Update docs

- Update port forwarding instructions in advanced usage.
  Ref: #1467

docs/advanced-usage-zh.md

@@ -221,22 +221,22 @@ sh vpn.sh
 
 **警告：** 端口转发会将 VPN 客户端上的端口暴露给整个因特网，这可能会带来**安全风险**！**不建议**这样做，除非你的用例需要它。
 
-**注：** 为 VPN 客户端分配的内网 IP 是动态的，而且客户端设备上的防火墙可能会阻止转发的流量。如果要将静态 IP 分配给 VPN 客户端，请参见上一节。要找到为特定的客户端分配的 IP，可以查看该 VPN 客户端上的连接状态。
+**注：** 为 VPN 客户端分配的内网 IP 是动态的，而且客户端设备上的防火墙可能会阻止转发的流量。如果要将静态 IP 分配给 VPN 客户端，请参见 [VPN 内网 IP 和流量](#vpn-内网-ip-和流量)。要找到为特定的客户端分配的 IP，可以查看该 VPN 客户端上的连接状态。
 
 示例 1：将 VPN 服务器上的 TCP 端口 443 转发到位于 `192.168.42.10` 的 IPsec/L2TP 客户端。
 ```
 # 获取默认网络接口名称
 netif=$(ip -4 route list 0/0 | grep -m 1 -Po '(?<=dev )(\S+)')
 iptables -I FORWARD 2 -i "$netif" -o ppp+ -p tcp --dport 443 -j ACCEPT
-iptables -t nat -A PREROUTING -p tcp --dport 443 -j DNAT --to 192.168.42.10
+iptables -t nat -A PREROUTING -i "$netif" -p tcp --dport 443 -j DNAT --to 192.168.42.10
 ```
 
 示例 2：将 VPN 服务器上的 UDP 端口 123 转发到位于 `192.168.43.10` 的 IKEv2（或 IPsec/XAuth）客户端。
 ```
 # 获取默认网络接口名称
 netif=$(ip -4 route list 0/0 | grep -m 1 -Po '(?<=dev )(\S+)')
 iptables -I FORWARD 2 -i "$netif" -d 192.168.43.0/24 -p udp --dport 123 -j ACCEPT
-iptables -t nat -A PREROUTING -p udp --dport 123 -j DNAT --to 192.168.43.10
+iptables -t nat -A PREROUTING -i "$netif" ! -s 192.168.43.0/24 -p udp --dport 123 -j DNAT --to 192.168.43.10
 ```
 
 如果你想要这些规则在重启后仍然有效，可以将这些命令添加到 `/etc/rc.local`。要删除添加的 IPTables 规则，请再次运行这些命令，但是将 `-I FORWARD 2` 替换为 `-D FORWARD`，并且将 `-A PREROUTING` 替换为 `-D PREROUTING`。

docs/advanced-usage.md

@@ -221,22 +221,22 @@ In certain circumstances, you may want to forward port(s) on the VPN server to a
 
 **Warning:** Port forwarding will expose port(s) on the VPN client to the entire Internet, which could be a **security risk**! This is NOT recommended, unless your use case requires it.
 
-**Note:** The internal VPN IPs assigned to VPN clients are dynamic, and firewalls on client devices may block forwarded traffic. To assign static IPs to VPN clients, refer to the previous section. To check which IP is assigned to a client, view the connection status on the VPN client.
+**Note:** The internal VPN IPs assigned to VPN clients are dynamic, and firewalls on client devices may block forwarded traffic. To assign static IPs to VPN clients, see [Internal VPN IPs and traffic](#internal-vpn-ips-and-traffic). To check which IP is assigned to a client, view the connection status on the VPN client.
 
 Example 1: Forward TCP port 443 on the VPN server to the IPsec/L2TP client at `192.168.42.10`.
 ```
 # Get default network interface name
 netif=$(ip -4 route list 0/0 | grep -m 1 -Po '(?<=dev )(\S+)')
 iptables -I FORWARD 2 -i "$netif" -o ppp+ -p tcp --dport 443 -j ACCEPT
-iptables -t nat -A PREROUTING -p tcp --dport 443 -j DNAT --to 192.168.42.10
+iptables -t nat -A PREROUTING -i "$netif" -p tcp --dport 443 -j DNAT --to 192.168.42.10
 ```
 
 Example 2: Forward UDP port 123 on the VPN server to the IKEv2 (or IPsec/XAuth) client at `192.168.43.10`.
 ```
 # Get default network interface name
 netif=$(ip -4 route list 0/0 | grep -m 1 -Po '(?<=dev )(\S+)')
 iptables -I FORWARD 2 -i "$netif" -d 192.168.43.0/24 -p udp --dport 123 -j ACCEPT
-iptables -t nat -A PREROUTING -p udp --dport 123 -j DNAT --to 192.168.43.10
+iptables -t nat -A PREROUTING -i "$netif" ! -s 192.168.43.0/24 -p udp --dport 123 -j DNAT --to 192.168.43.10
 ```
 
 If you want the rules to persist after reboot, you may add these commands to `/etc/rc.local`. To remove the added IPTables rules, run the commands again, but replace `-I FORWARD 2` with `-D FORWARD`, and replace `-A PREROUTING` with `-D PREROUTING`.