3 files changed, 178 insertions, 0 deletions
diff --git a/internal/failover/failover.go b/internal/failover/failover.go
new file mode 100644
index 0000000..f239eeb
--- /dev/null
+++ b/internal/failover/failover.go
@@ -0,0 +1,21 @@
+package failover
+
+import "time"
+
+const (
+	// Send a ping every 2 seconds to the gateway
+	pInterval time.Duration = 2 * time.Second
+
+	// pAlive is how many pings we need to have sent to check if the connection is alive
+	pAlive int = 3
+
+	// pDropped is how many pings we need to have sent to check if the connection is dropped
+	pDropped int = 5
+)
+
+// New creates a failover monitor for the gateway and the rx bytes function reader
+// This is a simple wrapper over `NewDroppedMonitor` to create one with the default settings
+// If this function returns True, the connection is dropped. False means it has exited and we don't know for sure if it's dropped or not
+func New(readRxBytes func() (int64, error)) (*DroppedConMon, error) {
+	return NewDroppedMonitor(pInterval, pAlive, pDropped, readRxBytes)
+}
diff --git a/internal/failover/monitor.go b/internal/failover/monitor.go
new file mode 100644
index 0000000..d14fb9e
--- /dev/null
+++ b/internal/failover/monitor.go
@@ -0,0 +1,106 @@
+package failover
+
+import (
+	"context"
+	"time"
+
+	"github.com/go-errors/errors"
+)
+
+// The DroppedConMon is a connection monitor that checks for an increase in rx bytes in certain intervals
+type DroppedConMon struct {
+	// pInterval means how the interval in which to send pings
+	pInterval time.Duration
+	// pAlive means how many pings need to be send before checking if the connection is alive
+	pAlive int
+	// pDropped means how many pings need to be send before checking if the connection is dropped
+	pDropped int
+	// The function that reads Rx bytes
+	// If this function returns an error, the monitor exits
+	readRxBytes func() (int64, error)
+	// The cancel context
+	// This is used to cancel the dropped connection monitor
+	cancel context.CancelFunc
+}
+
+func NewDroppedMonitor(pingInterval time.Duration, pAlive int, pDropped int, readRxBytes func() (int64, error)) (*DroppedConMon, error) {
+	if pAlive >= pDropped {
+		return nil, errors.New("pAlive must be smaller than pDropped")
+	}
+	return &DroppedConMon{pInterval: pingInterval, pAlive: pAlive, pDropped: pDropped, readRxBytes: readRxBytes}, nil
+}
+
+// Dropped checks whether or not the connection is 'dropped'
+// In other words, it checks if rx bytes has increased
+func (m *DroppedConMon) dropped(startBytes int64) (bool, error) {
+	b, err := m.readRxBytes()
+	if err != nil {
+		return false, err
+	}
+	return b <= startBytes, nil
+}
+
+// Start starts ticking every ping interval and check if the connection is dropped or alive
+// This does not check Rx bytes every tick, but rather when pAlive or pDropped is reached
+// It returns an error if there was an invalid input or a ping was failed to be sent
+func (m *DroppedConMon) Start(gateway string, mtuSize int) (bool, error) {
+	if mtuSize <= 0 {
+		return false, errors.New("invalid mtu size given")
+	}
+
+	// Create a context and save the cancel function
+	ctx, cancel := context.WithCancel(context.Background())
+	m.cancel = cancel
+	defer m.cancel()
+
+	// Create a ping struct with our mtu size
+	p, err := NewPinger(mtuSize)
+	if err != nil {
+		return false, err
+	}
+
+	// Read the start Rx bytes
+	b, err := m.readRxBytes()
+	if err != nil {
+		return false, err
+	}
+
+	// Create a new ticker that executes our ping function every 'interval' seconds
+	// It starts immediately and stops when we reach the end
+	ticker := time.NewTicker(m.pInterval)
+	defer ticker.Stop()
+
+	// Loop until the max drop counter
+	// We begin with 1 as this is used as the sequence number for ping
+	for s := 1; s <= m.pDropped; s++ {
+		// Send a ping and return if an error occurs
+		if err := p.Send(gateway, s); err != nil {
+			return false, err
+		}
+
+		// Early alive check
+		// If not dropped, return
+		if s == m.pAlive {
+			if d, err := m.dropped(b); !d {
+				return false, err
+			}
+		}
+		// Wait for the next tick to continue
+		select {
+		case <-ticker.C:
+			continue
+		case <-ctx.Done():
+			return false, errors.New("failover was cancelled")
+		}
+	}
+
+	// Dropped check if we have not returned early
+	return m.dropped(b)
+}
+
+// Cancel cancels the dropped connection failover monitor if there is one
+func (m *DroppedConMon) Cancel() {
+	if m.cancel != nil {
+		m.cancel()
+	}
+}
diff --git a/internal/failover/ping.go b/internal/failover/ping.go
new file mode 100644
index 0000000..2ffedd4
--- /dev/null
+++ b/internal/failover/ping.go
@@ -0,0 +1,51 @@
+package failover
+
+import (
+	"fmt"
+	"net"
+	"os"
+
+	"golang.org/x/net/icmp"
+	"golang.org/x/net/ipv4"
+
+	"github.com/go-errors/errors"
+)
+
+// mtuOverhead defines the total MTU overhead for an ICMP ECHO message: 20 bytes IP header + 8 bytes ICMP header
+var mtuOverhead = 28
+
+type Pinger struct {
+	listener net.PacketConn
+	buffer   []byte
+}
+
+func NewPinger(size int) (*Pinger, error) {
+	l, err := icmp.ListenPacket("udp4", "0.0.0.0")
+	if err != nil {
+		return nil, errors.WrapPrefix(err, "failed creating ping", 0)
+	}
+	return &Pinger{listener: l, buffer: make([]byte, size-mtuOverhead)}, nil
+}
+
+func (p Pinger) Send(gateway string, seq int) error {
+	errorMessage := fmt.Sprintf("failed sending ping, seq %d", seq)
+	// Make a new ICMP message
+	m := icmp.Message{
+		Type: ipv4.ICMPTypeEcho, Code: 0,
+		Body: &icmp.Echo{
+			ID: os.Getpid() & 0xffff, Seq: seq,
+			Data: p.buffer,
+		},
+	}
+	// Marshal the message to bytes
+	b, err := m.Marshal(nil)
+	if err != nil {
+		return errors.WrapPrefix(err, errorMessage, 0)
+	}
+	// And send it to the gateway IP!
+	_, err = p.listener.WriteTo(b, &net.UDPAddr{IP: net.ParseIP(gateway)})
+	if err != nil {
+		return errors.WrapPrefix(err, errorMessage, 0)
+	}
+	return nil
+}