fix(nat): 修复 ICMP NAT 映射只增不减导致内存无界增长

每个 ping 包都会以 (对端地址, identifier, sequence) 为键新增一条
永久映射，运行时间越长内存占用越大。

修复：
- 收到 EchoReply 命中映射即移除该条目（一次 echo 交换完成）。
- 映射值携带创建时间，select 循环中加入 60s 周期的 GC 分支，
  清理超时（60s）未收到应答的残留条目。

测试：test_evict_expired 验证超时条目被清理、未超时条目保留。

vnt-core/src/nat/internal_nat/icmp_nat.rs

@@ -7,10 +7,18 @@ use pnet_packet::icmp::{IcmpPacket, IcmpTypes};
 use pnet_packet::ipv4::Ipv4Packet;
 use std::collections::HashMap;
 use std::net::{IpAddr, Ipv4Addr, SocketAddr, SocketAddrV4};
+use std::time::{Duration, Instant};
 use tcp_ip::IpStack;
 use tcp_ip::icmp::IcmpSocket;
 use tokio::net::UdpSocket;
 
+/// ICMP echo 映射超时：正常 ping 应答在秒级返回，超时条目视为无应答残留
+const ICMP_NAT_TIMEOUT: Duration = Duration::from_secs(60);
+const ICMP_NAT_GC_INTERVAL: Duration = Duration::from_secs(60);
+
+/// (对端地址, identifier, sequence) -> (内网客户端地址, 创建时间)
+type IcmpNatMap = HashMap<(Ipv4Addr, Identifier, SequenceNumber), (Ipv4Addr, Instant)>;
+
 pub async fn start_icmp_nat(
     task_group: &TaskGroup,
     ip_stack: &IpStack,
@@ -49,24 +57,38 @@ async fn task(
 ) -> anyhow::Result<()> {
     let mut buf1 = vec![0u8; 65536];
     let mut buf2 = vec![0u8; 65536];
-    let mut map = HashMap::new();
+    let mut map: IcmpNatMap = HashMap::new();
+    let mut gc_interval = tokio::time::interval(ICMP_NAT_GC_INTERVAL);
     loop {
         tokio::select! {
             rs = tokio_icmp_socket.recv(&mut buf1) => {
                 let len = rs?;
-                tokio_icmp_socket_recv(&buf1[..len],&inner_icmp_socket,&map,no_tun,&network).await?;
+                tokio_icmp_socket_recv(&buf1[..len],&inner_icmp_socket,&mut map,no_tun,&network).await?;
             }
             rs = inner_icmp_socket.recv_from_to(&mut buf2) => {
                 let (len,src,dst) = rs?;
                 inner_icmp_socket_recv(&buf2[..len],src,dst,&tokio_icmp_socket,&mut map,no_tun,&network).await?;
             }
+            _ = gc_interval.tick() => {
+                evict_expired(&mut map, Instant::now(), ICMP_NAT_TIMEOUT);
+            }
         }
     }
 }
+
+/// 清理超时未收到应答的映射条目，防止 map 无界增长
+fn evict_expired(map: &mut IcmpNatMap, now: Instant, timeout: Duration) {
+    let before = map.len();
+    map.retain(|_, (_, created)| now.duration_since(*created) < timeout);
+    let evicted = before - map.len();
+    if evicted > 0 {
+        log::debug!("icmp nat evicted {} expired entries", evicted);
+    }
+}
 async fn tokio_icmp_socket_recv(
     buf: &[u8],
     inner_icmp_socket: &IcmpSocket,
-    map: &HashMap<(Ipv4Addr, Identifier, SequenceNumber), Ipv4Addr>,
+    map: &mut IcmpNatMap,
     no_tun: bool,
     network: &SharedNetworkAddr,
 ) -> anyhow::Result<()> {
@@ -88,7 +110,8 @@ async fn tokio_icmp_socket_recv(
     let mut src = ipv4.get_source();
     let identifier = Identifier::new(u16::from_be_bytes([payload[0], payload[1]]));
     let sequence_number = SequenceNumber::new(u16::from_be_bytes([payload[2], payload[3]]));
-    let Some(dst) = map.get(&(src, identifier, sequence_number)) else {
+    // 收到应答即完成一次 echo 交换，移除映射，避免条目残留
+    let Some((dst, _)) = map.remove(&(src, identifier, sequence_number)) else {
         return Ok(());
     };
     if no_tun && src == Ipv4Addr::LOCALHOST {
@@ -96,7 +119,7 @@ async fn tokio_icmp_socket_recv(
     }
 
     inner_icmp_socket
-        .send_from_to(ipv4.payload(), src.into(), (*dst).into())
+        .send_from_to(ipv4.payload(), src.into(), dst.into())
         .await
         .context("sending ICMPv4 failed")?;
     Ok(())
@@ -106,7 +129,7 @@ async fn inner_icmp_socket_recv(
     src: IpAddr,
     dst: IpAddr,
     tokio_icmp_socket: &UdpSocket,
-    map: &mut HashMap<(Ipv4Addr, Identifier, SequenceNumber), Ipv4Addr>,
+    map: &mut IcmpNatMap,
     no_tun: bool,
     network: &SharedNetworkAddr,
 ) -> anyhow::Result<()> {
@@ -131,9 +154,49 @@ async fn inner_icmp_socket_recv(
 
     let identifier = Identifier::new(u16::from_be_bytes([payload[0], payload[1]]));
     let sequence_number = SequenceNumber::new(u16::from_be_bytes([payload[2], payload[3]]));
-    map.insert((dst, identifier, sequence_number), src);
+    map.insert((dst, identifier, sequence_number), (src, Instant::now()));
     tokio_icmp_socket
         .send_to(buf, SocketAddr::new(dst.into(), 0))
         .await?;
     Ok(())
 }
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn entry(
+        ip: &str,
+        id: u16,
+        seq: u16,
+        created: Instant,
+    ) -> ((Ipv4Addr, Identifier, SequenceNumber), (Ipv4Addr, Instant)) {
+        (
+            (
+                ip.parse().unwrap(),
+                Identifier::new(id),
+                SequenceNumber::new(seq),
+            ),
+            (Ipv4Addr::new(10, 0, 0, 1), created),
+        )
+    }
+
+    /// 超时未应答的条目必须被清理，未超时的保留，map 不会无界增长
+    #[test]
+    fn test_evict_expired() {
+        let now = Instant::now();
+        let mut map: IcmpNatMap = HashMap::new();
+        let (k_fresh, v_fresh) = entry("8.8.8.8", 1, 1, now);
+        let (k_stale, v_stale) =
+            entry("1.1.1.1", 2, 2, now - ICMP_NAT_TIMEOUT - Duration::from_secs(1));
+        map.insert(k_fresh, v_fresh);
+        map.insert(k_stale, v_stale);
+
+        evict_expired(&mut map, now, ICMP_NAT_TIMEOUT);
+
+        assert_eq!(map.len(), 1);
+        assert!(map.contains_key(&k_fresh));
+        assert!(!map.contains_key(&k_stale));
+    }
+}