初版功能完成

internal/subnetgateway/pingrelay/relay.go

@@ -0,0 +1,85 @@
+// Package pingrelay probes a Site target and preserves Echo ID/Sequence.
+package pingrelay
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"net/netip"
+	"time"
+
+	"golang.org/x/net/icmp"
+	"golang.org/x/net/ipv4"
+)
+
+const DefaultTimeout = 3 * time.Second
+
+type Relay struct {
+	Timeout time.Duration
+}
+
+// Echo sends one host ICMP Echo and waits for the matching target reply.
+func (r Relay) Echo(ctx context.Context, target netip.Addr, id, sequence int, data []byte) error {
+	if !target.Is4() || id < 0 || id > 65535 || sequence < 0 || sequence > 65535 {
+		return errors.New("PingRelay target, ID, or Sequence is invalid")
+	}
+	timeout := r.Timeout
+	if timeout <= 0 {
+		timeout = DefaultTimeout
+	}
+	connection, err := icmp.ListenPacket("ip4:icmp", "0.0.0.0")
+	if err != nil {
+		return fmt.Errorf("open host ICMP socket: %w", err)
+	}
+	defer connection.Close()
+	deadline := time.Now().Add(timeout)
+	if contextDeadline, ok := ctx.Deadline(); ok && contextDeadline.Before(deadline) {
+		deadline = contextDeadline
+	}
+	if err := connection.SetDeadline(deadline); err != nil {
+		return err
+	}
+	stopClose := make(chan struct{})
+	go func() {
+		select {
+		case <-ctx.Done():
+			_ = connection.Close()
+		case <-stopClose:
+		}
+	}()
+	defer close(stopClose)
+	message := icmp.Message{
+		Type: ipv4.ICMPTypeEcho, Code: 0,
+		Body: &icmp.Echo{ID: id, Seq: sequence, Data: append([]byte(nil), data...)},
+	}
+	encoded, err := message.Marshal(nil)
+	if err != nil {
+		return fmt.Errorf("marshal ICMP Echo: %w", err)
+	}
+	if _, err := connection.WriteTo(encoded, &net.IPAddr{IP: net.IP(target.AsSlice())}); err != nil {
+		return fmt.Errorf("send ICMP Echo to %s: %w", target, err)
+	}
+	buffer := make([]byte, 1500)
+	for {
+		count, peer, err := connection.ReadFrom(buffer)
+		if err != nil {
+			if ctx.Err() != nil {
+				return ctx.Err()
+			}
+			return fmt.Errorf("wait for ICMP Echo Reply from %s: %w", target, err)
+		}
+		peerIP, ok := netip.AddrFromSlice(peer.(*net.IPAddr).IP)
+		if !ok || peerIP.Unmap() != target {
+			continue
+		}
+		parsed, err := icmp.ParseMessage(1, buffer[:count])
+		if err != nil || parsed.Type != ipv4.ICMPTypeEchoReply {
+			continue
+		}
+		echo, ok := parsed.Body.(*icmp.Echo)
+		if ok && echo.ID == id && echo.Seq == sequence {
+			return nil
+		}
+	}
+}