Failover: Initial implementation

1 files changed, 51 insertions, 0 deletions

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
+}