *StackIP) SetAcceptMulticast(accept bool) {
+	sb.acceptMulticast = accept
+}
+
 func (sb *StackIP) SetLogger(logger *slog.Logger) {
 	sb.handlers.log = logger
 }
@@ -79,8 +85,10 @@ func (sb *StackIP) Demux(carrierData []byte, offset int) error {
 	}
 	dst := ifrm.DestinationAddr()
 	if sb.ip != ([4]byte{}) && *dst != sb.ip {
-		sb.handlers.debug("ip:not-for-us")
-		return lneto.ErrPacketDrop // Not meant for us.
+		if !sb.acceptMulticast || dst[0]&0xF0 != 0xE0 {
+			sb.handlers.debug("ip:not-for-us")
+			return lneto.ErrPacketDrop // Not meant for us.
+		}
 	}
 
 	sb.validator.ResetErr()