Upstreaming DHCP changes from Fuchsia

PiperOrigin-RevId: 210221388
Change-Id: Ic82d592b8c4778855fa55ba913f6b9a10b2d511f

6 files changed, 685 insertions, 217 deletions

diff --git a/pkg/dhcp/BUILD b/pkg/dhcp/BUILD
index bd9f592b4..711a72c99 100644
--- a/pkg/dhcp/BUILD
+++ b/pkg/dhcp/BUILD
@@ -7,12 +7,14 @@ go_library(
     srcs = [
         "client.go",
         "dhcp.go",
+        "dhcp_string.go",
         "server.go",
     ],
     importpath = "gvisor.googlesource.com/gvisor/pkg/dhcp",
     deps = [
         "//pkg/rand",
         "//pkg/tcpip",
+        "//pkg/tcpip/buffer",
         "//pkg/tcpip/network/ipv4",
         "//pkg/tcpip/stack",
         "//pkg/tcpip/transport/udp",
@@ -33,5 +35,6 @@ go_test(
         "//pkg/tcpip/network/ipv4",
         "//pkg/tcpip/stack",
         "//pkg/tcpip/transport/udp",
+        "//pkg/waiter",
     ],
 )
diff --git a/pkg/dhcp/client.go b/pkg/dhcp/client.go
index 8b5fc0452..909040e79 100644
--- a/pkg/dhcp/client.go
+++ b/pkg/dhcp/client.go
@@ -18,7 +18,6 @@ import (
 	"bytes"
 	"context"
 	"fmt"
-	"log"
 	"sync"
 	"time"
 
@@ -32,9 +31,10 @@ import (
 
 // Client is a DHCP client.
 type Client struct {
-	stack    *stack.Stack
-	nicid    tcpip.NICID
-	linkAddr tcpip.LinkAddress
+	stack        *stack.Stack
+	nicid        tcpip.NICID
+	linkAddr     tcpip.LinkAddress
+	acquiredFunc func(old, new tcpip.Address, cfg Config)
 
 	mu          sync.Mutex
 	addr        tcpip.Address
@@ -46,29 +46,57 @@ type Client struct {
 // NewClient creates a DHCP client.
 //
 // TODO: add s.LinkAddr(nicid) to *stack.Stack.
-func NewClient(s *stack.Stack, nicid tcpip.NICID, linkAddr tcpip.LinkAddress) *Client {
+func NewClient(s *stack.Stack, nicid tcpip.NICID, linkAddr tcpip.LinkAddress, acquiredFunc func(old, new tcpip.Address, cfg Config)) *Client {
 	return &Client{
-		stack:    s,
-		nicid:    nicid,
-		linkAddr: linkAddr,
+		stack:        s,
+		nicid:        nicid,
+		linkAddr:     linkAddr,
+		acquiredFunc: acquiredFunc,
 	}
 }
 
-// Start starts the DHCP client.
+// Run starts the DHCP client.
 // It will periodically search for an IP address using the Request method.
-func (c *Client) Start() {
-	go func() {
-		for {
-			log.Print("DHCP request")
-			ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
-			err := c.Request(ctx, "")
-			cancel()
-			if err == nil {
-				break
-			}
+func (c *Client) Run(ctx context.Context) {
+	go c.run(ctx)
+}
+
+func (c *Client) run(ctx context.Context) {
+	defer func() {
+		c.mu.Lock()
+		defer c.mu.Unlock()
+		if c.addr != "" {
+			c.stack.RemoveAddress(c.nicid, c.addr)
 		}
-		log.Printf("DHCP acquired IP %s for %s", c.Address(), c.Config().LeaseLength)
 	}()
+
+	var renewAddr tcpip.Address
+	for {
+		reqCtx, cancel := context.WithTimeout(ctx, 3*time.Second)
+		cfg, err := c.Request(reqCtx, renewAddr)
+		cancel()
+		if err != nil {
+			select {
+			case <-time.After(1 * time.Second):
+				// loop and try again
+			case <-ctx.Done():
+				return
+			}
+		}
+
+		c.mu.Lock()
+		renewAddr = c.addr
+		c.mu.Unlock()
+
+		timer := time.NewTimer(cfg.LeaseLength)
+		select {
+		case <-ctx.Done():
+			timer.Stop()
+			return
+		case <-timer.C:
+			// loop and make a renewal request
+		}
+	}
 }
 
 // Address reports the IP address acquired by the DHCP client.
@@ -85,56 +113,53 @@ func (c *Client) Config() Config {
 	return c.cfg
 }
 
-// Shutdown relinquishes any lease and ends any outstanding renewal timers.
-func (c *Client) Shutdown() {
-	c.mu.Lock()
-	defer c.mu.Unlock()
-	if c.addr != "" {
-		c.stack.RemoveAddress(c.nicid, c.addr)
-	}
-	if c.cancelRenew != nil {
-		c.cancelRenew()
-	}
-}
-
 // Request executes a DHCP request session.
 //
 // On success, it adds a new address to this client's TCPIP stack.
 // If the server sets a lease limit a timer is set to automatically
 // renew it.
-func (c *Client) Request(ctx context.Context, requestedAddr tcpip.Address) error {
+func (c *Client) Request(ctx context.Context, requestedAddr tcpip.Address) (cfg Config, reterr error) {
+	if err := c.stack.AddAddress(c.nicid, ipv4.ProtocolNumber, "\xff\xff\xff\xff"); err != nil && err != tcpip.ErrDuplicateAddress {
+		return Config{}, fmt.Errorf("dhcp: %v", err)
+	}
+	if err := c.stack.AddAddress(c.nicid, ipv4.ProtocolNumber, "\x00\x00\x00\x00"); err != nil && err != tcpip.ErrDuplicateAddress {
+		return Config{}, fmt.Errorf("dhcp: %v", err)
+	}
+	defer c.stack.RemoveAddress(c.nicid, "\xff\xff\xff\xff")
+	defer c.stack.RemoveAddress(c.nicid, "\x00\x00\x00\x00")
+
 	var wq waiter.Queue
 	ep, err := c.stack.NewEndpoint(udp.ProtocolNumber, ipv4.ProtocolNumber, &wq)
 	if err != nil {
-		return fmt.Errorf("dhcp: outbound endpoint: %v", err)
+		return Config{}, fmt.Errorf("dhcp: outbound endpoint: %v", err)
 	}
-	err = ep.Bind(tcpip.FullAddress{
-		Addr: "\x00\x00\x00\x00",
-		Port: clientPort,
-	}, nil)
 	defer ep.Close()
-	if err != nil {
-		return fmt.Errorf("dhcp: connect failed: %v", err)
+	if err := ep.Bind(tcpip.FullAddress{
+		Addr: "\x00\x00\x00\x00",
+		Port: ClientPort,
+		NIC:  c.nicid,
+	}, nil); err != nil {
+		return Config{}, fmt.Errorf("dhcp: connect failed: %v", err)
 	}
 
 	epin, err := c.stack.NewEndpoint(udp.ProtocolNumber, ipv4.ProtocolNumber, &wq)
 	if err != nil {
-		return fmt.Errorf("dhcp: inbound endpoint: %v", err)
+		return Config{}, fmt.Errorf("dhcp: inbound endpoint: %v", err)
 	}
-	err = epin.Bind(tcpip.FullAddress{
-		Addr: "\xff\xff\xff\xff",
-		Port: clientPort,
-	}, nil)
 	defer epin.Close()
-	if err != nil {
-		return fmt.Errorf("dhcp: connect failed: %v", err)
+	if err := epin.Bind(tcpip.FullAddress{
+		Addr: "\xff\xff\xff\xff",
+		Port: ClientPort,
+		NIC:  c.nicid,
+	}, nil); err != nil {
+		return Config{}, fmt.Errorf("dhcp: connect failed: %v", err)
 	}
 
 	var xid [4]byte
 	rand.Read(xid[:])
 
 	// DHCPDISCOVERY
-	options := options{
+	discOpts := options{
 		{optDHCPMsgType, []byte{byte(dhcpDISCOVER)}},
 		{optParamReq, []byte{
 			1,  // request subnet mask
@@ -144,25 +169,34 @@ func (c *Client) Request(ctx context.Context, requestedAddr tcpip.Address) error
 		}},
 	}
 	if requestedAddr != "" {
-		options = append(options, option{optReqIPAddr, []byte(requestedAddr)})
+		discOpts = append(discOpts, option{optReqIPAddr, []byte(requestedAddr)})
 	}
-	h := make(header, headerBaseSize+options.len())
+	var clientID []byte
+	if len(c.linkAddr) == 6 {
+		clientID = append(
+			[]byte{1}, // RFC 1700: Hardware Type [Ethernet = 1]
+			c.linkAddr...,
+		)
+		discOpts = append(discOpts, option{optClientID, clientID})
+	}
+	h := make(header, headerBaseSize+discOpts.len()+1)
 	h.init()
 	h.setOp(opRequest)
 	copy(h.xidbytes(), xid[:])
 	h.setBroadcast()
 	copy(h.chaddr(), c.linkAddr)
-	h.setOptions(options)
+	h.setOptions(discOpts)
 
 	serverAddr := &tcpip.FullAddress{
 		Addr: "\xff\xff\xff\xff",
-		Port: serverPort,
+		Port: ServerPort,
+		NIC:  c.nicid,
 	}
 	wopts := tcpip.WriteOptions{
 		To: serverAddr,
 	}
 	if _, err := ep.Write(tcpip.SlicePayload(h), wopts); err != nil {
-		return fmt.Errorf("dhcp discovery write: %v", err)
+		return Config{}, fmt.Errorf("dhcp discovery write: %v", err)
 	}
 
 	we, ch := waiter.NewChannelEntry(nil)
@@ -170,6 +204,7 @@ func (c *Client) Request(ctx context.Context, requestedAddr tcpip.Address) error
 	defer wq.EventUnregister(&we)
 
 	// DHCPOFFER
+	var opts options
 	for {
 		var addr tcpip.FullAddress
 		v, _, err := epin.Read(&addr)
@@ -178,49 +213,84 @@ func (c *Client) Request(ctx context.Context, requestedAddr tcpip.Address) error
 			case <-ch:
 				continue
 			case <-ctx.Done():
-				return fmt.Errorf("reading dhcp offer: %v", tcpip.ErrAborted)
+				return Config{}, fmt.Errorf("reading dhcp offer: %v", tcpip.ErrAborted)
 			}
 		}
 		h = header(v)
-		if h.isValid() && h.op() == opReply && bytes.Equal(h.xidbytes(), xid[:]) {
-			break
+		var valid bool
+		var e error
+		opts, valid, e = loadDHCPReply(h, dhcpOFFER, xid[:])
+		if !valid {
+			if e != nil {
+				// TODO: handle all the errors?
+				// TODO: report malformed server responses
+			}
+			continue
 		}
-	}
-	if _, err := h.options(); err != nil {
-		return fmt.Errorf("dhcp offer: %v", err)
+		break
 	}
 
 	var ack bool
-	var cfg Config
+	if err := cfg.decode(opts); err != nil {
+		return Config{}, fmt.Errorf("dhcp offer: %v", err)
+	}
 
 	// DHCPREQUEST
 	addr := tcpip.Address(h.yiaddr())
 	if err := c.stack.AddAddress(c.nicid, ipv4.ProtocolNumber, addr); err != nil {
 		if err != tcpip.ErrDuplicateAddress {
-			return fmt.Errorf("adding address: %v", err)
+			return Config{}, fmt.Errorf("adding address: %v", err)
 		}
 	}
 	defer func() {
-		if ack {
-			c.mu.Lock()
-			c.addr = addr
-			c.cfg = cfg
-			c.mu.Unlock()
-		} else {
+		if !ack || reterr != nil {
 			c.stack.RemoveAddress(c.nicid, addr)
+			addr = ""
+			cfg = Config{Error: reterr}
+		}
+
+		c.mu.Lock()
+		oldAddr := c.addr
+		c.addr = addr
+		c.cfg = cfg
+		c.mu.Unlock()
+
+		// Clean up broadcast addresses before calling acquiredFunc
+		// so nothing else uses them by mistake.
+		//
+		// (The deferred RemoveAddress calls above silently error.)
+		c.stack.RemoveAddress(c.nicid, "\xff\xff\xff\xff")
+		c.stack.RemoveAddress(c.nicid, "\x00\x00\x00\x00")
+
+		if c.acquiredFunc != nil {
+			c.acquiredFunc(oldAddr, addr, cfg)
+		}
+		if requestedAddr != "" && requestedAddr != addr {
+			c.stack.RemoveAddress(c.nicid, requestedAddr)
 		}
 	}()
+	h.init()
 	h.setOp(opRequest)
 	for i, b := 0, h.yiaddr(); i < len(b); i++ {
 		b[i] = 0
 	}
-	h.setOptions([]option{
+	for i, b := 0, h.siaddr(); i < len(b); i++ {
+		b[i] = 0
+	}
+	for i, b := 0, h.giaddr(); i < len(b); i++ {
+		b[i] = 0
+	}
+	reqOpts := []option{
 		{optDHCPMsgType, []byte{byte(dhcpREQUEST)}},
 		{optReqIPAddr, []byte(addr)},
-		{optDHCPServer, h.siaddr()},
-	})
+		{optDHCPServer, []byte(cfg.ServerAddress)},
+	}
+	if len(clientID) != 0 {
+		reqOpts = append(reqOpts, option{optClientID, clientID})
+	}
+	h.setOptions(reqOpts)
 	if _, err := ep.Write(tcpip.SlicePayload(h), wopts); err != nil {
-		return fmt.Errorf("dhcp discovery write: %v", err)
+		return Config{}, fmt.Errorf("dhcp discovery write: %v", err)
 	}
 
 	// DHCPACK
@@ -232,53 +302,46 @@ func (c *Client) Request(ctx context.Context, requestedAddr tcpip.Address) error
 			case <-ch:
 				continue
 			case <-ctx.Done():
-				return fmt.Errorf("reading dhcp ack: %v", tcpip.ErrAborted)
+				return Config{}, fmt.Errorf("reading dhcp ack: %v", tcpip.ErrAborted)
 			}
 		}
 		h = header(v)
-		if h.isValid() && h.op() == opReply && bytes.Equal(h.xidbytes(), xid[:]) {
-			break
+		var valid bool
+		var e error
+		opts, valid, e = loadDHCPReply(h, dhcpACK, xid[:])
+		if !valid {
+			if e != nil {
+				// TODO: handle all the errors?
+				// TODO: report malformed server responses
+			}
+			if opts, valid, _ = loadDHCPReply(h, dhcpNAK, xid[:]); valid {
+				if msg := opts.message(); msg != "" {
+					return Config{}, fmt.Errorf("dhcp: NAK %q", msg)
+				}
+				return Config{}, fmt.Errorf("dhcp: NAK with no message")
+			}
+			continue
 		}
+		break
 	}
-	opts, e := h.options()
-	if e != nil {
-		return fmt.Errorf("dhcp ack: %v", e)
-	}
-	if err := cfg.decode(opts); err != nil {
-		return fmt.Errorf("dhcp ack bad options: %v", err)
+	ack = true
+	return cfg, nil
+}
+
+func loadDHCPReply(h header, typ dhcpMsgType, xid []byte) (opts options, valid bool, err error) {
+	if !h.isValid() || h.op() != opReply || !bytes.Equal(h.xidbytes(), xid[:]) {
+		return nil, false, nil
 	}
-	msgtype, e := opts.dhcpMsgType()
-	if e != nil {
-		return fmt.Errorf("dhcp ack: %v", e)
+	opts, err = h.options()
+	if err != nil {
+		return nil, false, err
 	}
-	ack = msgtype == dhcpACK
-	if !ack {
-		return fmt.Errorf("dhcp: request not acknowledged")
+	msgtype, err := opts.dhcpMsgType()
+	if err != nil {
+		return nil, false, err
 	}
-	if cfg.LeaseLength != 0 {
-		go c.renewAfter(cfg.LeaseLength)
+	if msgtype != typ {
+		return nil, false, nil
 	}
-	return nil
-}
-
-func (c *Client) renewAfter(d time.Duration) {
-	c.mu.Lock()
-	defer c.mu.Unlock()
-	if c.cancelRenew != nil {
-		c.cancelRenew()
-	}
-	ctx, cancel := context.WithCancel(context.Background())
-	c.cancelRenew = cancel
-	go func() {
-		timer := time.NewTimer(d)
-		defer timer.Stop()
-		select {
-		case <-ctx.Done():
-		case <-timer.C:
-			if err := c.Request(ctx, c.addr); err != nil {
-				log.Printf("address renewal failed: %v", err)
-				go c.renewAfter(1 * time.Minute)
-			}
-		}
-	}()
+	return opts, true, nil
 }
diff --git a/pkg/dhcp/dhcp.go b/pkg/dhcp/dhcp.go
index 18c318fc8..ceaba34c3 100644
--- a/pkg/dhcp/dhcp.go
+++ b/pkg/dhcp/dhcp.go
@@ -26,19 +26,21 @@ import (
 
 // Config is standard DHCP configuration.
 type Config struct {
-	ServerAddress    tcpip.Address     // address of the server
-	SubnetMask       tcpip.AddressMask // client address subnet mask
-	Gateway          tcpip.Address     // client default gateway
-	DomainNameServer tcpip.Address     // client domain name server
-	LeaseLength      time.Duration     // length of the address lease
+	Error         error
+	ServerAddress tcpip.Address     // address of the server
+	SubnetMask    tcpip.AddressMask // client address subnet mask
+	Gateway       tcpip.Address     // client default gateway
+	DNS           []tcpip.Address   // client DNS server addresses
+	LeaseLength   time.Duration     // length of the address lease
 }
 
 func (cfg *Config) decode(opts []option) error {
 	*cfg = Config{}
 	for _, opt := range opts {
 		b := opt.body
-		if l := opt.code.len(); l != -1 && l != len(b) {
-			return fmt.Errorf("%s bad length: %d", opt.code, len(b))
+		if !opt.code.lenValid(len(b)) {
+			// TODO: s/%v/%s/ when `go vet` is smarter.
+			return fmt.Errorf("%v: bad length: %d", opt.code, len(b))
 		}
 		switch opt.code {
 		case optLeaseTime:
@@ -51,7 +53,12 @@ func (cfg *Config) decode(opts []option) error {
 		case optDefaultGateway:
 			cfg.Gateway = tcpip.Address(b)
 		case optDomainNameServer:
-			cfg.DomainNameServer = tcpip.Address(b)
+			for ; len(b) > 0; b = b[4:] {
+				if len(b) < 4 {
+					return fmt.Errorf("DNS bad length: %d", len(b))
+				}
+				cfg.DNS = append(cfg.DNS, tcpip.Address(b[:4]))
+			}
 		}
 	}
 	return nil
@@ -67,8 +74,12 @@ func (cfg Config) encode() (opts []option) {
 	if cfg.Gateway != "" {
 		opts = append(opts, option{optDefaultGateway, []byte(cfg.Gateway)})
 	}
-	if cfg.DomainNameServer != "" {
-		opts = append(opts, option{optDomainNameServer, []byte(cfg.DomainNameServer)})
+	if len(cfg.DNS) > 0 {
+		dns := make([]byte, 0, 4*len(cfg.DNS))
+		for _, addr := range cfg.DNS {
+			dns = append(dns, addr...)
+		}
+		opts = append(opts, option{optDomainNameServer, dns})
 	}
 	if l := cfg.LeaseLength / time.Second; l != 0 {
 		v := make([]byte, 4)
@@ -82,8 +93,10 @@ func (cfg Config) encode() (opts []option) {
 }
 
 const (
-	serverPort = 67
-	clientPort = 68
+	// ServerPort is the well-known UDP port number for a DHCP server.
+	ServerPort = 67
+	// ClientPort is the well-known UDP port number for a DHCP client.
+	ClientPort = 68
 )
 
 var magicCookie = []byte{99, 130, 83, 99} // RFC 1497
@@ -107,10 +120,10 @@ func (h header) isValid() bool {
 	if o := h.op(); o != opRequest && o != opReply {
 		return false
 	}
-	if h[1] != 0x01 || h[2] != 0x06 || h[3] != 0x00 {
+	if h[1] != 0x01 || h[2] != 0x06 {
 		return false
 	}
-	return bytes.Equal(h[236:240], magicCookie) && h[len(h)-1] == 0
+	return bytes.Equal(h[236:240], magicCookie)
 }
 
 func (h header) op() op           { return op(h[0]) }
@@ -141,7 +154,7 @@ func (h header) options() (opts options, err error) {
 		}
 		optlen := int(h[i+1])
 		if len(h) < i+2+optlen {
-			return nil, fmt.Errorf("option too long")
+			return nil, fmt.Errorf("option %v too long i=%d, optlen=%d", optionCode(h[i]), i, optlen)
 		}
 		opts = append(opts, option{
 			code: optionCode(h[i]),
@@ -160,6 +173,8 @@ func (h header) setOptions(opts []option) {
 		copy(h[i+2:i+2+len(opt.body)], opt.body)
 		i += 2 + len(opt.body)
 	}
+	h[i] = 255 // End option
+	i++
 	for ; i < len(h); i++ {
 		h[i] = 0
 	}
@@ -182,47 +197,31 @@ const (
 	optSubnetMask       optionCode = 1
 	optDefaultGateway   optionCode = 3
 	optDomainNameServer optionCode = 6
+	optDomainName       optionCode = 15
 	optReqIPAddr        optionCode = 50
 	optLeaseTime        optionCode = 51
 	optDHCPMsgType      optionCode = 53 // dhcpMsgType
 	optDHCPServer       optionCode = 54
 	optParamReq         optionCode = 55
+	optMessage          optionCode = 56
+	optClientID         optionCode = 61
 )
 
-func (code optionCode) len() int {
+func (code optionCode) lenValid(l int) bool {
 	switch code {
-	case optSubnetMask, optDefaultGateway, optDomainNameServer,
+	case optSubnetMask, optDefaultGateway,
 		optReqIPAddr, optLeaseTime, optDHCPServer:
-		return 4
+		return l == 4
 	case optDHCPMsgType:
-		return 1
-	case optParamReq:
-		return -1 // no fixed length
-	default:
-		return -1
-	}
-}
-
-func (code optionCode) String() string {
-	switch code {
-	case optSubnetMask:
-		return "option(subnet-mask)"
-	case optDefaultGateway:
-		return "option(default-gateway)"
+		return l == 1
 	case optDomainNameServer:
-		return "option(dns)"
-	case optReqIPAddr:
-		return "option(request-ip-address)"
-	case optLeaseTime:
-		return "option(least-time)"
-	case optDHCPMsgType:
-		return "option(message-type)"
-	case optDHCPServer:
-		return "option(server)"
+		return l%4 == 0
+	case optMessage, optDomainName, optClientID:
+		return l >= 1
 	case optParamReq:
-		return "option(parameter-request)"
+		return true // no fixed length
 	default:
-		return fmt.Sprintf("option(%d)", code)
+		return true // unknown option, assume ok
 	}
 }
 
@@ -232,11 +231,12 @@ func (opts options) dhcpMsgType() (dhcpMsgType, error) {
 	for _, opt := range opts {
 		if opt.code == optDHCPMsgType {
 			if len(opt.body) != 1 {
-				return 0, fmt.Errorf("%s: bad length: %d", optDHCPMsgType, len(opt.body))
+				// TODO: s/%v/%s/ when `go vet` is smarter.
+				return 0, fmt.Errorf("%v: bad length: %d", opt.code, len(opt.body))
 			}
 			v := opt.body[0]
 			if v <= 0 || v >= 8 {
-				return 0, fmt.Errorf("%s: unknown value: %d", optDHCPMsgType, v)
+				return 0, fmt.Errorf("DHCP bad length: %d", len(opt.body))
 			}
 			return dhcpMsgType(v), nil
 		}
@@ -244,6 +244,15 @@ func (opts options) dhcpMsgType() (dhcpMsgType, error) {
 	return 0, nil
 }
 
+func (opts options) message() string {
+	for _, opt := range opts {
+		if opt.code == optMessage {
+			return string(opt.body)
+		}
+	}
+	return ""
+}
+
 func (opts options) len() int {
 	l := 0
 	for _, opt := range opts {
diff --git a/pkg/dhcp/dhcp_string.go b/pkg/dhcp/dhcp_string.go
new file mode 100644
index 000000000..7cabed29e
--- /dev/null
+++ b/pkg/dhcp/dhcp_string.go
@@ -0,0 +1,115 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package dhcp
+
+import (
+	"bytes"
+	"fmt"
+
+	"gvisor.googlesource.com/gvisor/pkg/tcpip"
+)
+
+func (h header) String() string {
+	opts, err := h.options()
+	var msgtype dhcpMsgType
+	if err == nil {
+		msgtype, err = opts.dhcpMsgType()
+	}
+	if !h.isValid() || err != nil {
+		return fmt.Sprintf("DHCP invalid, %v %v h[1:4]=%x cookie=%x len=%d (%v)", h.op(), h.xid(), []byte(h[1:4]), []byte(h[236:240]), len(h), err)
+	}
+	buf := new(bytes.Buffer)
+	fmt.Fprintf(buf, "%v %v len=%d\n", msgtype, h.xid(), len(h))
+	fmt.Fprintf(buf, "\tciaddr:%v yiaddr:%v siaddr:%v giaddr:%v\n",
+		tcpip.Address(h.ciaddr()),
+		tcpip.Address(h.yiaddr()),
+		tcpip.Address(h.siaddr()),
+		tcpip.Address(h.giaddr()))
+	fmt.Fprintf(buf, "\tchaddr:%x", h.chaddr())
+	for _, opt := range opts {
+		fmt.Fprintf(buf, "\n\t%v", opt)
+	}
+	return buf.String()
+}
+
+func (opt option) String() string {
+	buf := new(bytes.Buffer)
+	fmt.Fprintf(buf, "%v: ", opt.code)
+	fmt.Fprintf(buf, "%x", opt.body)
+	return buf.String()
+}
+
+func (code optionCode) String() string {
+	switch code {
+	case optSubnetMask:
+		return "option(subnet-mask)"
+	case optDefaultGateway:
+		return "option(default-gateway)"
+	case optDomainNameServer:
+		return "option(dns)"
+	case optDomainName:
+		return "option(domain-name)"
+	case optReqIPAddr:
+		return "option(request-ip-address)"
+	case optLeaseTime:
+		return "option(lease-time)"
+	case optDHCPMsgType:
+		return "option(message-type)"
+	case optDHCPServer:
+		return "option(server)"
+	case optParamReq:
+		return "option(parameter-request)"
+	case optMessage:
+		return "option(message)"
+	case optClientID:
+		return "option(client-id)"
+	default:
+		return fmt.Sprintf("option(%d)", code)
+	}
+}
+
+func (o op) String() string {
+	switch o {
+	case opRequest:
+		return "op(request)"
+	case opReply:
+		return "op(reply)"
+	}
+	return fmt.Sprintf("op(UNKNOWN:%d)", int(o))
+}
+
+func (t dhcpMsgType) String() string {
+	switch t {
+	case dhcpDISCOVER:
+		return "DHCPDISCOVER"
+	case dhcpOFFER:
+		return "DHCPOFFER"
+	case dhcpREQUEST:
+		return "DHCPREQUEST"
+	case dhcpDECLINE:
+		return "DHCPDECLINE"
+	case dhcpACK:
+		return "DHCPACK"
+	case dhcpNAK:
+		return "DHCPNAK"
+	case dhcpRELEASE:
+		return "DHCPRELEASE"
+	}
+	return fmt.Sprintf("DHCP(%d)", int(t))
+}
+
+func (v xid) String() string {
+	return fmt.Sprintf("xid:%x", uint32(v))
+}
diff --git a/pkg/dhcp/dhcp_test.go b/pkg/dhcp/dhcp_test.go
index 731ed61a5..67814683a 100644
--- a/pkg/dhcp/dhcp_test.go
+++ b/pkg/dhcp/dhcp_test.go
@@ -27,9 +27,13 @@ import (
 	"gvisor.googlesource.com/gvisor/pkg/tcpip/network/ipv4"
 	"gvisor.googlesource.com/gvisor/pkg/tcpip/stack"
 	"gvisor.googlesource.com/gvisor/pkg/tcpip/transport/udp"
+	"gvisor.googlesource.com/gvisor/pkg/waiter"
 )
 
-func TestDHCP(t *testing.T) {
+const nicid = tcpip.NICID(1)
+const serverAddr = tcpip.Address("\xc0\xa8\x03\x01")
+
+func createStack(t *testing.T) *stack.Stack {
 	const defaultMTU = 65536
 	id, linkEP := channel.New(256, defaultMTU, "")
 	if testing.Verbose() {
@@ -48,17 +52,9 @@ func TestDHCP(t *testing.T) {
 
 	s := stack.New([]string{ipv4.ProtocolName}, []string{udp.ProtocolName}, stack.Options{})
 
-	const nicid tcpip.NICID = 1
 	if err := s.CreateNIC(nicid, id); err != nil {
 		t.Fatal(err)
 	}
-	if err := s.AddAddress(nicid, ipv4.ProtocolNumber, "\x00\x00\x00\x00"); err != nil {
-		t.Fatal(err)
-	}
-	if err := s.AddAddress(nicid, ipv4.ProtocolNumber, "\xff\xff\xff\xff"); err != nil {
-		t.Fatal(err)
-	}
-	const serverAddr = tcpip.Address("\xc0\xa8\x03\x01")
 	if err := s.AddAddress(nicid, ipv4.ProtocolNumber, serverAddr); err != nil {
 		t.Fatal(err)
 	}
@@ -70,31 +66,38 @@ func TestDHCP(t *testing.T) {
 		NIC:         nicid,
 	}})
 
-	var clientAddrs = []tcpip.Address{"\xc0\xa8\x03\x02", "\xc0\xa8\x03\x03"}
+	return s
+}
+
+func TestDHCP(t *testing.T) {
+	s := createStack(t)
+	clientAddrs := []tcpip.Address{"\xc0\xa8\x03\x02", "\xc0\xa8\x03\x03"}
 
 	serverCfg := Config{
-		ServerAddress:    serverAddr,
-		SubnetMask:       "\xff\xff\xff\x00",
-		Gateway:          "\xc0\xa8\x03\xF0",
-		DomainNameServer: "\x08\x08\x08\x08",
-		LeaseLength:      24 * time.Hour,
+		ServerAddress: serverAddr,
+		SubnetMask:    "\xff\xff\xff\x00",
+		Gateway:       "\xc0\xa8\x03\xF0",
+		DNS: []tcpip.Address{
+			"\x08\x08\x08\x08", "\x08\x08\x04\x04",
+		},
+		LeaseLength: 24 * time.Hour,
 	}
 	serverCtx, cancel := context.WithCancel(context.Background())
 	defer cancel()
-	_, err := NewServer(serverCtx, s, clientAddrs, serverCfg)
+	_, err := newEPConnServer(serverCtx, s, clientAddrs, serverCfg)
 	if err != nil {
 		t.Fatal(err)
 	}
 
 	const clientLinkAddr0 = tcpip.LinkAddress("\x52\x11\x22\x33\x44\x52")
-	c0 := NewClient(s, nicid, clientLinkAddr0)
-	if err := c0.Request(context.Background(), ""); err != nil {
+	c0 := NewClient(s, nicid, clientLinkAddr0, nil)
+	if _, err := c0.Request(context.Background(), ""); err != nil {
 		t.Fatal(err)
 	}
 	if got, want := c0.Address(), clientAddrs[0]; got != want {
 		t.Errorf("c.Addr()=%s, want=%s", got, want)
 	}
-	if err := c0.Request(context.Background(), ""); err != nil {
+	if _, err := c0.Request(context.Background(), ""); err != nil {
 		t.Fatal(err)
 	}
 	if got, want := c0.Address(), clientAddrs[0]; got != want {
@@ -102,22 +105,219 @@ func TestDHCP(t *testing.T) {
 	}
 
 	const clientLinkAddr1 = tcpip.LinkAddress("\x52\x11\x22\x33\x44\x53")
-	c1 := NewClient(s, nicid, clientLinkAddr1)
-	if err := c1.Request(context.Background(), ""); err != nil {
+	c1 := NewClient(s, nicid, clientLinkAddr1, nil)
+	if _, err := c1.Request(context.Background(), ""); err != nil {
 		t.Fatal(err)
 	}
 	if got, want := c1.Address(), clientAddrs[1]; got != want {
 		t.Errorf("c.Addr()=%s, want=%s", got, want)
 	}
 
-	if err := c0.Request(context.Background(), ""); err != nil {
+	if _, err := c0.Request(context.Background(), ""); err != nil {
 		t.Fatal(err)
 	}
 	if got, want := c0.Address(), clientAddrs[0]; got != want {
 		t.Errorf("c.Addr()=%s, want=%s", got, want)
 	}
 
-	if got, want := c0.Config(), serverCfg; got != want {
+	if got, want := c0.Config(), serverCfg; !equalConfig(got, want) {
 		t.Errorf("client config:\n\t%#+v\nwant:\n\t%#+v", got, want)
 	}
 }
+
+func equalConfig(c0, c1 Config) bool {
+	if c0.Error != c1.Error || c0.ServerAddress != c1.ServerAddress || c0.SubnetMask != c1.SubnetMask || c0.Gateway != c1.Gateway || c0.LeaseLength != c1.LeaseLength {
+		return false
+	}
+	if len(c0.DNS) != len(c1.DNS) {
+		return false
+	}
+	for i := 0; i < len(c0.DNS); i++ {
+		if c0.DNS[i] != c1.DNS[i] {
+			return false
+		}
+	}
+	return true
+}
+
+func TestRenew(t *testing.T) {
+	s := createStack(t)
+	clientAddrs := []tcpip.Address{"\xc0\xa8\x03\x02"}
+
+	serverCfg := Config{
+		ServerAddress: serverAddr,
+		SubnetMask:    "\xff\xff\xff\x00",
+		Gateway:       "\xc0\xa8\x03\xF0",
+		DNS:           []tcpip.Address{"\x08\x08\x08\x08"},
+		LeaseLength:   1 * time.Second,
+	}
+	serverCtx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	_, err := newEPConnServer(serverCtx, s, clientAddrs, serverCfg)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	count := 0
+	var curAddr tcpip.Address
+	addrCh := make(chan tcpip.Address)
+	acquiredFunc := func(oldAddr, newAddr tcpip.Address, cfg Config) {
+		if err := cfg.Error; err != nil {
+			t.Fatalf("acquisition %d failed: %v", count, err)
+		}
+		if oldAddr != curAddr {
+			t.Fatalf("aquisition %d: curAddr=%v, oldAddr=%v", count, curAddr, oldAddr)
+		}
+		if cfg.LeaseLength != time.Second {
+			t.Fatalf("aquisition %d: lease length: %v, want %v", count, cfg.LeaseLength, time.Second)
+		}
+		count++
+		curAddr = newAddr
+		addrCh <- newAddr
+	}
+
+	clientCtx, cancel := context.WithCancel(context.Background())
+	const clientLinkAddr0 = tcpip.LinkAddress("\x52\x11\x22\x33\x44\x52")
+	c := NewClient(s, nicid, clientLinkAddr0, acquiredFunc)
+	c.Run(clientCtx)
+
+	var addr tcpip.Address
+	select {
+	case addr = <-addrCh:
+		t.Logf("got first address: %v", addr)
+	case <-time.After(5 * time.Second):
+		t.Fatal("timeout acquiring initial address")
+	}
+
+	select {
+	case newAddr := <-addrCh:
+		t.Logf("got renewal: %v", newAddr)
+		if newAddr != addr {
+			t.Fatalf("renewal address is %v, want %v", newAddr, addr)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("timeout waiting for address renewal")
+	}
+
+	cancel()
+}
+
+// Regression test for https://fuchsia.atlassian.net/browse/NET-17
+func TestNoNullTerminator(t *testing.T) {
+	v := "\x02\x01\x06\x00" +
+		"\xc8\x37\xbe\x73\x00\x00\x80\x00\x00\x00\x00\x00\xc0\xa8\x2b\x92" +
+		"\xc0\xa8\x2b\x01\x00\x00\x00\x00\x00\x0f\x60\x0a\x23\x93\x00\x00" +
+		"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" +
+		"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" +
+		"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" +
+		"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" +
+		"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" +
+		"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" +
+		"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" +
+		"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" +
+		"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" +
+		"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" +
+		"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" +
+		"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" +
+		"\x00\x00\x00\x00\x00\x00\x00\x00\x63\x82\x53\x63\x35\x01\x02\x36" +
+		"\x04\xc0\xa8\x2b\x01\x33\x04\x00\x00\x0e\x10\x3a\x04\x00\x00\x07" +
+		"\x08\x3b\x04\x00\x00\x0c\x4e\x01\x04\xff\xff\xff\x00\x1c\x04\xc0" +
+		"\xa8\x2b\xff\x03\x04\xc0\xa8\x2b\x01\x06\x04\xc0\xa8\x2b\x01\x2b" +
+		"\x0f\x41\x4e\x44\x52\x4f\x49\x44\x5f\x4d\x45\x54\x45\x52\x45\x44" +
+		"\xff"
+	h := header(v)
+	if !h.isValid() {
+		t.Error("failed to decode header")
+	}
+
+	if got, want := h.op(), opReply; got != want {
+		t.Errorf("h.op()=%v, want=%v", got, want)
+	}
+
+	if _, err := h.options(); err != nil {
+		t.Errorf("bad options: %v", err)
+	}
+}
+
+func teeConn(c conn) (conn, conn) {
+	dup1 := &dupConn{
+		c:   c,
+		dup: make(chan connMsg, 8),
+	}
+	dup2 := &chConn{
+		c:  c,
+		ch: dup1.dup,
+	}
+	return dup1, dup2
+}
+
+type connMsg struct {
+	buf  buffer.View
+	addr tcpip.FullAddress
+	err  error
+}
+
+type dupConn struct {
+	c   conn
+	dup chan connMsg
+}
+
+func (c *dupConn) Read() (buffer.View, tcpip.FullAddress, error) {
+	v, addr, err := c.c.Read()
+	c.dup <- connMsg{v, addr, err}
+	return v, addr, err
+}
+func (c *dupConn) Write(b []byte, addr *tcpip.FullAddress) error { return c.c.Write(b, addr) }
+
+type chConn struct {
+	ch chan connMsg
+	c  conn
+}
+
+func (c *chConn) Read() (buffer.View, tcpip.FullAddress, error) {
+	msg := <-c.ch
+	return msg.buf, msg.addr, msg.err
+}
+func (c *chConn) Write(b []byte, addr *tcpip.FullAddress) error { return c.c.Write(b, addr) }
+
+func TestTwoServers(t *testing.T) {
+	s := createStack(t)
+
+	wq := new(waiter.Queue)
+	ep, err := s.NewEndpoint(udp.ProtocolNumber, ipv4.ProtocolNumber, wq)
+	if err != nil {
+		t.Fatalf("dhcp: server endpoint: %v", err)
+	}
+	if err = ep.Bind(tcpip.FullAddress{Port: ServerPort}, nil); err != nil {
+		t.Fatalf("dhcp: server bind: %v", err)
+	}
+
+	serverCtx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	c1, c2 := teeConn(newEPConn(serverCtx, wq, ep))
+
+	if _, err := NewServer(serverCtx, c1, []tcpip.Address{"\xc0\xa8\x03\x02"}, Config{
+		ServerAddress: "\xc0\xa8\x03\x01",
+		SubnetMask:    "\xff\xff\xff\x00",
+		Gateway:       "\xc0\xa8\x03\xF0",
+		DNS:           []tcpip.Address{"\x08\x08\x08\x08"},
+		LeaseLength:   30 * time.Minute,
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := NewServer(serverCtx, c2, []tcpip.Address{"\xc0\xa8\x04\x02"}, Config{
+		ServerAddress: "\xc0\xa8\x04\x01",
+		SubnetMask:    "\xff\xff\xff\x00",
+		Gateway:       "\xc0\xa8\x03\xF0",
+		DNS:           []tcpip.Address{"\x08\x08\x08\x08"},
+		LeaseLength:   30 * time.Minute,
+	}); err != nil {
+		t.Fatal(err)
+	}
+
+	const clientLinkAddr0 = tcpip.LinkAddress("\x52\x11\x22\x33\x44\x52")
+	c := NewClient(s, nicid, clientLinkAddr0, nil)
+	if _, err := c.Request(context.Background(), ""); err != nil {
+		t.Fatal(err)
+	}
+}
diff --git a/pkg/dhcp/server.go b/pkg/dhcp/server.go
index 0beac7782..003e272b2 100644
--- a/pkg/dhcp/server.go
+++ b/pkg/dhcp/server.go
@@ -17,11 +17,13 @@ package dhcp
 import (
 	"context"
 	"fmt"
+	"io"
 	"log"
 	"sync"
 	"time"
 
 	"gvisor.googlesource.com/gvisor/pkg/tcpip"
+	"gvisor.googlesource.com/gvisor/pkg/tcpip/buffer"
 	"gvisor.googlesource.com/gvisor/pkg/tcpip/network/ipv4"
 	"gvisor.googlesource.com/gvisor/pkg/tcpip/stack"
 	"gvisor.googlesource.com/gvisor/pkg/tcpip/transport/udp"
@@ -30,10 +32,8 @@ import (
 
 // Server is a DHCP server.
 type Server struct {
-	stack     *stack.Stack
+	conn      conn
 	broadcast tcpip.FullAddress
-	wq        waiter.Queue
-	ep        tcpip.Endpoint
 	addrs     []tcpip.Address // TODO: use a tcpip.AddressMask or range structure
 	cfg       Config
 	cfgopts   []option // cfg to send to client
@@ -44,36 +44,96 @@ type Server struct {
 	leases map[tcpip.LinkAddress]serverLease
 }
 
+// conn is a blocking read/write network endpoint.
+type conn interface {
+	Read() (buffer.View, tcpip.FullAddress, error)
+	Write([]byte, *tcpip.FullAddress) error
+}
+
+type epConn struct {
+	ctx  context.Context
+	wq   *waiter.Queue
+	ep   tcpip.Endpoint
+	we   waiter.Entry
+	inCh chan struct{}
+}
+
+func newEPConn(ctx context.Context, wq *waiter.Queue, ep tcpip.Endpoint) *epConn {
+	c := &epConn{
+		ctx: ctx,
+		wq:  wq,
+		ep:  ep,
+	}
+	c.we, c.inCh = waiter.NewChannelEntry(nil)
+	wq.EventRegister(&c.we, waiter.EventIn)
+
+	go func() {
+		<-ctx.Done()
+		wq.EventUnregister(&c.we)
+	}()
+
+	return c
+}
+
+func (c *epConn) Read() (buffer.View, tcpip.FullAddress, error) {
+	for {
+		var addr tcpip.FullAddress
+		v, _, err := c.ep.Read(&addr)
+		if err == tcpip.ErrWouldBlock {
+			select {
+			case <-c.inCh:
+				continue
+			case <-c.ctx.Done():
+				return nil, tcpip.FullAddress{}, io.EOF
+			}
+		}
+		if err != nil {
+			return v, addr, fmt.Errorf("read: %v", err)
+		}
+		return v, addr, nil
+	}
+}
+
+func (c *epConn) Write(b []byte, addr *tcpip.FullAddress) error {
+	if _, err := c.ep.Write(tcpip.SlicePayload(b), tcpip.WriteOptions{To: addr}); err != nil {
+		return fmt.Errorf("write: %v", err)
+	}
+	return nil
+}
+
+func newEPConnServer(ctx context.Context, stack *stack.Stack, addrs []tcpip.Address, cfg Config) (*Server, error) {
+	wq := new(waiter.Queue)
+	ep, err := stack.NewEndpoint(udp.ProtocolNumber, ipv4.ProtocolNumber, wq)
+	if err != nil {
+		return nil, fmt.Errorf("dhcp: server endpoint: %v", err)
+	}
+	if err := ep.Bind(tcpip.FullAddress{Port: ServerPort}, nil); err != nil {
+		return nil, fmt.Errorf("dhcp: server bind: %v", err)
+	}
+	c := newEPConn(ctx, wq, ep)
+	return NewServer(ctx, c, addrs, cfg)
+}
+
 // NewServer creates a new DHCP server and begins serving.
 // The server continues serving until ctx is done.
-func NewServer(ctx context.Context, stack *stack.Stack, addrs []tcpip.Address, cfg Config) (*Server, error) {
+func NewServer(ctx context.Context, c conn, addrs []tcpip.Address, cfg Config) (*Server, error) {
+	if cfg.ServerAddress == "" {
+		return nil, fmt.Errorf("dhcp: server requires explicit server address")
+	}
 	s := &Server{
-		stack:   stack,
+		conn:    c,
 		addrs:   addrs,
 		cfg:     cfg,
 		cfgopts: cfg.encode(),
 		broadcast: tcpip.FullAddress{
 			Addr: "\xff\xff\xff\xff",
-			Port: clientPort,
+			Port: ClientPort,
 		},
 
 		handlers: make([]chan header, 8),
 		leases:   make(map[tcpip.LinkAddress]serverLease),
 	}
 
-	var err *tcpip.Error
-	s.ep, err = s.stack.NewEndpoint(udp.ProtocolNumber, ipv4.ProtocolNumber, &s.wq)
-	if err != nil {
-		return nil, fmt.Errorf("dhcp: server endpoint: %v", err)
-	}
-	serverBroadcast := tcpip.FullAddress{
-		Addr: "",
-		Port: serverPort,
-	}
-	if err := s.ep.Bind(serverBroadcast, nil); err != nil {
-		return nil, fmt.Errorf("dhcp: server bind: %v", err)
-	}
-
 	for i := 0; i < len(s.handlers); i++ {
 		ch := make(chan header, 8)
 		s.handlers[i] = ch
@@ -108,20 +168,10 @@ func (s *Server) expirer(ctx context.Context) {
 // reader listens for all incoming DHCP packets and fans them out to
 // handling goroutines based on XID as session identifiers.
 func (s *Server) reader(ctx context.Context) {
-	we, ch := waiter.NewChannelEntry(nil)
-	s.wq.EventRegister(&we, waiter.EventIn)
-	defer s.wq.EventUnregister(&we)
-
 	for {
-		var addr tcpip.FullAddress
-		v, _, err := s.ep.Read(&addr)
-		if err == tcpip.ErrWouldBlock {
-			select {
-			case <-ch:
-				continue
-			case <-ctx.Done():
-				return
-			}
+		v, _, err := s.conn.Read()
+		if err != nil {
+			return
 		}
 
 		h := header(v)
@@ -234,21 +284,50 @@ func (s *Server) handleDiscover(hreq header, opts options) {
 	// DHCPOFFER
 	opts = options{{optDHCPMsgType, []byte{byte(dhcpOFFER)}}}
 	opts = append(opts, s.cfgopts...)
-	h := make(header, headerBaseSize+opts.len())
+	h := make(header, headerBaseSize+opts.len()+1)
 	h.init()
 	h.setOp(opReply)
 	copy(h.xidbytes(), hreq.xidbytes())
 	copy(h.yiaddr(), lease.addr)
-	copy(h.siaddr(), s.cfg.ServerAddress)
 	copy(h.chaddr(), hreq.chaddr())
 	h.setOptions(opts)
-	s.ep.Write(tcpip.SlicePayload(h), tcpip.WriteOptions{To: &s.broadcast})
+	s.conn.Write([]byte(h), &s.broadcast)
+}
+
+func (s *Server) nack(hreq header) {
+	// DHCPNACK
+	opts := options([]option{
+		{optDHCPMsgType, []byte{byte(dhcpNAK)}},
+		{optDHCPServer, []byte(s.cfg.ServerAddress)},
+	})
+	h := make(header, headerBaseSize+opts.len()+1)
+	h.init()
+	h.setOp(opReply)
+	copy(h.xidbytes(), hreq.xidbytes())
+	copy(h.chaddr(), hreq.chaddr())
+	h.setOptions(opts)
+	s.conn.Write([]byte(h), &s.broadcast)
 }
 
 func (s *Server) handleRequest(hreq header, opts options) {
 	linkAddr := tcpip.LinkAddress(hreq.chaddr()[:6])
 	xid := hreq.xid()
 
+	reqopts, err := hreq.options()
+	if err != nil {
+		s.nack(hreq)
+		return
+	}
+	var reqcfg Config
+	if err := reqcfg.decode(reqopts); err != nil {
+		s.nack(hreq)
+		return
+	}
+	if reqcfg.ServerAddress != s.cfg.ServerAddress {
+		// This request is for a different DHCP server. Ignore it.
+		return
+	}
+
 	s.mu.Lock()
 	lease := s.leases[linkAddr]
 	switch lease.state {
@@ -271,15 +350,14 @@ func (s *Server) handleRequest(hreq header, opts options) {
 	// DHCPACK
 	opts = []option{{optDHCPMsgType, []byte{byte(dhcpACK)}}}
 	opts = append(opts, s.cfgopts...)
-	h := make(header, headerBaseSize+opts.len())
+	h := make(header, headerBaseSize+opts.len()+1)
 	h.init()
 	h.setOp(opReply)
 	copy(h.xidbytes(), hreq.xidbytes())
 	copy(h.yiaddr(), lease.addr)
-	copy(h.siaddr(), s.cfg.ServerAddress)
 	copy(h.chaddr(), hreq.chaddr())
 	h.setOptions(opts)
-	s.ep.Write(tcpip.SlicePayload(h), tcpip.WriteOptions{To: &s.broadcast})
+	s.conn.Write([]byte(h), &s.broadcast)
 }
 
 type leaseState int