Add per-node ping monitoring with unreachable error tracking

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ping.go

@@ -0,0 +1,89 @@
+package tendrils
+
+import (
+	"net"
+	"time"
+
+	"golang.org/x/net/icmp"
+	"golang.org/x/net/ipv4"
+)
+
+func (t *Tendrils) pingNode(node *Node) {
+	t.nodes.mu.RLock()
+	var ips []string
+	for _, iface := range node.Interfaces {
+		for ipStr := range iface.IPs {
+			ip := net.ParseIP(ipStr)
+			if ip != nil && ip.To4() != nil {
+				ips = append(ips, ipStr)
+			}
+		}
+	}
+	t.nodes.mu.RUnlock()
+
+	if len(ips) == 0 {
+		return
+	}
+
+	for _, ipStr := range ips {
+		reachable := t.pingIP(ipStr)
+		if reachable {
+			t.errors.ClearUnreachable(node, ipStr)
+		} else {
+			t.errors.SetUnreachable(node, ipStr)
+		}
+	}
+}
+
+func (t *Tendrils) pingIP(ipStr string) bool {
+	conn, err := icmp.ListenPacket("ip4:icmp", "0.0.0.0")
+	if err != nil {
+		return false
+	}
+	defer conn.Close()
+
+	conn.SetDeadline(time.Now().Add(500 * time.Millisecond))
+
+	ip := net.ParseIP(ipStr)
+	seq := uint16(time.Now().UnixNano() & 0xFFFF)
+
+	msg := icmp.Message{
+		Type: ipv4.ICMPTypeEcho,
+		Code: 0,
+		Body: &icmp.Echo{
+			ID:   int(seq),
+			Seq:  1,
+			Data: []byte("tendrils"),
+		},
+	}
+	msgBytes, err := msg.Marshal(nil)
+	if err != nil {
+		return false
+	}
+
+	_, err = conn.WriteTo(msgBytes, &net.IPAddr{IP: ip})
+	if err != nil {
+		return false
+	}
+
+	buf := make([]byte, 1500)
+	for {
+		n, peer, err := conn.ReadFrom(buf)
+		if err != nil {
+			return false
+		}
+
+		parsed, err := icmp.ParseMessage(1, buf[:n])
+		if err != nil {
+			continue
+		}
+
+		if parsed.Type == ipv4.ICMPTypeEchoReply {
+			if ipAddr, ok := peer.(*net.IPAddr); ok {
+				if ipAddr.IP.String() == ipStr {
+					return true
+				}
+			}
+		}
+	}
+}