Initial commit

Signed-off-by: Evgenii Stratonikov <stratonikov@runbox.com>

Makefile

@@ -0,0 +1,3 @@
+test:
+	# TODO figure out needed capabilities
+	sudo go test -count=1 -v ./...

README.md

@@ -0,0 +1,41 @@
+# Source-based routing in Golang
+
+
+Consider this routing table:
+```
+10.11.70.0/23 dev data0 proto kernel scope link src 10.11.70.42
+10.11.70.0/23 dev data1 proto kernel scope link src 10.11.71.42
+192.168.123.0/24 dev internal0 proto kernel scope link src 192.168.123.42
+192.168.123.0/24 dev internal1 proto kernel scope link src 192.168.123.142
+```
+
+Simple `net.Dial` to either `10.11.70.42` or `10.11.71.42` will match the first subnet and be routed via data0.
+This problems is usually solved by bonds.
+But sometimes you need to invent a bicycle.
+
+## Usage
+
+```golang
+import "git.frostfs.info/TrueCloudLab/multinet"
+
+d, err := multinet.NewDialer(Config{
+    Subnets:  []string{"10.11.70.0/23", "192.168.123.0/24"},
+    Balancer: multinet.BalancerTypeRoundRobin,
+})
+if err != nil {
+    // handle error
+}
+
+conn, err := d.DialContext(ctx, "tcp", "10.11.70.42")
+if err != nil {
+    // handle error
+}
+// do stuff
+```
+
+### Updating interface state
+
+`Multidialer` exposes `UpdateInterface()` method for updating state of a single link.
+`NetlinkWatcher` can wrap `Multidialer` type and perform all updates automatically.
+
+TODO: describe needed capabilities here.

balancer.go

@@ -0,0 +1,61 @@
+package multinet
+
+import (
+	"context"
+	"errors"
+	"net"
+	"sync/atomic"
+)
+
+// BalancerType reperents the algorithm which is used to pick source address.
+type BalancerType string
+
+const (
+	// BalancerTypeNoop picks first address for which link is up.
+	BalancerTypeNoop BalancerType = ""
+	// BalancerTypeNoop implements simple round-robin between up links.
+	// It is not fair in case some links are down.
+	BalancerTypeRoundRobin BalancerType = "roundrobin"
+)
+
+type balancer interface {
+	DialContext(ctx context.Context, s *Subnet, network, address string) (net.Conn, error)
+}
+
+type roundRobin struct {
+	d *dialer
+	i atomic.Uint32
+}
+
+func (r *roundRobin) DialContext(ctx context.Context, s *Subnet, network, address string) (net.Conn, error) {
+	next := int(r.i.Add(1))
+	for i := range s.Interfaces {
+		ii := s.Interfaces[(i+next)%len(s.Interfaces)]
+		if ii.Down {
+			continue
+		}
+
+		dd := r.d.dialer
+		dd.LocalAddr = ii.LocalAddr
+		return r.d.dialContext(&dd, ctx, "tcp", address)
+	}
+	return nil, errors.New("(*roundRobin).DialContext: no suitale node found")
+}
+
+type firstEnabled struct {
+	d *dialer
+}
+
+func (r *firstEnabled) DialContext(ctx context.Context, s *Subnet, network, address string) (net.Conn, error) {
+	for i := range s.Interfaces {
+		ii := s.Interfaces[i%len(s.Interfaces)]
+		if ii.Down {
+			continue
+		}
+
+		dd := r.d.dialer
+		dd.LocalAddr = ii.LocalAddr
+		return r.d.dialContext(&dd, ctx, "tcp", address)
+	}
+	return nil, errors.New("(*firstEnabled).DialContext: no suitale node found")
+}

dialer.go

@@ -0,0 +1,219 @@
+package multinet
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"net"
+	"net/netip"
+	"sort"
+	"sync"
+)
+
+// Dialer contains the single most important method from the net.Dialer.
+type Dialer interface {
+	DialContext(ctx context.Context, network, address string) (net.Conn, error)
+}
+
+// Multidialer is like Dialer, but supports link state updates.
+type Multidialer interface {
+	Dialer
+	UpdateInterface(name string, addr netip.Addr, status bool) error
+}
+
+var (
+	_ Dialer = (*net.Dialer)(nil)
+	_ Dialer = (*dialer)(nil)
+)
+
+type dialer struct {
+	// Protects subnets field (recursively).
+	mtx     sync.RWMutex
+	subnets []Subnet
+	// Default options for the net.Dialer.
+	dialer net.Dialer
+	// If true, allow to dial only configured subnets.
+	restrict bool
+	// Algorithm to which picks source address for each ip.
+	balancer balancer
+	// Hook used in tests, overrides `net.Dialer.DialContext()`
+	testHookDialContext func(d *net.Dialer, ctx context.Context, network, address string) (net.Conn, error)
+}
+
+// Subnet represents a single subnet, possibly routable from multiple interfaces.
+type Subnet struct {
+	Mask       netip.Prefix
+	Interfaces []Source
+}
+
+// Source represents a single source IP belonging to a particular subnet.
+type Source struct {
+	Name      string
+	LocalAddr *net.TCPAddr
+	Down      bool
+}
+
+// Config contains Multidialer configuration.
+type Config struct {
+	// Routable subnets to prioritize in CIDR format.
+	Subnets []string
+	// If true, the only configurd subnets available through this dialer.
+	// Otherwise, a failback to the net.DefaultDialer.
+	Restrict bool
+	// Dialer containes default options for the net.Dialer to use.
+	// LocalAddr is overriden.
+	Dialer net.Dialer
+	// Balancer specifies algorithm used to pick source address.
+	Balancer BalancerType
+}
+
+// NewDialer ...
+func NewDialer(c Config) (Multidialer, error) {
+	ifaces, err := net.Interfaces()
+	if err != nil {
+		return nil, err
+	}
+	sort.Slice(ifaces, func(i, j int) bool {
+		return ifaces[i].Name < ifaces[j].Name
+	})
+
+	var sources []iface
+	for i := range ifaces {
+		info, err := processIface(ifaces[i])
+		if err != nil {
+			return nil, err
+		}
+		sources = append(sources, info)
+	}
+
+	var d dialer
+	for _, subnet := range c.Subnets {
+		s, err := processSubnet(subnet, sources)
+		if err != nil {
+			return nil, err
+		}
+		d.subnets = append(d.subnets, s)
+	}
+
+	switch c.Balancer {
+	case BalancerTypeNoop:
+		d.balancer = &firstEnabled{d: &d}
+	case BalancerTypeRoundRobin:
+		d.balancer = &roundRobin{d: &d}
+	default:
+		return nil, fmt.Errorf("invalid balancer type: %s", c.Balancer)
+	}
+
+	d.restrict = c.Restrict
+
+	return &d, nil
+}
+
+type iface struct {
+	info  net.Interface
+	addrs []netip.Prefix
+}
+
+func processIface(info net.Interface) (iface, error) {
+	ips, err := info.Addrs()
+	if err != nil {
+		return iface{}, err
+	}
+
+	var addrs []netip.Prefix
+	for i := range ips {
+		p, err := netip.ParsePrefix(ips[i].String())
+		if err != nil {
+			return iface{}, err
+		}
+
+		addrs = append(addrs, p)
+	}
+	return iface{info: info, addrs: addrs}, nil
+}
+
+func processSubnet(subnet string, sources []iface) (Subnet, error) {
+	s, err := netip.ParsePrefix(subnet)
+	if err != nil {
+		return Subnet{}, err
+	}
+
+	var ifs []Source
+	for _, source := range sources {
+		for i := range source.addrs {
+			src := source.addrs[i].Addr()
+			if s.Contains(src) {
+				ifs = append(ifs, Source{
+					Name:      source.info.Name,
+					LocalAddr: &net.TCPAddr{IP: net.IP(src.AsSlice())},
+				})
+			}
+		}
+	}
+
+	sort.Slice(ifs, func(i, j int) bool {
+		if ifs[i].Name != ifs[j].Name {
+			return ifs[i].Name < ifs[j].Name
+		}
+		return bytes.Compare(ifs[i].LocalAddr.IP, ifs[j].LocalAddr.IP) == -1
+	})
+
+	return Subnet{
+		Mask:       s,
+		Interfaces: ifs,
+	}, nil
+}
+
+// DialContext implements the Dialer interface.
+// Hostnames for address are currently not supported.
+func (d *dialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
+	addr, err := netip.ParseAddrPort(address)
+	if err != nil {
+		return nil, err
+	}
+
+	d.mtx.RLock()
+	defer d.mtx.RUnlock()
+
+	for i := range d.subnets {
+		if d.subnets[i].Mask.Contains(addr.Addr()) {
+			return d.balancer.DialContext(ctx, &d.subnets[i], network, address)
+		}
+	}
+
+	if d.restrict {
+		return nil, fmt.Errorf("no suitable interface for: [%s]%s", network, address)
+	}
+	return d.dialer.DialContext(ctx, network, address)
+}
+
+func (d *dialer) dialContext(nd *net.Dialer, ctx context.Context, network, address string) (net.Conn, error) {
+	if h := d.testHookDialContext; h != nil {
+		return h(nd, ctx, "tcp", address)
+	}
+	return nd.DialContext(ctx, "tcp", address)
+}
+
+// UpdateInterface implements the Multidialer interface.
+// Updating address on a specific interface is currently not supported.
+func (d *dialer) UpdateInterface(iface string, addr netip.Addr, up bool) error {
+	d.mtx.Lock()
+	defer d.mtx.Unlock()
+
+	for i := range d.subnets {
+		for j := range d.subnets[i].Interfaces {
+			matchIface := d.subnets[i].Interfaces[j].Name == iface
+			if matchIface {
+				d.subnets[i].Interfaces[j].Down = !up
+				continue
+			}
+
+			a, _ := netip.AddrFromSlice(d.subnets[i].Interfaces[j].LocalAddr.IP)
+			matchAddr := a.IsUnspecified() || addr == a
+			if matchAddr {
+				d.subnets[i].Interfaces[j].Down = !up
+			}
+		}
+	}
+	return nil
+}

dialer_test.go

@@ -0,0 +1,164 @@
+package multinet
+
+import (
+	"net"
+	"net/netip"
+	"runtime"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"github.com/vishvananda/netlink"
+	"github.com/vishvananda/netns"
+)
+
+func TestDialer(t *testing.T) {
+	runInNewNamespace(t, "2 interfaces with multiple routes in different subnets", func(t *testing.T, ns netns.NsHandle) {
+		setup(t, map[string][]string{
+			"testdev1": {"1.2.30.10/23", "4.4.4.4/8"},
+			"testdev2": {"1.2.30.11/23", "4.4.4.5/8"},
+		})
+
+		// Do not use `t.Run` because everything should be executed in a single OS thread.
+
+		{ // Restrict to a single subnet.
+			d, err := NewDialer(Config{
+				Subnets: []string{"1.2.30.0/23"},
+			})
+			require.NoError(t, err)
+			require.Equal(t, []Subnet{
+				{
+					Mask: netip.MustParsePrefix("1.2.30.0/23"),
+					Interfaces: []Source{
+						{Name: "testdev1", LocalAddr: &net.TCPAddr{IP: net.IP{1, 2, 30, 10}}},
+						{Name: "testdev2", LocalAddr: &net.TCPAddr{IP: net.IP{1, 2, 30, 11}}},
+					},
+				},
+			}, d.(*dialer).subnets)
+		}
+
+		{ // Restrict to two subnets.
+			d, err := NewDialer(Config{
+				Subnets: []string{"1.2.30.0/23", "4.0.0.0/8"},
+			})
+			require.NoError(t, err)
+			require.Equal(t, []Subnet{
+				{
+					Mask: netip.MustParsePrefix("1.2.30.0/23"),
+					Interfaces: []Source{
+						{Name: "testdev1", LocalAddr: &net.TCPAddr{IP: net.IP{1, 2, 30, 10}}},
+						{Name: "testdev2", LocalAddr: &net.TCPAddr{IP: net.IP{1, 2, 30, 11}}},
+					},
+				},
+				{
+					Mask: netip.MustParsePrefix("4.0.0.0/8"),
+					Interfaces: []Source{
+						{Name: "testdev1", LocalAddr: &net.TCPAddr{IP: net.IP{4, 4, 4, 4}}},
+						{Name: "testdev2", LocalAddr: &net.TCPAddr{IP: net.IP{4, 4, 4, 5}}},
+					},
+				},
+			}, d.(*dialer).subnets)
+		}
+	})
+	runInNewNamespace(t, "4 interfaces, 2 for data, 2 internal", func(t *testing.T, ns netns.NsHandle) {
+		setup(t, map[string][]string{
+			"internal1": {"192.168.0.1/16"},
+			"internal2": {"192.168.0.2/16"},
+			"data1":     {"10.11.12.101/24"},
+			"data2":     {"10.11.12.102/24"},
+		})
+
+		d, err := NewDialer(Config{
+			Subnets: []string{"10.11.12.0/24", "192.168.0.0/16"},
+		})
+		require.NoError(t, err)
+		require.Equal(t, []Subnet{
+			{
+				Mask: netip.MustParsePrefix("10.11.12.0/24"),
+				Interfaces: []Source{
+					{Name: "data1", LocalAddr: &net.TCPAddr{IP: net.IP{10, 11, 12, 101}}},
+					{Name: "data2", LocalAddr: &net.TCPAddr{IP: net.IP{10, 11, 12, 102}}},
+				},
+			},
+			{
+				Mask: netip.MustParsePrefix("192.168.0.0/16"),
+				Interfaces: []Source{
+					{Name: "internal1", LocalAddr: &net.TCPAddr{IP: net.IP{192, 168, 0, 1}}},
+					{Name: "internal2", LocalAddr: &net.TCPAddr{IP: net.IP{192, 168, 0, 2}}},
+				},
+			},
+		}, d.(*dialer).subnets)
+	})
+	runInNewNamespace(t, "with ipv6", func(t *testing.T, ns netns.NsHandle) {
+		addr1 := "2001:db8:85a3:8d3:1319:8a2e:370:7348/64"
+		addr2 := "2001:db8:85a3:8d3:1319:8a2e:370:8192/64"
+		setup(t, map[string][]string{
+			"testdev1": {addr1},
+			"testdev2": {addr2},
+		})
+
+		// Do not use `t.Run` because everything should be executed in a single OS thread.
+
+		{ // Restrict to a single subnet.
+			d, err := NewDialer(Config{
+				Subnets: []string{"2001:db8:85a3:8d3::/64"},
+			})
+			require.NoError(t, err)
+			require.Equal(t, []Subnet{
+				{
+					Mask: netip.MustParsePrefix("2001:db8:85a3:8d3::/64"),
+					Interfaces: []Source{
+						{Name: "testdev1", LocalAddr: mustParseIPv6(t, addr1)},
+						{Name: "testdev2", LocalAddr: mustParseIPv6(t, addr2)},
+					},
+				},
+			}, d.(*dialer).subnets)
+		}
+	})
+}
+
+func mustParseIPv6(t *testing.T, s string) *net.TCPAddr {
+	ip, _, err := net.ParseCIDR(s)
+	require.NoError(t, err)
+	return &net.TCPAddr{IP: ip}
+}
+
+func setup(t *testing.T, config map[string][]string) {
+	for name, ips := range config {
+		link := createLink(t, name)
+		for i := range ips {
+			ip, err := netlink.ParseIPNet(ips[i])
+			require.NoError(t, err)
+			require.NoError(t, netlink.AddrAdd(link, &netlink.Addr{IPNet: ip}))
+		}
+	}
+}
+
+func createLink(t *testing.T, name string) netlink.Link {
+	require.NoError(t, netlink.LinkAdd(&netlink.Dummy{LinkAttrs: netlink.LinkAttrs{Name: name}}))
+
+	link, err := netlink.LinkByName(name)
+	require.NoError(t, err)
+	require.NoError(t, netlink.LinkSetUp(link))
+	return link
+}
+
+func runInNewNamespace(t *testing.T, name string, f func(t *testing.T, ns netns.NsHandle)) {
+	t.Run(name, func(t *testing.T) {
+		// To avoid messing with host network settings,
+		// we create a new names space and execute tests in it.
+		// Switching thread can move us to a different namespace, thus this line.
+		runtime.LockOSThread()
+		defer runtime.UnlockOSThread()
+
+		origns, err := netns.Get()
+		require.NoError(t, err)
+		defer origns.Close()
+		defer netns.Set(origns)
+
+		newns, err := netns.New()
+		require.NoError(t, err)
+		defer newns.Close()
+
+		f(t, newns)
+	})
+}

go.mod

@@ -0,0 +1,16 @@
+module git.frostfs.info/TrueCloudLab/multinet
+
+go 1.21.0
+
+require (
+	github.com/stretchr/testify v1.8.4
+	github.com/vishvananda/netlink v1.1.0
+	github.com/vishvananda/netns v0.0.4
+	golang.org/x/sys v0.2.0
+)
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)

health.go

@@ -0,0 +1,73 @@
+package multinet
+
+import (
+	"net/netip"
+	"sync"
+
+	"github.com/vishvananda/netlink"
+	"github.com/vishvananda/netns"
+	"golang.org/x/sys/unix"
+)
+
+type NetlinkWatcher struct {
+	d           Multidialer
+	linkUpdates chan netlink.LinkUpdate
+	addrUpdates chan netlink.AddrUpdate
+	done        chan struct{}
+	wg          sync.WaitGroup
+}
+
+func NewNetlinkWatcher(d Multidialer) *NetlinkWatcher {
+	return &NetlinkWatcher{
+		d:           d,
+		addrUpdates: make(chan netlink.AddrUpdate, 1),
+		linkUpdates: make(chan netlink.LinkUpdate, 1),
+		done:        make(chan struct{}),
+	}
+}
+
+func (w *NetlinkWatcher) Start() error {
+	ns, err := netns.Get()
+	if err != nil {
+		return err
+	}
+
+	if err := netlink.LinkSubscribe(w.linkUpdates, w.done); err != nil {
+		return err
+	}
+	if err := netlink.AddrSubscribe(w.addrUpdates, w.done); err != nil {
+		close(w.done)
+		return err
+	}
+
+	w.wg.Add(1)
+	go w.watch(ns)
+	return nil
+}
+
+func (w *NetlinkWatcher) watch(ns netns.NsHandle) {
+	defer w.wg.Done()
+
+	for {
+		select {
+		case <-w.done:
+			return
+		case update := <-w.addrUpdates:
+			// Wont work if an multiple interfaces share IP address.
+			// Should not happen in practice.
+			ip, ok := netip.AddrFromSlice(update.LinkAddress.IP)
+			if !ok {
+				continue
+			}
+			w.d.UpdateInterface("", ip, update.NewAddr)
+		case update := <-w.linkUpdates:
+			up := update.Flags&unix.IFF_UP != 0
+			w.d.UpdateInterface(update.Link.Attrs().Name, netip.Addr{}, up)
+		}
+	}
+}
+
+func (w *NetlinkWatcher) Stop() {
+	close(w.done)
+	w.wg.Wait()
+}

health_test.go

@@ -0,0 +1,158 @@
+package multinet
+
+import (
+	"context"
+	"net"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+	"github.com/vishvananda/netlink"
+	"github.com/vishvananda/netns"
+)
+
+func Test_NetlinkWatcher(t *testing.T) {
+	runInNewNamespace(t, "noop balancer, disable interface", func(t *testing.T, ns netns.NsHandle) {
+		setup(t, map[string][]string{
+			"testdev1": {"1.2.30.11/23"},
+			"testdev2": {"1.2.30.12/23"},
+		})
+
+		addr1 := &net.TCPAddr{IP: net.IP{1, 2, 30, 11}}
+		addr2 := &net.TCPAddr{IP: net.IP{1, 2, 30, 12}}
+
+		d, err := NewDialer(Config{
+			Subnets: []string{"1.2.30.0/23"},
+		})
+		require.NoError(t, err)
+
+		w := NewNetlinkWatcher(d)
+		require.NoError(t, w.Start())
+		t.Cleanup(w.Stop)
+
+		result := make(chan net.Addr, 1)
+		d.(*dialer).testHookDialContext = func(d *net.Dialer, _ context.Context, _, _ string) (net.Conn, error) {
+			result <- d.LocalAddr
+			return nil, nil
+		}
+
+		checkDialAddr(t, d, result, addr1)
+		checkDialAddr(t, d, result, addr1)
+
+		link, err := netlink.LinkByName("testdev1")
+		require.NoError(t, err)
+		require.NoError(t, netlink.LinkSetDown(link))
+		time.Sleep(time.Second)
+
+		checkDialAddr(t, d, result, addr2)
+		checkDialAddr(t, d, result, addr2)
+
+		require.NoError(t, netlink.LinkSetUp(link))
+		time.Sleep(time.Second)
+
+		checkDialAddr(t, d, result, addr1)
+	})
+
+	runInNewNamespace(t, "noop balancer, remove address", func(t *testing.T, ns netns.NsHandle) {
+		setup(t, map[string][]string{
+			"testdev1": {"1.2.30.11/23"},
+			"testdev2": {"1.2.30.12/23"},
+		})
+
+		addr1 := &net.TCPAddr{IP: net.IP{1, 2, 30, 11}}
+		addr2 := &net.TCPAddr{IP: net.IP{1, 2, 30, 12}}
+
+		d, err := NewDialer(Config{
+			Subnets: []string{"1.2.30.0/23"},
+		})
+		require.NoError(t, err)
+
+		w := NewNetlinkWatcher(d)
+		require.NoError(t, w.Start())
+		t.Cleanup(w.Stop)
+
+		result := make(chan net.Addr, 1)
+		d.(*dialer).testHookDialContext = func(d *net.Dialer, _ context.Context, _, _ string) (net.Conn, error) {
+			result <- d.LocalAddr
+			return nil, nil
+		}
+
+		checkDialAddr(t, d, result, addr1)
+		checkDialAddr(t, d, result, addr1)
+
+		link, err := netlink.LinkByName("testdev1")
+		require.NoError(t, err)
+
+		ip, err := netlink.ParseIPNet("1.2.30.11/23")
+		require.NoError(t, err)
+		require.NoError(t, netlink.AddrDel(link, &netlink.Addr{IPNet: ip}))
+		time.Sleep(time.Second)
+
+		checkDialAddr(t, d, result, addr2)
+		checkDialAddr(t, d, result, addr2)
+
+		require.NoError(t, netlink.AddrAdd(link, &netlink.Addr{IPNet: ip}))
+		time.Sleep(time.Second)
+
+		checkDialAddr(t, d, result, addr1)
+	})
+
+	runInNewNamespace(t, "round-robin balancer, disable interface", func(t *testing.T, ns netns.NsHandle) {
+		setup(t, map[string][]string{
+			"testdev1": {"1.2.30.11/23"},
+			"testdev2": {"1.2.30.12/23"},
+		})
+
+		addr1 := &net.TCPAddr{IP: net.IP{1, 2, 30, 11}}
+		addr2 := &net.TCPAddr{IP: net.IP{1, 2, 30, 12}}
+
+		d, err := NewDialer(Config{
+			Subnets:  []string{"1.2.30.0/23"},
+			Balancer: BalancerTypeRoundRobin,
+		})
+		require.NoError(t, err)
+
+		w := NewNetlinkWatcher(d)
+		require.NoError(t, w.Start())
+		t.Cleanup(w.Stop)
+
+		result := make(chan net.Addr, 1)
+		d.(*dialer).testHookDialContext = func(d *net.Dialer, _ context.Context, _, _ string) (net.Conn, error) {
+			result <- d.LocalAddr
+			return nil, nil
+		}
+
+		checkDialAddr(t, d, result, addr2)
+		checkDialAddr(t, d, result, addr1)
+		checkDialAddr(t, d, result, addr2)
+
+		link, err := netlink.LinkByName("testdev1")
+		require.NoError(t, err)
+		require.NoError(t, netlink.LinkSetDown(link))
+		time.Sleep(time.Second)
+
+		checkDialAddr(t, d, result, addr2)
+		checkDialAddr(t, d, result, addr2)
+
+		require.NoError(t, netlink.LinkSetUp(link))
+		time.Sleep(time.Second)
+
+		checkDialAddr(t, d, result, addr1)
+		checkDialAddr(t, d, result, addr2)
+	})
+}
+
+func checkDialAddr(t *testing.T, d Multidialer, ch chan net.Addr, expected net.Addr) {
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
+
+	_, err := d.DialContext(ctx, "tcp", "1.2.30.42:12345")
+	require.NoError(t, err)
+
+	select {
+	case addr := <-ch:
+		require.Equal(t, expected, addr)
+	default:
+		require.Fail(t, "DialContext() was not called")
+	}
+}