//go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris
// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris

package dhcpd

import (
	"fmt"
	"net"
	"strings"
	"testing"
	"time"

	"github.com/AdguardTeam/AdGuardHome/internal/aghnet"
	"github.com/AdguardTeam/golibs/stringutil"
	"github.com/AdguardTeam/golibs/testutil"
	"github.com/insomniacslk/dhcp/dhcpv4"
	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/require"

	//lint:ignore SA1019 See the TODO in go.mod.
	"github.com/mdlayher/raw"
)

var (
	DefaultRangeStart = net.IP{192, 168, 10, 100}
	DefaultRangeEnd   = net.IP{192, 168, 10, 200}
	DefaultGatewayIP  = net.IP{192, 168, 10, 1}
	DefaultSelfIP     = net.IP{192, 168, 10, 2}
	DefaultSubnetMask = net.IP{255, 255, 255, 0}
)

func notify4(flags uint32) {
}

// defaultV4ServerConf returns the default configuration for *v4Server to use in
// tests.
func defaultV4ServerConf() (conf V4ServerConf) {
	return V4ServerConf{
		Enabled:    true,
		RangeStart: DefaultRangeStart,
		RangeEnd:   DefaultRangeEnd,
		GatewayIP:  DefaultGatewayIP,
		SubnetMask: DefaultSubnetMask,
		notify:     notify4,
		dnsIPAddrs: []net.IP{DefaultSelfIP},
	}
}

// defaultSrv prepares the default DHCPServer to use in tests.  The underlying
// type of s is *v4Server.
func defaultSrv(t *testing.T) (s DHCPServer) {
	t.Helper()

	var err error
	s, err = v4Create(defaultV4ServerConf())
	require.NoError(t, err)

	return s
}

func TestV4Server_leasing(t *testing.T) {
	const (
		staticName  = "static-client"
		anotherName = "another-client"
	)

	staticIP := net.IP{192, 168, 10, 10}
	anotherIP := DefaultRangeStart
	staticMAC := net.HardwareAddr{0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA}
	anotherMAC := net.HardwareAddr{0xBB, 0xBB, 0xBB, 0xBB, 0xBB, 0xBB}

	s := defaultSrv(t)

	t.Run("add_static", func(t *testing.T) {
		err := s.AddStaticLease(&Lease{
			Expiry:   time.Unix(leaseExpireStatic, 0),
			Hostname: staticName,
			HWAddr:   staticMAC,
			IP:       staticIP,
		})
		require.NoError(t, err)

		t.Run("same_name", func(t *testing.T) {
			err = s.AddStaticLease(&Lease{
				Expiry:   time.Unix(leaseExpireStatic, 0),
				Hostname: staticName,
				HWAddr:   anotherMAC,
				IP:       anotherIP,
			})
			assert.ErrorIs(t, err, ErrDupHostname)
		})

		t.Run("same_mac", func(t *testing.T) {
			wantErrMsg := "dhcpv4: adding static lease: removing " +
				"dynamic leases for " + anotherIP.String() +
				" (" + staticMAC.String() + "): static lease already exists"

			err = s.AddStaticLease(&Lease{
				Expiry:   time.Unix(leaseExpireStatic, 0),
				Hostname: anotherName,
				HWAddr:   staticMAC,
				IP:       anotherIP,
			})
			testutil.AssertErrorMsg(t, wantErrMsg, err)
		})

		t.Run("same_ip", func(t *testing.T) {
			wantErrMsg := "dhcpv4: adding static lease: removing " +
				"dynamic leases for " + staticIP.String() +
				" (" + anotherMAC.String() + "): static lease already exists"

			err = s.AddStaticLease(&Lease{
				Expiry:   time.Unix(leaseExpireStatic, 0),
				Hostname: anotherName,
				HWAddr:   anotherMAC,
				IP:       staticIP,
			})
			testutil.AssertErrorMsg(t, wantErrMsg, err)
		})
	})

	t.Run("add_dynamic", func(t *testing.T) {
		s4, ok := s.(*v4Server)
		require.True(t, ok)

		discoverAnOffer := func(
			t *testing.T,
			name string,
			ip net.IP,
			mac net.HardwareAddr,
		) (resp *dhcpv4.DHCPv4) {
			testutil.CleanupAndRequireSuccess(t, func() (err error) {
				return s.ResetLeases(s.GetLeases(LeasesStatic))
			})

			req, err := dhcpv4.NewDiscovery(
				mac,
				dhcpv4.WithOption(dhcpv4.OptHostName(name)),
				dhcpv4.WithOption(dhcpv4.OptRequestedIPAddress(ip)),
				dhcpv4.WithOption(dhcpv4.OptClientIdentifier([]byte{1, 2, 3, 4, 5, 6, 8})),
				dhcpv4.WithGatewayIP(DefaultGatewayIP),
			)
			require.NoError(t, err)

			resp = &dhcpv4.DHCPv4{}
			res := s4.handle(req, resp)
			require.Positive(t, res)
			require.Equal(t, dhcpv4.MessageTypeOffer, resp.MessageType())

			resp.ClientHWAddr = mac

			return resp
		}

		t.Run("same_name", func(t *testing.T) {
			resp := discoverAnOffer(t, staticName, anotherIP, anotherMAC)

			req, err := dhcpv4.NewRequestFromOffer(resp, dhcpv4.WithOption(
				dhcpv4.OptHostName(staticName),
			))
			require.NoError(t, err)

			res := s4.handle(req, resp)
			require.Positive(t, res)

			assert.Equal(t, aghnet.GenerateHostname(resp.YourIPAddr), resp.HostName())
		})

		t.Run("same_mac", func(t *testing.T) {
			resp := discoverAnOffer(t, anotherName, anotherIP, staticMAC)

			req, err := dhcpv4.NewRequestFromOffer(resp, dhcpv4.WithOption(
				dhcpv4.OptHostName(anotherName),
			))
			require.NoError(t, err)

			res := s4.handle(req, resp)
			require.Positive(t, res)

			fqdnOptData := resp.Options.Get(dhcpv4.OptionFQDN)
			require.Len(t, fqdnOptData, 3+len(staticName))
			assert.Equal(t, []uint8(staticName), fqdnOptData[3:])

			assert.Equal(t, staticIP, resp.YourIPAddr)
		})

		t.Run("same_ip", func(t *testing.T) {
			resp := discoverAnOffer(t, anotherName, staticIP, anotherMAC)

			req, err := dhcpv4.NewRequestFromOffer(resp, dhcpv4.WithOption(
				dhcpv4.OptHostName(anotherName),
			))
			require.NoError(t, err)

			res := s4.handle(req, resp)
			require.Positive(t, res)

			assert.NotEqual(t, staticIP, resp.YourIPAddr)
		})
	})
}

func TestV4Server_AddRemove_static(t *testing.T) {
	s := defaultSrv(t)

	ls := s.GetLeases(LeasesStatic)
	require.Empty(t, ls)

	testCases := []struct {
		lease      *Lease
		name       string
		wantErrMsg string
	}{{
		lease: &Lease{
			Hostname: "success.local",
			HWAddr:   net.HardwareAddr{0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA},
			IP:       net.IP{192, 168, 10, 150},
		},
		name:       "success",
		wantErrMsg: "",
	}, {
		lease: &Lease{
			Hostname: "probably-router.local",
			HWAddr:   net.HardwareAddr{0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA},
			IP:       DefaultGatewayIP,
		},
		name: "with_gateway_ip",
		wantErrMsg: "dhcpv4: adding static lease: " +
			"can't assign the gateway IP 192.168.10.1 to the lease",
	}, {
		lease: &Lease{
			Hostname: "ip6.local",
			HWAddr:   net.HardwareAddr{0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA},
			IP:       net.ParseIP("ffff::1"),
		},
		name: "ipv6",
		wantErrMsg: `dhcpv4: adding static lease: ` +
			`invalid ip "ffff::1", only ipv4 is supported`,
	}, {
		lease: &Lease{
			Hostname: "bad-mac.local",
			HWAddr:   net.HardwareAddr{0xAA, 0xAA},
			IP:       net.IP{192, 168, 10, 150},
		},
		name: "bad_mac",
		wantErrMsg: `dhcpv4: adding static lease: bad mac address "aa:aa": ` +
			`bad mac address length 2, allowed: [6 8 20]`,
	}, {
		lease: &Lease{
			Hostname: "bad-lbl-.local",
			HWAddr:   net.HardwareAddr{0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA},
			IP:       net.IP{192, 168, 10, 150},
		},
		name: "bad_hostname",
		wantErrMsg: `dhcpv4: adding static lease: validating hostname: ` +
			`bad domain name "bad-lbl-.local": ` +
			`bad domain name label "bad-lbl-": bad domain name label rune '-'`,
	}}

	for _, tc := range testCases {
		t.Run(tc.name, func(t *testing.T) {
			err := s.AddStaticLease(tc.lease)
			testutil.AssertErrorMsg(t, tc.wantErrMsg, err)
			if tc.wantErrMsg != "" {
				return
			}

			err = s.RemoveStaticLease(&Lease{
				IP:     tc.lease.IP,
				HWAddr: tc.lease.HWAddr,
			})
			diffErrMsg := fmt.Sprintf("dhcpv4: lease for ip %s is different: %+v", tc.lease.IP, tc.lease)
			testutil.AssertErrorMsg(t, diffErrMsg, err)

			// Remove static lease.
			err = s.RemoveStaticLease(tc.lease)
			require.NoError(t, err)
		})

		ls = s.GetLeases(LeasesStatic)
		require.Emptyf(t, ls, "after %s", tc.name)
	}
}

func TestV4_AddReplace(t *testing.T) {
	sIface := defaultSrv(t)

	s, ok := sIface.(*v4Server)
	require.True(t, ok)

	dynLeases := []Lease{{
		Hostname: "dynamic-1.local",
		HWAddr:   net.HardwareAddr{0x11, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA},
		IP:       net.IP{192, 168, 10, 150},
	}, {
		Hostname: "dynamic-2.local",
		HWAddr:   net.HardwareAddr{0x22, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA},
		IP:       net.IP{192, 168, 10, 151},
	}}

	for i := range dynLeases {
		err := s.addLease(&dynLeases[i])
		require.NoError(t, err)
	}

	stLeases := []*Lease{{
		Hostname: "static-1.local",
		HWAddr:   net.HardwareAddr{0x33, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA},
		IP:       net.IP{192, 168, 10, 150},
	}, {
		Hostname: "static-2.local",
		HWAddr:   net.HardwareAddr{0x22, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA},
		IP:       net.IP{192, 168, 10, 152},
	}}

	for _, l := range stLeases {
		err := s.AddStaticLease(l)
		require.NoError(t, err)
	}

	ls := s.GetLeases(LeasesStatic)
	require.Len(t, ls, 2)

	for i, l := range ls {
		assert.True(t, stLeases[i].IP.Equal(l.IP))
		assert.Equal(t, stLeases[i].HWAddr, l.HWAddr)
		assert.True(t, l.IsStatic())
	}
}

func TestV4Server_handle_optionsPriority(t *testing.T) {
	defaultIP := net.IP{192, 168, 1, 1}
	knownIP := net.IP{1, 2, 3, 4}

	// prepareSrv creates a *v4Server and sets the opt6IPs in the initial
	// configuration of the server as the value for DHCP option 6.
	prepareSrv := func(t *testing.T, opt6IPs []net.IP) (s *v4Server) {
		t.Helper()

		conf := defaultV4ServerConf()
		if len(opt6IPs) > 0 {
			b := &strings.Builder{}
			stringutil.WriteToBuilder(b, "6 ips ", opt6IPs[0].String())
			for _, ip := range opt6IPs[1:] {
				stringutil.WriteToBuilder(b, ",", ip.String())
			}
			conf.Options = []string{b.String()}
		} else {
			defer func() { s.options.Update(dhcpv4.OptDNS(defaultIP)) }()
		}

		ss, err := v4Create(conf)
		require.NoError(t, err)

		var ok bool
		s, ok = ss.(*v4Server)
		require.True(t, ok)

		s.conf.dnsIPAddrs = []net.IP{defaultIP}

		return s
	}

	// checkResp creates a discovery message with DHCP option 6 requested amd
	// asserts the response to contain wantIPs in this option.
	checkResp := func(t *testing.T, s *v4Server, wantIPs []net.IP) {
		t.Helper()

		mac := net.HardwareAddr{0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA}
		req, err := dhcpv4.NewDiscovery(mac, dhcpv4.WithRequestedOptions(
			dhcpv4.OptionDomainNameServer,
		))
		require.NoError(t, err)

		var resp *dhcpv4.DHCPv4
		resp, err = dhcpv4.NewReplyFromRequest(req)
		require.NoError(t, err)

		res := s.handle(req, resp)
		require.Equal(t, 1, res)

		o := resp.GetOneOption(dhcpv4.OptionDomainNameServer)
		require.NotEmpty(t, o)

		wantData := []byte{}
		for _, ip := range wantIPs {
			wantData = append(wantData, ip...)
		}
		assert.Equal(t, o, wantData)
	}

	t.Run("default", func(t *testing.T) {
		s := prepareSrv(t, nil)

		checkResp(t, s, []net.IP{defaultIP})
	})

	t.Run("explicitly_configured", func(t *testing.T) {
		s := prepareSrv(t, []net.IP{knownIP, knownIP})

		checkResp(t, s, []net.IP{knownIP, knownIP})
	})
}

func TestV4StaticLease_Get(t *testing.T) {
	sIface := defaultSrv(t)

	s, ok := sIface.(*v4Server)
	require.True(t, ok)

	s.conf.dnsIPAddrs = []net.IP{{192, 168, 10, 1}}
	s.options.Update(dhcpv4.OptDNS(s.conf.dnsIPAddrs...))

	l := &Lease{
		Hostname: "static-1.local",
		HWAddr:   net.HardwareAddr{0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA},
		IP:       net.IP{192, 168, 10, 150},
	}
	err := s.AddStaticLease(l)
	require.NoError(t, err)

	var req, resp *dhcpv4.DHCPv4
	mac := net.HardwareAddr{0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA}

	t.Run("discover", func(t *testing.T) {
		req, err = dhcpv4.NewDiscovery(mac, dhcpv4.WithRequestedOptions(
			dhcpv4.OptionDomainNameServer,
		))
		require.NoError(t, err)

		resp, err = dhcpv4.NewReplyFromRequest(req)
		require.NoError(t, err)

		assert.Equal(t, 1, s.handle(req, resp))
	})

	// Don't continue if we got any errors in the previous subtest.
	require.NoError(t, err)

	t.Run("offer", func(t *testing.T) {
		assert.Equal(t, dhcpv4.MessageTypeOffer, resp.MessageType())
		assert.Equal(t, mac, resp.ClientHWAddr)
		assert.True(t, l.IP.Equal(resp.YourIPAddr))
		assert.True(t, s.conf.GatewayIP.Equal(resp.Router()[0]))
		assert.True(t, s.conf.GatewayIP.Equal(resp.ServerIdentifier()))
		assert.Equal(t, s.conf.subnet.Mask, resp.SubnetMask())
		assert.Equal(t, s.conf.leaseTime.Seconds(), resp.IPAddressLeaseTime(-1).Seconds())
	})

	t.Run("request", func(t *testing.T) {
		req, err = dhcpv4.NewRequestFromOffer(resp)
		require.NoError(t, err)

		resp, err = dhcpv4.NewReplyFromRequest(req)
		require.NoError(t, err)

		assert.Equal(t, 1, s.handle(req, resp))
	})

	require.NoError(t, err)

	t.Run("ack", func(t *testing.T) {
		assert.Equal(t, dhcpv4.MessageTypeAck, resp.MessageType())
		assert.Equal(t, mac, resp.ClientHWAddr)
		assert.True(t, l.IP.Equal(resp.YourIPAddr))
		assert.True(t, s.conf.GatewayIP.Equal(resp.Router()[0]))
		assert.True(t, s.conf.GatewayIP.Equal(resp.ServerIdentifier()))
		assert.Equal(t, s.conf.subnet.Mask, resp.SubnetMask())
		assert.Equal(t, s.conf.leaseTime.Seconds(), resp.IPAddressLeaseTime(-1).Seconds())
	})

	dnsAddrs := resp.DNS()
	require.Len(t, dnsAddrs, 1)

	assert.True(t, s.conf.GatewayIP.Equal(dnsAddrs[0]))

	t.Run("check_lease", func(t *testing.T) {
		ls := s.GetLeases(LeasesStatic)
		require.Len(t, ls, 1)

		assert.True(t, l.IP.Equal(ls[0].IP))
		assert.Equal(t, mac, ls[0].HWAddr)
	})
}

func TestV4DynamicLease_Get(t *testing.T) {
	conf := defaultV4ServerConf()
	conf.Options = []string{
		"81 hex 303132",
		"82 ip 1.2.3.4",
	}

	var err error
	sIface, err := v4Create(conf)
	require.NoError(t, err)

	s, ok := sIface.(*v4Server)
	require.True(t, ok)

	s.conf.dnsIPAddrs = []net.IP{{192, 168, 10, 1}}
	s.options.Update(dhcpv4.OptDNS(s.conf.dnsIPAddrs...))

	var req, resp *dhcpv4.DHCPv4
	mac := net.HardwareAddr{0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA}

	t.Run("discover", func(t *testing.T) {
		req, err = dhcpv4.NewDiscovery(mac, dhcpv4.WithRequestedOptions(
			dhcpv4.OptionFQDN,
			dhcpv4.OptionRelayAgentInformation,
		))
		require.NoError(t, err)

		resp, err = dhcpv4.NewReplyFromRequest(req)
		require.NoError(t, err)

		assert.Equal(t, 1, s.handle(req, resp))
	})

	// Don't continue if we got any errors in the previous subtest.
	require.NoError(t, err)

	t.Run("offer", func(t *testing.T) {
		assert.Equal(t, dhcpv4.MessageTypeOffer, resp.MessageType())
		assert.Equal(t, mac, resp.ClientHWAddr)

		assert.Equal(t, s.conf.RangeStart, resp.YourIPAddr)
		assert.Equal(t, s.conf.GatewayIP, resp.ServerIdentifier())

		router := resp.Router()
		require.Len(t, router, 1)

		assert.Equal(t, s.conf.GatewayIP, router[0])

		assert.Equal(t, s.conf.subnet.Mask, resp.SubnetMask())
		assert.Equal(t, s.conf.leaseTime.Seconds(), resp.IPAddressLeaseTime(-1).Seconds())
		assert.Equal(t, []byte("012"), resp.Options.Get(dhcpv4.OptionFQDN))

		rai := resp.RelayAgentInfo()
		require.NotNil(t, rai)
		assert.Equal(t, net.IP{1, 2, 3, 4}, net.IP(rai.ToBytes()))
	})

	t.Run("request", func(t *testing.T) {
		req, err = dhcpv4.NewRequestFromOffer(resp)
		require.NoError(t, err)

		resp, err = dhcpv4.NewReplyFromRequest(req)
		require.NoError(t, err)

		assert.Equal(t, 1, s.handle(req, resp))
	})

	require.NoError(t, err)

	t.Run("ack", func(t *testing.T) {
		assert.Equal(t, dhcpv4.MessageTypeAck, resp.MessageType())
		assert.Equal(t, mac, resp.ClientHWAddr)
		assert.True(t, s.conf.RangeStart.Equal(resp.YourIPAddr))

		router := resp.Router()
		require.Len(t, router, 1)

		assert.Equal(t, s.conf.GatewayIP, router[0])

		assert.True(t, s.conf.GatewayIP.Equal(resp.ServerIdentifier()))
		assert.Equal(t, s.conf.subnet.Mask, resp.SubnetMask())
		assert.Equal(t, s.conf.leaseTime.Seconds(), resp.IPAddressLeaseTime(-1).Seconds())
	})

	dnsAddrs := resp.DNS()
	require.Len(t, dnsAddrs, 1)

	assert.True(t, net.IP{192, 168, 10, 1}.Equal(dnsAddrs[0]))

	// check lease
	t.Run("check_lease", func(t *testing.T) {
		ls := s.GetLeases(LeasesDynamic)
		require.Len(t, ls, 1)

		assert.True(t, net.IP{192, 168, 10, 100}.Equal(ls[0].IP))
		assert.Equal(t, mac, ls[0].HWAddr)
	})
}

func TestNormalizeHostname(t *testing.T) {
	testCases := []struct {
		name       string
		hostname   string
		wantErrMsg string
		want       string
	}{{
		name:       "success",
		hostname:   "example.com",
		wantErrMsg: "",
		want:       "example.com",
	}, {
		name:       "success_empty",
		hostname:   "",
		wantErrMsg: "",
		want:       "",
	}, {
		name:       "success_spaces",
		hostname:   "my device 01",
		wantErrMsg: "",
		want:       "my-device-01",
	}, {
		name:       "success_underscores",
		hostname:   "my_device_01",
		wantErrMsg: "",
		want:       "my-device-01",
	}, {
		name:       "error_part",
		hostname:   "device !!!",
		wantErrMsg: "",
		want:       "device",
	}, {
		name:       "error_part_spaces",
		hostname:   "device ! ! !",
		wantErrMsg: "",
		want:       "device",
	}, {
		name:       "error",
		hostname:   "!!!",
		wantErrMsg: `normalizing "!!!": no valid parts`,
		want:       "",
	}, {
		name:       "error_spaces",
		hostname:   "! ! !",
		wantErrMsg: `normalizing "! ! !": no valid parts`,
		want:       "",
	}}

	for _, tc := range testCases {
		t.Run(tc.name, func(t *testing.T) {
			got, err := normalizeHostname(tc.hostname)
			testutil.AssertErrorMsg(t, tc.wantErrMsg, err)
			assert.Equal(t, tc.want, got)
		})
	}
}

// fakePacketConn is a mock implementation of net.PacketConn to simplify
// testing.
type fakePacketConn struct {
	// writeTo is used to substitute net.PacketConn's WriteTo method.
	writeTo func(p []byte, addr net.Addr) (n int, err error)
	// net.PacketConn is embedded here simply to make *fakePacketConn a
	// net.PacketConn without actually implementing all methods.
	net.PacketConn
}

// WriteTo implements net.PacketConn interface for *fakePacketConn.
func (fc *fakePacketConn) WriteTo(p []byte, addr net.Addr) (n int, err error) {
	return fc.writeTo(p, addr)
}

func TestV4Server_Send(t *testing.T) {
	s := &v4Server{}

	var (
		defaultIP = net.IP{99, 99, 99, 99}
		knownIP   = net.IP{4, 2, 4, 2}
		knownMAC  = net.HardwareAddr{6, 5, 4, 3, 2, 1}
	)

	defaultPeer := &net.UDPAddr{
		IP: defaultIP,
		// Use neither client nor server port to check it actually
		// changed.
		Port: dhcpv4.ClientPort + dhcpv4.ServerPort,
	}
	defaultResp := &dhcpv4.DHCPv4{}

	testCases := []struct {
		want net.Addr
		req  *dhcpv4.DHCPv4
		resp *dhcpv4.DHCPv4
		name string
	}{{
		name: "giaddr",
		req:  &dhcpv4.DHCPv4{GatewayIPAddr: knownIP},
		resp: defaultResp,
		want: &net.UDPAddr{
			IP:   knownIP,
			Port: dhcpv4.ServerPort,
		},
	}, {
		name: "nak",
		req:  &dhcpv4.DHCPv4{},
		resp: &dhcpv4.DHCPv4{
			Options: dhcpv4.OptionsFromList(
				dhcpv4.OptMessageType(dhcpv4.MessageTypeNak),
			),
		},
		want: defaultPeer,
	}, {
		name: "ciaddr",
		req:  &dhcpv4.DHCPv4{ClientIPAddr: knownIP},
		resp: &dhcpv4.DHCPv4{},
		want: &net.UDPAddr{
			IP:   knownIP,
			Port: dhcpv4.ClientPort,
		},
	}, {
		name: "chaddr",
		req:  &dhcpv4.DHCPv4{ClientHWAddr: knownMAC},
		resp: &dhcpv4.DHCPv4{YourIPAddr: knownIP},
		want: &dhcpUnicastAddr{
			Addr:   raw.Addr{HardwareAddr: knownMAC},
			yiaddr: knownIP,
		},
	}, {
		name: "who_are_you",
		req:  &dhcpv4.DHCPv4{},
		resp: &dhcpv4.DHCPv4{},
		want: defaultPeer,
	}}

	for _, tc := range testCases {
		t.Run(tc.name, func(t *testing.T) {
			conn := &fakePacketConn{
				writeTo: func(_ []byte, addr net.Addr) (_ int, _ error) {
					assert.Equal(t, tc.want, addr)

					return 0, nil
				},
			}

			s.send(cloneUDPAddr(defaultPeer), conn, tc.req, tc.resp)
		})
	}

	t.Run("giaddr_nak", func(t *testing.T) {
		req := &dhcpv4.DHCPv4{
			GatewayIPAddr: knownIP,
		}
		// Ensure the request is for unicast.
		req.SetUnicast()
		resp := &dhcpv4.DHCPv4{
			Options: dhcpv4.OptionsFromList(
				dhcpv4.OptMessageType(dhcpv4.MessageTypeNak),
			),
		}
		want := &net.UDPAddr{
			IP:   req.GatewayIPAddr,
			Port: dhcpv4.ServerPort,
		}

		conn := &fakePacketConn{
			writeTo: func(_ []byte, addr net.Addr) (n int, err error) {
				assert.Equal(t, want, addr)

				return 0, nil
			},
		}

		s.send(cloneUDPAddr(defaultPeer), conn, req, resp)
		assert.True(t, resp.IsBroadcast())
	})
}