HTTP server example and DHCP Server rewrite (#38)

* improvements to dhcpv4 server

* begin adding examples/httpserver

* add better VLAN tagging methods

* pcap: add error printing; fix bug in CRC

* ipv4: ToS and Flags construction and flag manipulation improvements

* add dhcp interception to httptap and improve httpserver example

.gitignore

@@ -25,6 +25,8 @@ vendor/
 /xcurl
 /xnet
 /httpclient
+/httpserver
+**/local*
 /stack
 **__debug_bin*
 # `__debug_bin` Debug binary generated in VSCode when using the built-in debugger.
@@ -33,6 +35,7 @@ vendor/
 /bridge
 # IDE
 .vscode/
+agents.md
 
 # For local development and testing create `local` directories.
 local

dhcpv4/dhcp_test.go

@@ -2,6 +2,7 @@ package dhcpv4
 
 import (
 	"bytes"
+	"net/netip"
 	"testing"
 )
 
@@ -25,7 +26,10 @@ func TestClientServer(t *testing.T) {
 			t.Errorf("want client state %s, got %s", state.String(), cl.State().String())
 		}
 	}
-	sv.Reset(svAddr, DefaultServerPort)
+	sv.Configure(ServerConfig{
+		ServerAddr: svAddr,
+		Subnet:     netip.PrefixFrom(netip.AddrFrom4(svAddr), 24),
+	})
 	// CLIENT DISCOVER.
 	assertClState(StateInit)
 	var buf [1024]byte

dhcpv4/server.go

@@ -11,15 +11,33 @@ import (
 )
 
 type Server struct {
-	connID   uint64
-	nextAddr netip.Addr
-	prefix   netip.Prefix
-	hosts    map[[36]byte]serverEntry
-	vld      lneto.Validator
-	pending  int
-	port     uint16
-	siaddr   [4]byte
-	gwaddr   [4]byte
+	connID       uint64
+	nextAddr     netip.Addr
+	prefix       netip.Prefix
+	hosts        map[[36]byte]serverEntry
+	vld          lneto.Validator
+	pending      int
+	leaseSeconds uint32
+	port         uint16
+	siaddr       [4]byte
+	gwaddr       [4]byte
+	dns          [4]byte
+}
+
+// ServerConfig contains configuration parameters for [Server.Configure].
+type ServerConfig struct {
+	// ServerAddr is the DHCP server's own IPv4 address.
+	ServerAddr [4]byte
+	// Gateway advertised to clients as default router. Zero value omits the option.
+	Gateway [4]byte
+	// DNS server address advertised to clients. Zero value omits the option.
+	DNS [4]byte
+	// Subnet defines the network prefix for address allocation and subnet mask responses.
+	Subnet netip.Prefix
+	// LeaseSeconds is the lease duration. Zero defaults to 3600.
+	LeaseSeconds uint32
+	// Port is the server listening port. Zero defaults to DefaultServerPort.
+	Port uint16
 }
 
 type serverEntry struct {
@@ -35,30 +53,53 @@ type serverEntry struct {
 	//  - Init: Server received discover, pending Offer sent out.
 	//  - Selecting: Server sent out offer, request not received.
 	//  - Requesting: Request received, pending Ack sent out.
-	//  - Bound: Request sent out, no more pending data to be sent.
+	//  - Bound: Ack sent out, no more pending data to be sent.
 	state ClientState
 }
 
-func (sv *Server) Reset(serverAddr [4]byte, port uint16) {
-	*sv = Server{
-		connID:   sv.connID + 1,
-		siaddr:   serverAddr,
-		port:     port,
-		hosts:    sv.hosts,
-		nextAddr: netip.AddrFrom4(serverAddr),
+// Configure resets and configures the server with the given configuration.
+// The connection ID is incremented on each call to invalidate existing connections.
+// The hosts map is reused across calls to avoid reallocation.
+func (sv *Server) Configure(cfg ServerConfig) error {
+	svAddr := netip.AddrFrom4(cfg.ServerAddr)
+	if !cfg.Subnet.IsValid() {
+		return errors.New("dhcpv4 server: invalid subnet")
+	} else if !cfg.Subnet.Contains(svAddr) {
+		return errors.New("dhcpv4 server: server address outside subnet")
 	}
-	if sv.hosts == nil {
-		sv.hosts = make(map[[36]byte]serverEntry)
+	port := cfg.Port
+	if port == 0 {
+		port = DefaultServerPort
+	}
+	lease := cfg.LeaseSeconds
+	if lease == 0 {
+		lease = 3600
+	}
+	hosts := sv.hosts
+	if hosts == nil {
+		hosts = make(map[[36]byte]serverEntry)
 	} else {
-		for k := range sv.hosts {
-			delete(sv.hosts, k)
+		for k := range hosts {
+			delete(hosts, k)
 		}
 	}
+	*sv = Server{
+		connID:       sv.connID + 1,
+		siaddr:       cfg.ServerAddr,
+		gwaddr:       cfg.Gateway,
+		dns:          cfg.DNS,
+		prefix:       cfg.Subnet,
+		port:         port,
+		leaseSeconds: lease,
+		nextAddr:     svAddr,
+		hosts:        hosts,
+	}
+	return nil
 }
 
 func (sv *Server) ConnectionID() *uint64 { return &sv.connID }
 func (sv *Server) Protocol() uint64      { return uint64(lneto.IPProtoUDP) }
-func (sv *Server) Port() uint16          { return sv.port }
+func (sv *Server) LocalPort() uint16     { return sv.port }
 
 func (sv *Server) Demux(carrierData []byte, frameOffset int) error {
 	isIPLayer := frameOffset >= 28
@@ -103,6 +144,9 @@ func (sv *Server) Demux(carrierData []byte, frameOffset int) error {
 		}
 		return nil
 	})
+	if err != nil {
+		return err
+	}
 	var clientIDRaw [36]byte
 	var client serverEntry
 	var clientExists bool
@@ -115,16 +159,17 @@ func (sv *Server) Demux(carrierData []byte, frameOffset int) error {
 
 	switch msgType {
 	case MsgDiscover:
-		if clientExists {
-			err = errors.New("DHCP Discover on initialized client")
-			break
+		if clientExists && (client.state == StateInit || client.state == StateRequesting) {
+			sv.pending-- // Cancel unfulfilled pending response.
 		}
-		if len(reqAddr) == 4 {
-			println("requested", reqAddr[0], reqAddr[1], reqAddr[2], reqAddr[3])
+		if !clientExists {
+			addr, ok := sv.allocAddr(reqAddr)
+			if !ok {
+				return errors.New("dhcpv4 server: address pool exhausted")
+			}
+			client.addr = addr
 		}
-		sv.nextAddr = sv.nextAddr.Next()
 		copy(client.requestlist[:], reqlist)
-		client.addr = sv.nextAddr.As4()
 		client.state = StateInit
 		client.hostname = string(hostname)
 		client.xid = dfrm.XID()
@@ -137,7 +182,7 @@ func (sv *Server) Demux(carrierData []byte, frameOffset int) error {
 
 	case MsgRequest:
 		if !clientExists {
-			err = errors.New("request for non existing client?")
+			err = errors.New("request for non existing client")
 		} else if dfrm.XID() != client.xid {
 			err = errors.New("unexpected XID for client")
 		} else if client.state != StateSelecting && client.state != StateRequesting {
@@ -146,11 +191,22 @@ func (sv *Server) Demux(carrierData []byte, frameOffset int) error {
 		if err != nil {
 			break
 		}
-		client.state = StateRequesting
-		sv.pending++
+		if client.state == StateSelecting {
+			client.state = StateRequesting
+			sv.pending++
+		}
+
+	case MsgRelease:
+		if clientExists {
+			if client.state == StateInit || client.state == StateRequesting {
+				sv.pending--
+			}
+			delete(sv.hosts, clientIDRaw)
+			return nil
+		}
 
 	default:
-		err = errors.New("unhandled message type")
+		err = fmt.Errorf("unhandled message type %s", msgType.String())
 	}
 	if err != nil {
 		return fmt.Errorf("msgtype=%s client=%+v: %w", msgType.String(), client, err)
@@ -169,7 +225,7 @@ func (sv *Server) Encapsulate(carrierData []byte, offsetToIP, offsetToFrame int)
 		return 0, errOptionNotFit
 	}
 	if sv.pending == 0 {
-		return 0, nil // No pending outgoing frames.a
+		return 0, nil // No pending outgoing frames.
 	}
 
 	var client serverEntry
@@ -205,6 +261,26 @@ func (sv *Server) Encapsulate(carrierData []byte, offsetToIP, offsetToFrame int)
 		n, _ = EncodeOption(optBuf[nopt:], OptRouter, sv.gwaddr[:]...)
 		nopt += n
 	}
+	if sv.prefix.IsValid() {
+		bits := uint(sv.prefix.Bits())
+		mask := ^uint32(0) << (32 - bits)
+		var maskBuf [4]byte
+		binary.BigEndian.PutUint32(maskBuf[:], mask)
+		n, _ = EncodeOption(optBuf[nopt:], OptSubnetMask, maskBuf[:]...)
+		nopt += n
+	}
+	if sv.dns != [4]byte{} {
+		n, _ = EncodeOption(optBuf[nopt:], OptDNSServers, sv.dns[:]...)
+		nopt += n
+	}
+	if sv.leaseSeconds > 0 {
+		n, _ = EncodeOption32(optBuf[nopt:], OptIPAddressLeaseTime, sv.leaseSeconds)
+		nopt += n
+		n, _ = EncodeOption32(optBuf[nopt:], OptRenewTimeValue, sv.leaseSeconds/2)
+		nopt += n
+		n, _ = EncodeOption32(optBuf[nopt:], OptRebindingTimeValue, sv.leaseSeconds*7/8)
+		nopt += n
+	}
 	optBuf[nopt] = byte(OptEnd)
 	nopt++
 
@@ -234,6 +310,40 @@ func (sv *Server) Encapsulate(carrierData []byte, offsetToIP, offsetToFrame int)
 	return OptionsOffset + nopt, nil
 }
 
+// allocAddr allocates the next available address from the pool.
+// If reqAddr is a valid 4-byte address within the subnet and not already assigned,
+// it is preferred. Returns false if the pool is exhausted.
+func (sv *Server) allocAddr(reqAddr []byte) ([4]byte, bool) {
+	if len(reqAddr) == 4 {
+		candidate := netip.AddrFrom4([4]byte(reqAddr))
+		if sv.prefix.Contains(candidate) && candidate.As4() != sv.siaddr && !sv.isAddrAssigned(candidate) {
+			return candidate.As4(), true
+		}
+	}
+	sv.nextAddr = sv.nextAddr.Next()
+	if !sv.prefix.Contains(sv.nextAddr) {
+		return [4]byte{}, false
+	}
+	// Reject broadcast address (all host bits set).
+	a := sv.nextAddr.As4()
+	hostBits := uint(32 - sv.prefix.Bits())
+	hostMask := ^uint32(0) >> (32 - hostBits)
+	if binary.BigEndian.Uint32(a[:])&hostMask == hostMask {
+		return [4]byte{}, false
+	}
+	return a, true
+}
+
+func (sv *Server) isAddrAssigned(addr netip.Addr) bool {
+	a4 := addr.As4()
+	for _, v := range sv.hosts {
+		if v.addr == a4 {
+			return true
+		}
+	}
+	return false
+}
+
 func (sv *Server) getClient(clientID [36]byte) (serverEntry, bool) {
 	entry, ok := sv.hosts[clientID]
 	return entry, ok
@@ -257,4 +367,4 @@ func getSrcIPPort(ipCarrier []byte) (srcaddr []byte, port uint16, err error) {
 	}
 	port = binary.BigEndian.Uint16(ipCarrier[off:]) // TCP and UDP share same port offsets.
 	return srcaddr, port, nil
-}
+}

dhcpv4/server_test.go

@@ -0,0 +1,358 @@
+package dhcpv4
+
+import (
+	"net/netip"
+	"testing"
+)
+
+func testServerConfig(svAddr [4]byte) ServerConfig {
+	return ServerConfig{
+		ServerAddr: svAddr,
+		Subnet:     netip.PrefixFrom(netip.AddrFrom4(svAddr), 24),
+	}
+}
+
+// TestServerMultipleClients verifies the server can handle multiple clients
+// going through the full DORA flow independently.
+func TestServerMultipleClients(t *testing.T) {
+	svAddr := [4]byte{192, 168, 1, 1}
+	var sv Server
+	sv.Configure(testServerConfig(svAddr))
+
+	const nClients = 3
+	var clients [nClients]Client
+	var bufs [nClients][1024]byte
+
+	for i := range clients {
+		err := clients[i].BeginRequest(uint32(100+i), RequestConfig{
+			ClientHardwareAddr: [6]byte{0, 0, 0, 0, 0, byte(i + 1)},
+			Hostname:           "host",
+			ClientID:           string([]byte{byte(i + 1)}),
+		})
+		if err != nil {
+			t.Fatalf("client %d BeginRequest: %v", i, err)
+		}
+	}
+
+	// Phase 1: All clients send DISCOVER.
+	for i := range clients {
+		n, err := clients[i].Encapsulate(bufs[i][:], -1, 0)
+		if err != nil {
+			t.Fatalf("client %d discover encapsulate: %v", i, err)
+		}
+		err = sv.Demux(bufs[i][:n], 0)
+		if err != nil {
+			t.Fatalf("client %d discover demux: %v", i, err)
+		}
+	}
+
+	// Route server responses to the correct client by XID (map iteration is non-deterministic).
+	clientByXID := make(map[uint32]int)
+	for i := range clients {
+		clientByXID[uint32(100+i)] = i
+	}
+
+	// Phase 2: Server sends all OFFERs, clients receive.
+	var assignedAddrs [nClients][4]byte
+	for range clients {
+		var buf [1024]byte
+		n, err := sv.Encapsulate(buf[:], -1, 0)
+		if err != nil {
+			t.Fatalf("offer encapsulate: %v", err)
+		} else if n == 0 {
+			t.Fatal("no offer from server")
+		}
+		frm, _ := NewFrame(buf[:n])
+		ci := clientByXID[frm.XID()]
+		assignedAddrs[ci] = *frm.YIAddr()
+		err = clients[ci].Demux(buf[:n], 0)
+		if err != nil {
+			t.Fatalf("client %d offer demux: %v", ci, err)
+		}
+	}
+
+	// Phase 3: All clients send REQUEST.
+	for i := range clients {
+		n, err := clients[i].Encapsulate(bufs[i][:], -1, 0)
+		if err != nil {
+			t.Fatalf("client %d request encapsulate: %v", i, err)
+		} else if n == 0 {
+			t.Fatalf("client %d: no request data", i)
+		}
+		err = sv.Demux(bufs[i][:n], 0)
+		if err != nil {
+			t.Fatalf("client %d request demux: %v", i, err)
+		}
+	}
+
+	// Phase 4: Server sends all ACKs, clients receive.
+	for range clients {
+		var buf [1024]byte
+		n, err := sv.Encapsulate(buf[:], -1, 0)
+		if err != nil {
+			t.Fatalf("ack encapsulate: %v", err)
+		} else if n == 0 {
+			t.Fatal("no ack from server")
+		}
+		frm, _ := NewFrame(buf[:n])
+		ci := clientByXID[frm.XID()]
+		err = clients[ci].Demux(buf[:n], 0)
+		if err != nil {
+			t.Fatalf("client %d ack demux: %v", ci, err)
+		}
+		if clients[ci].State() != StateBound {
+			t.Errorf("client %d: want StateBound, got %s", ci, clients[ci].State())
+		}
+	}
+
+	// All assigned addresses must be unique.
+	for i := 0; i < nClients; i++ {
+		for j := i + 1; j < nClients; j++ {
+			if assignedAddrs[i] == assignedAddrs[j] {
+				t.Errorf("clients %d and %d got same address %v", i, j, assignedAddrs[i])
+			}
+		}
+	}
+}
+
+// TestServerSequentialAddressAllocation verifies that the server allocates
+// addresses sequentially starting from serverAddr+1.
+func TestServerSequentialAddressAllocation(t *testing.T) {
+	svAddr := [4]byte{192, 168, 1, 1}
+	var sv Server
+	sv.Configure(testServerConfig(svAddr))
+
+	// Build raw DISCOVER frames for two clients.
+	for i := byte(0); i < 2; i++ {
+		var buf [512]byte
+		frm, _ := NewFrame(buf[:])
+		frm.ClearHeader()
+		frm.SetOp(OpRequest)
+		frm.SetHardware(1, 6, 0)
+		frm.SetXID(uint32(200 + i))
+		frm.SetSecs(1)
+		copy(frm.CHAddrAs6()[:], []byte{0, 0, 0, 0, 0, 10 + i})
+		frm.SetMagicCookie(MagicCookie)
+		opts := buf[OptionsOffset:]
+		n := writeOption(opts, OptMessageType, byte(MsgDiscover))
+		n += writeOption(opts[n:], OptClientIdentifier, 10+i)
+		opts[n] = byte(OptEnd)
+		n++
+
+		err := sv.Demux(buf[:OptionsOffset+n], 0)
+		if err != nil {
+			t.Fatalf("discover %d: %v", i, err)
+		}
+	}
+
+	// Encapsulate both OFFERs and verify addresses are in expected range.
+	var seen [2][4]byte
+	for i := byte(0); i < 2; i++ {
+		var buf [512]byte
+		n, err := sv.Encapsulate(buf[:], -1, 0)
+		if err != nil {
+			t.Fatalf("offer %d encapsulate: %v", i, err)
+		} else if n == 0 {
+			t.Fatalf("offer %d: no data", i)
+		}
+		frm, _ := NewFrame(buf[:n])
+		seen[i] = *frm.YIAddr()
+		if seen[i][0] != 192 || seen[i][1] != 168 || seen[i][2] != 1 {
+			t.Errorf("offer %d: unexpected subnet in %v", i, seen[i])
+		}
+		if seen[i][3] != 2 && seen[i][3] != 3 {
+			t.Errorf("offer %d: expected .2 or .3, got .%d", i, seen[i][3])
+		}
+	}
+	if seen[0] == seen[1] {
+		t.Errorf("both offers got same address %v", seen[0])
+	}
+}
+
+// TestServerOfferContainsOptions verifies that server OFFER responses
+// contain the expected DHCP options from the ServerConfig.
+func TestServerOfferContainsOptions(t *testing.T) {
+	svAddr := [4]byte{192, 168, 1, 1}
+	gwAddr := [4]byte{192, 168, 1, 254}
+	dnsAddr := [4]byte{8, 8, 8, 8}
+	var sv Server
+	sv.Configure(ServerConfig{
+		ServerAddr:   svAddr,
+		Gateway:      gwAddr,
+		DNS:          dnsAddr,
+		Subnet:       netip.PrefixFrom(netip.AddrFrom4(svAddr), 24),
+		LeaseSeconds: 7200,
+	})
+
+	var cl Client
+	err := cl.BeginRequest(500, RequestConfig{
+		ClientHardwareAddr: [6]byte{0xde, 0xad, 0xbe, 0xef, 0xca, 0xfe},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf [1024]byte
+	n, err := cl.Encapsulate(buf[:], -1, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = sv.Demux(buf[:n], 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	n, err = sv.Encapsulate(buf[:], -1, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	frm, _ := NewFrame(buf[:n])
+
+	var gotServerID, gotRouter, gotSubnet, gotDNS [4]byte
+	var gotLease, gotRenew, gotRebind uint32
+	var foundServerID, foundRouter, foundSubnet, foundDNS, foundLease bool
+	frm.ForEachOption(func(_ int, opt OptNum, data []byte) error {
+		switch opt {
+		case OptServerIdentification:
+			if len(data) == 4 {
+				foundServerID = true
+				copy(gotServerID[:], data)
+			}
+		case OptRouter:
+			if len(data) == 4 {
+				foundRouter = true
+				copy(gotRouter[:], data)
+			}
+		case OptSubnetMask:
+			if len(data) == 4 {
+				foundSubnet = true
+				copy(gotSubnet[:], data)
+			}
+		case OptDNSServers:
+			if len(data) == 4 {
+				foundDNS = true
+				copy(gotDNS[:], data)
+			}
+		case OptIPAddressLeaseTime:
+			if len(data) == 4 {
+				foundLease = true
+				gotLease = maybeU32(data)
+			}
+		case OptRenewTimeValue:
+			gotRenew = maybeU32(data)
+		case OptRebindingTimeValue:
+			gotRebind = maybeU32(data)
+		}
+		return nil
+	})
+	if !foundServerID || gotServerID != svAddr {
+		t.Errorf("server ID: found=%v got=%v want=%v", foundServerID, gotServerID, svAddr)
+	}
+	if !foundRouter || gotRouter != gwAddr {
+		t.Errorf("router: found=%v got=%v want=%v", foundRouter, gotRouter, gwAddr)
+	}
+	if !foundSubnet || gotSubnet != [4]byte{255, 255, 255, 0} {
+		t.Errorf("subnet: found=%v got=%v want=255.255.255.0", foundSubnet, gotSubnet)
+	}
+	if !foundDNS || gotDNS != dnsAddr {
+		t.Errorf("DNS: found=%v got=%v want=%v", foundDNS, gotDNS, dnsAddr)
+	}
+	if !foundLease || gotLease != 7200 {
+		t.Errorf("lease: found=%v got=%v want=7200", foundLease, gotLease)
+	}
+	if gotRenew != 3600 {
+		t.Errorf("renew T1: got %d want 3600", gotRenew)
+	}
+	if gotRebind != 6300 {
+		t.Errorf("rebind T2: got %d want 6300", gotRebind)
+	}
+}
+
+// TestServerEncapsulateNoPending verifies Encapsulate returns 0 bytes
+// when there are no pending responses.
+func TestServerEncapsulateNoPending(t *testing.T) {
+	var sv Server
+	sv.Configure(testServerConfig([4]byte{192, 168, 1, 1}))
+
+	var buf [512]byte
+	n, err := sv.Encapsulate(buf[:], -1, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 0 {
+		t.Errorf("expected 0 bytes from empty server, got %d", n)
+	}
+}
+
+// TestServerConfigValidation verifies that Configure rejects invalid configurations.
+func TestServerConfigValidation(t *testing.T) {
+	var sv Server
+	err := sv.Configure(ServerConfig{
+		ServerAddr: [4]byte{192, 168, 1, 1},
+	})
+	if err == nil {
+		t.Error("expected error for zero subnet")
+	}
+	err = sv.Configure(ServerConfig{
+		ServerAddr: [4]byte{10, 0, 0, 1},
+		Subnet:     netip.PrefixFrom(netip.AddrFrom4([4]byte{192, 168, 1, 0}), 24),
+	})
+	if err == nil {
+		t.Error("expected error for server address outside subnet")
+	}
+}
+
+// TestServerRediscover verifies that a client that was previously bound
+// can send a fresh DISCOVER and get re-served.
+func TestServerRediscover(t *testing.T) {
+	svAddr := [4]byte{192, 168, 1, 1}
+	var sv Server
+	sv.Configure(testServerConfig(svAddr))
+
+	// First DORA cycle.
+	var cl Client
+	cl.BeginRequest(1, RequestConfig{
+		ClientHardwareAddr: [6]byte{1, 2, 3, 4, 5, 6},
+		ClientID:           "rediscover-client",
+	})
+	var buf [1024]byte
+	n, _ := cl.Encapsulate(buf[:], -1, 0)
+	sv.Demux(buf[:n], 0)
+	n, _ = sv.Encapsulate(buf[:], -1, 0)
+	cl.Demux(buf[:n], 0)
+	n, _ = cl.Encapsulate(buf[:], -1, 0)
+	sv.Demux(buf[:n], 0)
+	n, _ = sv.Encapsulate(buf[:], -1, 0)
+	cl.Demux(buf[:n], 0)
+	if cl.State() != StateBound {
+		t.Fatalf("first DORA: want StateBound, got %s", cl.State())
+	}
+
+	// Client reboots and sends fresh DISCOVER.
+	cl.Reset()
+	cl.BeginRequest(2, RequestConfig{
+		ClientHardwareAddr: [6]byte{1, 2, 3, 4, 5, 6},
+		ClientID:           "rediscover-client",
+	})
+	n, _ = cl.Encapsulate(buf[:], -1, 0)
+	err := sv.Demux(buf[:n], 0)
+	if err != nil {
+		t.Fatalf("rediscover demux: %v", err)
+	}
+	n, _ = sv.Encapsulate(buf[:], -1, 0)
+	if n == 0 {
+		t.Fatal("no offer after rediscover")
+	}
+	err = cl.Demux(buf[:n], 0)
+	if err != nil {
+		t.Fatalf("rediscover offer demux: %v", err)
+	}
+	// Complete the second DORA.
+	n, _ = cl.Encapsulate(buf[:], -1, 0)
+	sv.Demux(buf[:n], 0)
+	n, _ = sv.Encapsulate(buf[:], -1, 0)
+	cl.Demux(buf[:n], 0)
+	if cl.State() != StateBound {
+		t.Errorf("second DORA: want StateBound, got %s", cl.State())
+	}
+}

ethernet/frame.go

@@ -79,6 +79,24 @@ func (efrm Frame) SetEtherType(v Type) {
 // VLANTag returns the VLAN tag field following the TPID=0x8100. See [VLANTag]. Call [Frame.ValidateSize] to ensure this function does not panic.
 func (efrm Frame) VLANTag() VLANTag { return VLANTag(binary.BigEndian.Uint16(efrm.buf[14:16])) }
 
+// SetVLAN sets following 3 fields:
+//   - 12:14 ethernet frame type set to constant [TypeVLAN].
+//   - 14:16 set to VLANTag argument value vt
+//   - 16:18 set to the VLAN ether type vlanType.
+func (efrm Frame) SetVLAN(tag VLANTag, vlanType Type) {
+	efrm.SetEtherType(TypeVLAN)
+	binary.BigEndian.PutUint16(efrm.buf[14:16], uint16(tag))
+	binary.BigEndian.PutUint16(efrm.buf[16:18], uint16(vlanType))
+}
+
+// VLAN returns fields 14:16 and 16:18. Does not check field 12:14 for correctness.
+// VLAN panics if length is insufficient.
+func (efrm Frame) VLAN() (VLANTag, Type) {
+	vt := binary.BigEndian.Uint16(efrm.buf[14:16])
+	et := binary.BigEndian.Uint16(efrm.buf[16:18])
+	return VLANTag(vt), Type(et)
+}
+
 // SetVLANTag sets the VLAN tag field of the Ethernet Header. See [VLANTag]. Call [Frame.ValidateSize] to ensure this function does not panic.
 func (efrm Frame) SetVLANTag(vt VLANTag) { binary.BigEndian.PutUint16(efrm.buf[14:16], uint16(vt)) }
 

examples/httpserver/index.html

@@ -0,0 +1,92 @@
+<!DOCTYPE html>
+<html>
+<head>
+	<title>Arbitrary Waveform Generator Control</title>
+	<meta name="viewport" content="width=device-width, initial-scale=1">
+	<style>
+		body {
+			font-family: Arial, sans-serif;
+			max-width: 800px;
+			margin: 50px auto;
+			padding: 20px;
+			background-color: #f5f5f5;
+		}
+		h1 {
+			color: #333;
+			text-align: center;
+		}
+		.chip-control {
+			background: white;
+			border-radius: 8px;
+			padding: 20px;
+			margin: 20px 0;
+			box-shadow: 0 2px 4px rgba(0,0,0,0.1);
+		}
+		.chip-name {
+			font-size: 18px;
+			font-weight: bold;
+			color: #2c3e50;
+			margin-bottom: 15px;
+		}
+		.form-group {
+			margin: 10px 0;
+		}
+		label {
+			display: inline-block;
+			width: 120px;
+			font-weight: bold;
+			color: #555;
+		}
+		input[type="number"] {
+			width: 200px;
+			padding: 8px;
+			border: 1px solid #ddd;
+			border-radius: 4px;
+			font-size: 14px;
+		}
+		button {
+			background-color: #3498db;
+			color: white;
+			border: none;
+			padding: 10px 20px;
+			border-radius: 4px;
+			cursor: pointer;
+			font-size: 14px;
+			margin-top: 10px;
+		}
+		button:hover {
+			background-color: #2980b9;
+		}
+		.current-value {
+			color: #888;
+			font-size: 12px;
+			margin-left: 130px;
+		}
+	</style>
+</head>
+<body>
+	<h1>AD9850 DDS Control</h1>
+	<div>
+	<!-- Begin Poetry -->
+	<p>
+	Whose woods these are I think I know.<br>
+	His house is in the village though;<br>
+	He will not see me stopping here<br>
+	To watch his woods fill up with snow.<br>
+	<br>
+	My little horse must think it queer<br>
+	To stop without a farmhouse near<br>
+	Between the woods and frozen lake<br>
+	The darkest evening of the year.<br>
+	<br>
+	He gives his harness bells a shake<br>
+	To ask if there is some mistake.<br>
+	The only other sound’s the sweep<br>
+	Of easy wind and downy flake.<br>
+	<br>
+	The woods are lovely, dark and deep,<br>
+	But I have promises to keep,<br>
+	And miles to go before I sleep,<br>
+	</div
+</body>
+</html>

examples/httpserver/main.go

@@ -0,0 +1,356 @@
+//go:build !tinygo && linux
+
+package main
+
+import (
+	"bytes"
+	_ "embed"
+	"encoding/hex"
+	"errors"
+	"flag"
+	"fmt"
+	"log"
+	"log/slog"
+	"math"
+	"net"
+	"net/netip"
+	"os"
+	"runtime"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/soypat/lneto"
+	"github.com/soypat/lneto/ethernet"
+	"github.com/soypat/lneto/http/httpraw"
+	"github.com/soypat/lneto/internal"
+	"github.com/soypat/lneto/internal/ltesto"
+	"github.com/soypat/lneto/internet/pcap"
+	"github.com/soypat/lneto/tcp"
+	"github.com/soypat/lneto/x/xnet"
+)
+
+//go:embed index.html
+var indexhtml string
+
+var softRand = time.Now().Unix()
+
+func main() {
+	err := run()
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+	fmt.Println("success")
+}
+
+func run() (err error) {
+	var (
+		flagInterface = "tap0"
+		flagUseHTTP   = false
+		flagNoPcap    = false
+		flagPort      = 80
+	)
+	flag.StringVar(&flagInterface, "i", flagInterface, "Interface to use. Either tap* or the name of an existing interface to bridge to.")
+	flag.BoolVar(&flagUseHTTP, "ihttp", flagUseHTTP, "Use HTTP tap interface.")
+	flag.BoolVar(&flagNoPcap, "nopcap", flagNoPcap, "Disable pcap logging.")
+	flag.IntVar(&flagPort, "port", flagPort, "Port to listen on.")
+	flag.Usage = func() {
+		fmt.Fprintf(os.Stderr, "httpserver is a minimal HTTP server using the lneto networking stack.\n")
+		flag.PrintDefaults()
+	}
+	flag.Parse()
+	fmt.Println("softrand", softRand)
+	var iface ltesto.Interface
+	if flagUseHTTP {
+		iface = ltesto.NewHTTPTapClient("http://127.0.0.1:7070")
+	} else {
+		if strings.HasPrefix(flagInterface, "tap") {
+			tap, err := internal.NewTap(flagInterface, netip.MustParsePrefix("192.168.1.1/24"))
+			if err != nil {
+				return err
+			}
+			iface = tap
+		} else {
+			bridge, err := internal.NewBridge(flagInterface)
+			if err != nil {
+				return err
+			}
+			err = bridge.SetReadTimeout(5 * time.Millisecond)
+			if err != nil {
+				return err
+			}
+			iface = bridge
+		}
+	}
+	defer iface.Close()
+
+	nicHW, err := iface.HardwareAddress6()
+	if err != nil {
+		return err
+	}
+	mtu, err := iface.MTU()
+	if err != nil {
+		return err
+	}
+	nicAddr, err := iface.IPMask()
+	if err != nil {
+		return err
+	}
+	fmt.Println("NIC hardware address:", net.HardwareAddr(nicHW[:]).String(), "mtu:", mtu, "addr:", nicAddr.String())
+
+	var stack xnet.StackAsync
+	err = stack.Reset(xnet.StackConfig{
+		Hostname:        "httpserver",
+		RandSeed:        softRand,
+		HardwareAddress: nicHW,
+		MTU:             uint16(mtu),
+		MaxTCPConns:     1000,
+	})
+	if err != nil {
+		return err
+	}
+
+	// Loop goroutine handles packet encapsulation/decapsulation.
+	go func() {
+		lastAction := time.Now()
+		buf := make([]byte, math.MaxUint16)
+		var cap pcap.PacketBreakdown
+		var frames []pcap.Frame
+		pf := pcap.Formatter{
+			FilterClasses: []pcap.FieldClass{pcap.FieldClassFlags, pcap.FieldClassOperation, pcap.FieldClassDst, pcap.FieldClassSrc, pcap.FieldClassAddress, pcap.FieldClassTimestamp},
+		}
+		var pfbuf []byte
+		logFrames := func(context string, pkt []byte) error {
+			if flagNoPcap {
+				return nil
+			}
+			frames, err = cap.CaptureEthernet(frames[:0], pkt, 0)
+			if err != nil {
+				pkt := hex.EncodeToString(pkt)
+				slog.Error(err.Error(), slog.Any("pkt", pkt))
+				return err
+			}
+			pfbuf = fmt.Appendf(pfbuf[:0], "%-3s %3d", context, len(pkt))
+			pfbuf = append(pfbuf, ' ', '[')
+			pfbuf, err = pf.FormatFrames(pfbuf, frames, pkt)
+			pfbuf = bytes.ReplaceAll(pfbuf, stack.Addr().AppendTo(nil), []byte("us"))
+			pfbuf = bytes.ReplaceAll(pfbuf, ethernet.AppendAddr(nil, stack.HardwareAddress()), []byte("us"))
+			pfbuf = append(pfbuf, ']', '\n')
+			if err != nil {
+				return err
+			}
+			_, err = os.Stdout.Write(pfbuf)
+			return err
+		}
+		for {
+			nwrite, err := stack.Encapsulate(buf[:], -1, 0)
+			if err != nil {
+				log.Println("ERR:ENCAPSULATE", err)
+			} else if nwrite > 0 {
+				err = logFrames("OUT", buf[:nwrite])
+				if err != nil {
+					log.Println("ERR:OUTLOG", err)
+				}
+				n, err := iface.Write(buf[:nwrite])
+				if err != nil {
+					log.Fatal("goroutine encapsulate:", err)
+				} else if n != nwrite {
+					log.Fatalf("mismatch written bytes %d!=%d", nwrite, n)
+				}
+			}
+
+			clear(buf[:nwrite])
+			ready, err := tryPoll(iface, 5*time.Millisecond)
+			if err != nil {
+				log.Fatal("goroutine poll:", err)
+			}
+			if !ready {
+				continue
+			}
+			nread, err := iface.Read(buf)
+			if err != nil {
+				log.Fatal("goroutine read:", err)
+			} else if nread > 0 {
+				err = stack.Demux(buf[:nread], 0)
+				if !errors.Is(err, lneto.ErrPacketDrop) {
+					err = logFrames("IN", buf[:nread])
+					if err != nil {
+						log.Println("ERR:INLOG", err)
+					}
+				}
+			}
+			clear(buf[:nread])
+			if nread == 0 && nwrite == 0 && time.Since(lastAction) > 4*time.Second {
+				time.Sleep(5 * time.Millisecond)
+			} else {
+				lastAction = time.Now()
+				runtime.Gosched()
+			}
+		}
+	}()
+
+	rstack := stack.StackRetrying(5 * time.Millisecond)
+
+	const (
+		dhcpTimeout = 6 * time.Second
+		dhcpRetries = 2
+	)
+	timeDHCP := timer("DHCP request completed")
+	results, err := rstack.DoDHCPv4([4]byte{192, 168, 1, 96}, dhcpTimeout, dhcpRetries)
+	if err != nil {
+		return fmt.Errorf("DHCP failed: %w", err)
+	}
+	timeDHCP()
+	err = stack.AssimilateDHCPResults(results)
+	if err != nil {
+		return fmt.Errorf("assimilating DHCP results: %w", err)
+	}
+	slog.Info("dhcp-complete", slog.String("assignedIP", results.AssignedAddr.String()), slog.String("routerIP", results.Router.String()))
+
+	const (
+		arpTimeout = 2 * time.Second
+		arpRetries = 2
+	)
+	timeResolveRouterHW := timer("Router ARP resolution")
+	routerHw, err := rstack.DoResolveHardwareAddress6(results.Router, arpTimeout, arpRetries)
+	if err != nil {
+		return fmt.Errorf("ARP resolution of router failed: %w", err)
+	}
+	timeResolveRouterHW()
+	stack.SetGateway6(routerHw)
+
+	svPort := uint16(flagPort)
+	fmt.Printf("Listening on %s:%d\n", stack.Addr().String(), svPort)
+
+	// Serve connections in a loop.
+	for {
+		var conn tcp.Conn
+		conn.Configure(tcp.ConnConfig{
+			RxBuf:             make([]byte, mtu),
+			TxBuf:             make([]byte, mtu),
+			TxPacketQueueSize: 3,
+		})
+		err = stack.ListenTCP(&conn, svPort)
+		if err != nil {
+			return fmt.Errorf("listen TCP: %w", err)
+		}
+		fmt.Println("waiting for connection...")
+
+		// Wait for TCP handshake to complete.
+		deadline := time.Now().Add(60 * time.Second)
+		for conn.State() != tcp.StateEstablished {
+			if time.Now().After(deadline) {
+				conn.Abort()
+				fmt.Println("listen timeout, retrying...")
+				break
+			}
+			time.Sleep(5 * time.Millisecond)
+		}
+		if conn.State() != tcp.StateEstablished {
+			continue
+		}
+		fmt.Println("connection established from", net.IP(conn.RemoteAddr()).String())
+		go func() {
+			err = handleConnection(&conn)
+			if err != nil {
+				fmt.Println("handle error:", err)
+			}
+		}()
+	}
+}
+
+func handleConnection(conn *tcp.Conn) error {
+	conn.SetDeadline(time.Now().Add(10 * time.Second))
+
+	// Read HTTP request.
+	var hdr httpraw.Header
+	var needMore bool = true
+	for needMore {
+		_, err := hdr.ReadFromLimited(conn, 1024)
+		if err != nil {
+			return fmt.Errorf("reading request: %w", err)
+		}
+		const asResponse = false
+		needMore, err = hdr.TryParse(asResponse)
+		if err != nil && !needMore {
+			return fmt.Errorf("parsing request: %w", err)
+		}
+	}
+
+	method := string(hdr.Method())
+	uri := string(hdr.RequestURI())
+	fmt.Printf("< %s %s\n", method, uri)
+
+	// Build response body.
+
+	// Build HTTP response.
+	var resp httpraw.Header
+	resp.SetProtocol("HTTP/1.1")
+	resp.SetStatus("200", "OK")
+	resp.Set("Content-Type", "text/html")
+	resp.Set("Content-Length", strconv.Itoa(len(indexhtml)))
+	resp.Set("Connection", "close")
+	response, err := resp.AppendResponse(nil)
+	if err != nil {
+		return fmt.Errorf("building response: %w", err)
+	}
+	response = append(response, indexhtml...)
+
+	// Send response.
+	_, err = conn.Write(response)
+	if err != nil {
+		return fmt.Errorf("writing response: %w", err)
+	}
+	err = conn.Flush()
+	if err != nil {
+		return fmt.Errorf("flushing response: %w", err)
+	}
+	fmt.Printf("> %d bytes sent\n", len(response))
+
+	conn.Close()
+	return nil
+}
+
+func clear(buf []byte) {
+	for i := range buf {
+		buf[i] = 0
+	}
+}
+
+func timer(context string) func() {
+	start := time.Now()
+	return func() {
+		elapsed := time.Since(start)
+		fmt.Printf("[%s] %s\n", prettyDuration(elapsed), context)
+	}
+}
+
+func prettyDuration(d time.Duration) string {
+	switch {
+	case d < time.Microsecond:
+		// Print as is.
+	case d < time.Millisecond:
+		d = d.Round(time.Microsecond)
+	case d < time.Second:
+		d = d.Round(time.Millisecond)
+	case d < 10*time.Second:
+		d = d.Round(100 * time.Millisecond)
+	case d < 10*time.Minute:
+		d = d.Round(1000 * time.Millisecond)
+	case d < time.Hour:
+		d = d.Round(time.Minute)
+	}
+	return d.String()
+}
+
+func tryPoll(iface ltesto.Interface, poll time.Duration) (dataMayBeReady bool, _ error) {
+	if poller, ok := iface.(interface {
+		Poll(time.Duration) (bool, error)
+	}); ok {
+		ready, err := poller.Poll(poll)
+		return ready, err
+	}
+	dataMayBeReady = true
+	return dataMayBeReady, nil
+}

examples/httptap/dhcp.go

@@ -0,0 +1,350 @@
+//go:build !tinygo && linux
+
+package main
+
+import (
+	"encoding/binary"
+	"fmt"
+	"net/netip"
+	"sync"
+
+	"github.com/soypat/lneto"
+	"github.com/soypat/lneto/arp"
+	"github.com/soypat/lneto/dhcpv4"
+	"github.com/soypat/lneto/ethernet"
+	"github.com/soypat/lneto/internal/ltesto"
+	"github.com/soypat/lneto/ipv4"
+	"github.com/soypat/lneto/udp"
+)
+
+const (
+	sizeEthernet = 14
+	sizeIPv4     = 20
+	sizeUDP      = 8
+	sizeARPv4    = 28
+	sizeDHCPMin  = dhcpv4.OptionsOffset + 256 // Minimum space for DHCP frame + options.
+)
+
+// dhcpInterceptor wraps an ltesto.Interface and intercepts DHCP traffic.
+// DHCP packets from the client are handled by an embedded dhcpv4.Server
+// and never forwarded to the real interface. DHCP responses are returned
+// on subsequent Read calls. All non-DHCP traffic passes through unchanged.
+type dhcpInterceptor struct {
+	mu    sync.Mutex
+	inner ltesto.Interface
+	sv    dhcpv4.Server
+
+	// Server network identity.
+	svMAC [6]byte
+	svIP  [4]byte
+
+	// Pending ARP reply.
+	arpReply [sizeEthernet + sizeARPv4]byte
+	arpReady bool
+
+	// ARP cache for gateway forwarding: maps IP→MAC from snooped traffic.
+	arpCache [8]arpEntry
+}
+
+type arpEntry struct {
+	mac [6]byte
+	ip  [4]byte
+}
+
+// newDHCPInterceptor creates a dhcpInterceptor that wraps iface and serves
+// DHCP from the given server address and subnet.
+func newDHCPInterceptor(iface ltesto.Interface, svIP [4]byte, svMAC [6]byte, subnet netip.Prefix) (*dhcpInterceptor, error) {
+	d := &dhcpInterceptor{
+		inner: iface,
+		svMAC: svMAC,
+		svIP:  svIP,
+	}
+	err := d.sv.Configure(dhcpv4.ServerConfig{
+		ServerAddr: svIP,
+		Gateway:    svIP,
+		DNS:        [4]byte{8, 8, 8, 8},
+		Subnet:     subnet,
+	})
+	return d, err
+}
+
+func (d *dhcpInterceptor) Write(b []byte) (int, error) {
+	if d.isARPRequestForUs(b) {
+		d.mu.Lock()
+		d.buildARPReply(b)
+		d.mu.Unlock()
+		return len(b), nil
+	}
+	if isDHCPRequest(b) {
+		d.mu.Lock()
+		defer d.mu.Unlock()
+		dhcpOff := dhcpOffset(b)
+		if dhcpOff < 0 {
+			return d.inner.Write(b) // Malformed, pass through.
+		}
+		err := d.sv.Demux(b, dhcpOff)
+		if err != nil {
+			return 0, fmt.Errorf("dhcp server demux: %w", err)
+		}
+		return len(b), nil // Consumed by DHCP server, don't forward.
+	}
+	d.rewriteEthernetDst(b)
+	return d.inner.Write(b)
+}
+
+func (d *dhcpInterceptor) Read(b []byte) (int, error) {
+	d.mu.Lock()
+	if d.arpReady {
+		n := copy(b, d.arpReply[:])
+		d.arpReady = false
+		d.mu.Unlock()
+		return n, nil
+	}
+	n, err := d.buildDHCPResponse(b)
+	d.mu.Unlock()
+	if n > 0 {
+		return n, nil
+	}
+	if err != nil {
+		return 0, err
+	}
+	n, err = d.inner.Read(b)
+	if n >= sizeEthernet+sizeARPv4 && binary.BigEndian.Uint16(b[12:14]) == uint16(ethernet.TypeARP) {
+		d.snoopARP(b[:n])
+	}
+	return n, err
+}
+
+// buildDHCPResponse tries to get a pending DHCP response from the server and
+// wraps it in Ethernet + IPv4 + UDP headers. Returns 0 if no response pending.
+// Caller must hold d.mu.
+func (d *dhcpInterceptor) buildDHCPResponse(buf []byte) (int, error) {
+	if len(buf) < sizeEthernet+sizeIPv4+sizeUDP+sizeDHCPMin {
+		return 0, nil
+	}
+	// Build Ethernet+IPv4 headers since DHCP server may use hardware/ip addr.
+	efrm, _ := ethernet.NewFrame(buf)
+	*efrm.DestinationHardwareAddr() = [6]byte{}
+	*efrm.SourceHardwareAddr() = d.svMAC
+	efrm.SetEtherType(ethernet.TypeIPv4)
+
+	ifrm, _ := ipv4.NewFrame(buf[sizeEthernet:])
+	ifrm.SetVersionAndIHL(4, 5)
+	ifrm.SetToS(0)
+	ifrm.SetFlags(ipv4.FlagDontFragment)
+	ifrm.SetTTL(64)
+	ifrm.SetProtocol(lneto.IPProtoUDP)
+	*ifrm.SourceAddr() = d.svIP
+	*ifrm.DestinationAddr() = [4]byte{}
+
+	// Build UDP header.
+	ufrm, _ := udp.NewFrame(buf[sizeEthernet+sizeIPv4:])
+	ufrm.SetSourcePort(dhcpv4.DefaultServerPort)
+	ufrm.SetDestinationPort(dhcpv4.DefaultClientPort)
+
+	dhcpStart := sizeEthernet + sizeIPv4 + sizeUDP
+	// Ask DHCP server to fill in the payload. offsetToIP=sizeEthernet so
+	// the server can set IP src/dst via internal.SetIPAddrs.
+	dhcpLen, err := d.sv.Encapsulate(buf, sizeEthernet, dhcpStart)
+	if err != nil {
+		return 0, fmt.Errorf("dhcp server encapsulate: %w", err)
+	}
+	if dhcpLen == 0 {
+		return 0, nil // No pending response.
+	}
+
+	totalIPLen := uint16(sizeIPv4 + sizeUDP + dhcpLen)
+	udpLen := uint16(sizeUDP + dhcpLen)
+	totalFrameLen := sizeEthernet + int(totalIPLen)
+
+	// DHCP responses must be broadcast since the client doesn't have
+	// an IP configured yet and the stack would drop unicast packets.
+	*efrm.DestinationHardwareAddr() = ethernet.BroadcastAddr()
+	*ifrm.DestinationAddr() = [4]byte{255, 255, 255, 255}
+	ifrm.SetTotalLength(totalIPLen)
+	ufrm.SetLength(udpLen)
+	// Source and destination IPs already set by dhcpv4.Server.Encapsulate.
+	ifrm.SetCRC(0)
+	prelimCRC := ifrm.CalculateHeaderCRC()
+	ifrm.SetID(^(^prelimCRC * 37))
+	ifrm.SetCRC(0)
+	ifrm.SetCRC(ifrm.CalculateHeaderCRC())
+	// Compute UDP checksum (required, the lneto stack validates it on Demux).
+	ufrm.SetCRC(0)
+	var udpCRC lneto.CRC791
+	ifrm.CRCWriteUDPPseudo(&udpCRC, udpLen)
+	ufrm.SetCRC(lneto.NeverZeroSum(udpCRC.PayloadSum16(ufrm.RawData()[:udpLen])))
+	return totalFrameLen, nil
+}
+
+// isDHCPRequest checks if a raw Ethernet frame is a DHCP request (client → server).
+// Checks: EtherType=IPv4, IP proto=UDP, UDP dst port=67, DHCP magic cookie.
+func isDHCPRequest(b []byte) bool {
+	if len(b) < sizeEthernet+sizeIPv4+sizeUDP+dhcpv4.OptionsOffset {
+		return false
+	}
+	// EtherType must be IPv4.
+	if binary.BigEndian.Uint16(b[12:14]) != uint16(ethernet.TypeIPv4) {
+		return false
+	}
+	// IP header length (IHL) to find UDP header.
+	ihl := int(b[sizeEthernet]&0xf) * 4
+	if ihl < sizeIPv4 {
+		return false
+	}
+	ipStart := sizeEthernet
+	// IP protocol must be UDP.
+	if b[ipStart+9] != uint8(lneto.IPProtoUDP) {
+		return false
+	}
+	udpStart := ipStart + ihl
+	if len(b) < udpStart+sizeUDP {
+		return false
+	}
+	// UDP destination port must be DHCP server port (67).
+	dstPort := binary.BigEndian.Uint16(b[udpStart+2 : udpStart+4])
+	if dstPort != dhcpv4.DefaultServerPort {
+		return false
+	}
+	// Verify DHCP magic cookie.
+	dhcpStart := udpStart + sizeUDP
+	return dhcpv4.PayloadIsDHCPv4(b[dhcpStart:])
+}
+
+// dhcpOffset returns the byte offset where the DHCP payload begins
+// within a raw Ethernet frame. Returns -1 if the frame is too short.
+func dhcpOffset(b []byte) int {
+	if len(b) < sizeEthernet+sizeIPv4+sizeUDP {
+		return -1
+	}
+	ihl := int(b[sizeEthernet]&0xf) * 4
+	off := sizeEthernet + ihl + sizeUDP
+	if off > len(b) {
+		return -1
+	}
+	return off
+}
+
+// isARPRequestForUs checks if b is an ARP request targeting d.svIP.
+func (d *dhcpInterceptor) isARPRequestForUs(b []byte) bool {
+	if len(b) < sizeEthernet+sizeARPv4 {
+		return false
+	}
+	if binary.BigEndian.Uint16(b[12:14]) != uint16(ethernet.TypeARP) {
+		return false
+	}
+	afrm, err := arp.NewFrame(b[sizeEthernet:])
+	if err != nil {
+		return false
+	}
+	if afrm.Operation() != arp.OpRequest {
+		return false
+	}
+	_, targetIP := afrm.Target4()
+	return *targetIP == d.svIP
+}
+
+// buildARPReply constructs an ARP reply in d.arpReply from the given ARP request.
+// Caller must hold d.mu.
+func (d *dhcpInterceptor) buildARPReply(request []byte) {
+	reqARP, _ := arp.NewFrame(request[sizeEthernet:])
+	senderHW, senderIP := reqARP.Sender4()
+
+	buf := d.arpReply[:]
+	// Ethernet header: reply to requester.
+	efrm, _ := ethernet.NewFrame(buf)
+	*efrm.DestinationHardwareAddr() = *senderHW
+	*efrm.SourceHardwareAddr() = d.svMAC
+	efrm.SetEtherType(ethernet.TypeARP)
+
+	// ARP reply.
+	afrm, _ := arp.NewFrame(buf[sizeEthernet:])
+	afrm.SetHardware(1, 6)                       // Ethernet, 6-byte addresses
+	afrm.SetProtocol(ethernet.TypeIPv4, 4)        // IPv4, 4-byte addresses
+	afrm.SetOperation(arp.OpReply)
+	replySndrHW, replySndrIP := afrm.Sender4()
+	*replySndrHW = d.svMAC
+	*replySndrIP = d.svIP
+	replyTgtHW, replyTgtIP := afrm.Target4()
+	*replyTgtHW = *senderHW
+	*replyTgtIP = *senderIP
+
+	d.arpReady = true
+}
+
+// snoopARP records the sender's IP→MAC mapping from an ARP packet.
+func (d *dhcpInterceptor) snoopARP(b []byte) {
+	afrm, err := arp.NewFrame(b[sizeEthernet:])
+	if err != nil {
+		return
+	}
+	senderHW, senderIP := afrm.Sender4()
+	if *senderIP == ([4]byte{}) {
+		return
+	}
+	d.mu.Lock()
+	d.arpCacheStore(*senderHW, *senderIP)
+	d.mu.Unlock()
+}
+
+// rewriteEthernetDst rewrites the Ethernet destination MAC for frames
+// addressed to the gateway (svMAC). Acts as a basic IP forwarder by
+// looking up the destination IP in the ARP cache.
+func (d *dhcpInterceptor) rewriteEthernetDst(b []byte) {
+	if len(b) < sizeEthernet+sizeIPv4 {
+		return
+	}
+	// Only rewrite frames addressed to the gateway.
+	if *(*[6]byte)(b[0:6]) != d.svMAC {
+		return
+	}
+	// Only rewrite IPv4 frames.
+	if binary.BigEndian.Uint16(b[12:14]) != uint16(ethernet.TypeIPv4) {
+		return
+	}
+	dstIP := *(*[4]byte)(b[sizeEthernet+16 : sizeEthernet+20])
+	d.mu.Lock()
+	mac, ok := d.arpCacheLookup(dstIP)
+	d.mu.Unlock()
+	if ok {
+		copy(b[0:6], mac[:])
+	}
+}
+
+// arpCacheLookup finds a MAC for the given IP. Caller must hold d.mu.
+func (d *dhcpInterceptor) arpCacheLookup(ip [4]byte) ([6]byte, bool) {
+	for i := range d.arpCache {
+		if d.arpCache[i].ip == ip {
+			return d.arpCache[i].mac, true
+		}
+	}
+	return [6]byte{}, false
+}
+
+// arpCacheStore adds or updates an IP→MAC entry. Caller must hold d.mu.
+func (d *dhcpInterceptor) arpCacheStore(mac [6]byte, ip [4]byte) {
+	// Update existing entry.
+	for i := range d.arpCache {
+		if d.arpCache[i].ip == ip {
+			d.arpCache[i].mac = mac
+			return
+		}
+	}
+	// Find empty slot.
+	for i := range d.arpCache {
+		if d.arpCache[i].ip == ([4]byte{}) {
+			d.arpCache[i] = arpEntry{mac: mac, ip: ip}
+			return
+		}
+	}
+	// Evict first entry.
+	copy(d.arpCache[:], d.arpCache[1:])
+	d.arpCache[len(d.arpCache)-1] = arpEntry{mac: mac, ip: ip}
+}
+
+// Delegate remaining ltesto.Interface methods to inner.
+
+func (d *dhcpInterceptor) Close() error                       { return d.inner.Close() }
+func (d *dhcpInterceptor) HardwareAddress6() ([6]byte, error) { return d.inner.HardwareAddress6() }
+func (d *dhcpInterceptor) MTU() (int, error)                  { return d.inner.MTU() }
+func (d *dhcpInterceptor) IPMask() (netip.Prefix, error)      { return d.inner.IPMask() }

examples/httptap/main.go

@@ -9,6 +9,7 @@ import (
 	"net"
 	"net/http"
 	"net/netip"
+	"strconv"
 	"strings"
 	"time"
 
@@ -61,6 +62,21 @@ func run() error {
 		iface = br
 	}
 
+	// Wrap interface with DHCP server interceptor.
+	hwaddr, err := iface.HardwareAddress6()
+	if err != nil {
+		return err
+	}
+	ipMask, err := iface.IPMask()
+	if err != nil {
+		return err
+	}
+	svIP := ipMask.Addr().As4()
+	iface, err = newDHCPInterceptor(iface, svIP, hwaddr, ipMask.Masked())
+	if err != nil {
+		return fmt.Errorf("DHCP interceptor: %w", err)
+	}
+
 	sv, err := ltesto.NewHTTPTapServer(iface, flagMinMTU, flagPacketQueueSize, flagPacketQueueSize)
 	if err != nil {
 		return err
@@ -72,6 +88,10 @@ func run() error {
 	}
 	var pfbuf []byte
 	sv.OnTransfer(func(channel int, pkt []byte) {
+		channelstr := "OS"
+		if channel != 0 {
+			channelstr = strconv.Itoa(channel) // Will not allocate for values 99 and under (stdlib).
+		}
 		captime := time.Now()
 		frames, err := cap.CaptureEthernet(nil, pkt, 0)
 		if err == nil {
@@ -79,15 +99,15 @@ func run() error {
 			pfbuf, err = pf.FormatFrames(pfbuf, frames, pkt)
 			pfbuf = append(pfbuf, ']')
 			if err != nil {
-				fmt.Printf("%d %s !err:%s\n", channel, captime.Format("15:04:05.000"), err)
+				fmt.Printf("%-2s %s !err:%s\n", channelstr, captime.Format("15:04:05.000"), err)
 			} else {
-				fmt.Printf("%d %s %s\n", channel, captime.Format("15:04:05.000"), pfbuf)
+				fmt.Printf("%-2s %s %s\n", channelstr, captime.Format("15:04:05.000"), pfbuf)
 			}
 		} else {
-			fmt.Println(channel, captime.Format("15:04:05.000"), "cap ERR", frames, err.Error())
+			fmt.Printf("%-2s %s %s %v %s\n", channelstr, captime.Format("15:04:05.000"), "cap ERR", frames, err.Error())
 		}
 	})
-	hwaddr, err := sv.HardwareAddress6()
+	hwaddr, err = sv.HardwareAddress6()
 	if err != nil {
 		return err
 	}

internet/pcap/capture.go

@@ -244,10 +244,12 @@ func (pc *PacketBreakdown) CaptureIPv4(dst []Frame, pkt []byte, bitOffset int) (
 				println("BAD UDP")
 				return dst, pc.vld.ErrPop()
 			}
-			frameLen := ufrm.Length()
-			ifrm4.CRCWriteUDPPseudo(&crc, frameLen)
-			if crc.PayloadSum16(ufrm.RawData()[:frameLen]) != 0 {
-				protoErrs = append(protoErrs, lneto.ErrBadCRC)
+			if ufrm.CRC() != 0 {
+				frameLen := ufrm.Length()
+				ifrm4.CRCWriteUDPPseudo(&crc, frameLen)
+				if crc.PayloadSum16(ufrm.RawData()[:frameLen]) != 0 {
+					protoErrs = append(protoErrs, lneto.ErrBadCRC)
+				}
 			}
 		}
 	case lneto.IPProtoICMP:

internet/pcap/format.go

@@ -80,6 +80,16 @@ func (f *Formatter) FormatFrame(dst []byte, frm Frame, pkt []byte) (_ []byte, er
 			return dst, err
 		}
 	}
+	if len(frm.Errors) > 0 {
+		dst = append(dst, " errs=("...)
+		for i, err := range frm.Errors {
+			if i != 0 {
+				dst = append(dst, ';')
+			}
+			dst = append(dst, err.Error()...)
+		}
+		dst = append(dst, ')')
+	}
 	return dst, nil
 }
 

ipv4/definitions.go

@@ -7,6 +7,14 @@ const (
 // ToS represents the Traffic Class (a.k.a Type of Service). It is 8 bits long. 6 MSB are Differentiated Services; 2 LSB are Explicit Congenstion Notification.
 type ToS uint8
 
+// NewToS returns a [ToS] from an Explicit Congestion Notification value and a Differentiated Services Field value.
+func NewToS(ECN, DS uint8) ToS {
+	if ECN > 0b11 || DS > 0b11_1111 {
+		panic("invalid ECN/DS value")
+	}
+	return ToS(ECN | (DS << 2))
+}
+
 // DS returns the top 6 bits of the IPv4 ToS holding the Differentiated Services field
 // which is used to classify packets.
 func (tos ToS) DS() uint8 { return uint8(tos) >> 2 }
@@ -17,22 +25,46 @@ func (tos ToS) ECN() uint8 { return uint8(tos & 0b11) }
 // Flags holds fragmentation field data of an IPv4 header. It is 16 bits long.
 type Flags uint16
 
+const (
+	flagIsEvilPos           = 13
+	flagDontFragPos         = 14
+	flagMoreFragPos         = 15
+	FlagOffsetMask          = (1 << flagIsEvilPos) - 1
+	flagIsEvil        Flags = 1 << flagIsEvilPos
+	FlagDontFragment  Flags = 1 << flagDontFragPos
+	FlagMoreFragments Flags = 1 << flagMoreFragPos
+)
+
+func NewFlags(fragOffset uint16, dontFrag, moreFrag bool) Flags {
+	if fragOffset > FlagOffsetMask {
+		panic("invalid NewFlags arg")
+	}
+	return Flags(fragOffset) | Flags(b2u8(dontFrag))<<flagDontFragPos | Flags(b2u8(moreFrag))<<flagMoreFragPos
+}
+
 // IsEvil returns true if evil bit set as per [RFC3514].
 //
 // [RFC3514]: https://datatracker.ietf.org/doc/html/rfc3514
-func (f Flags) IsEvil() bool { return f&2000 != 0 }
+func (f Flags) IsEvil() bool { return f&flagIsEvil != 0 }
 
 // DontFragment specifies whether the datagram can not be fragmented.
 // This can be used when sending packets to a host that does not have resources to perform reassembly of fragments.
 // If the DontFragment(DF) flag is set, and fragmentation is required to route the packet, then the packet is dropped.
-func (f Flags) DontFragment() bool { return f&0x4000 != 0 }
+func (f Flags) DontFragment() bool { return f&FlagDontFragment != 0 }
 
 // MoreFragments is cleared for unfragmented packets.
 // For fragmented packets, all fragments except the last have the MF flag set.
 // The last fragment has a non-zero Fragment Offset field, so it can still be differentiated from an unfragmented packet.
-func (f Flags) MoreFragments() bool { return f&0x8000 != 0 }
+func (f Flags) MoreFragments() bool { return f&FlagMoreFragments != 0 }
 
 // FragmentOffset specifies the offset of a particular fragment relative to the beginning of the original unfragmented IP datagram.
 // Fragments are specified in units of 8 bytes, which is why fragment lengths are always a multiple of 8; except the last, which may be smaller.
 // The fragmentation offset value for the first fragment is always 0.
-func (f Flags) FragmentOffset() uint16 { return uint16(f) & 0x1fff }
+func (f Flags) FragmentOffset() uint16 { return uint16(f) & FlagOffsetMask }
+
+func b2u8(b bool) uint8 {
+	if b {
+		return 1
+	}
+	return 0
+}