feat(rust-server, rust-client, ts-interfaces): add configurable packet forwarding with TUN and userspace NAT modes

rust/src/tunnel.rs

@@ -1,5 +1,5 @@
 use anyhow::Result;
-use std::net::Ipv4Addr;
+use std::net::{IpAddr, Ipv4Addr, Ipv6Addr};
 use tracing::info;
 
 /// Configuration for creating a TUN device.
@@ -80,6 +80,26 @@ pub fn check_tun_mtu(packet: &[u8], mtu_config: &crate::mtu::MtuConfig) -> TunMt
     }
 }
 
+/// Extract destination IP from a raw IP packet header.
+pub fn extract_dst_ip(packet: &[u8]) -> Option<IpAddr> {
+    if packet.is_empty() {
+        return None;
+    }
+    let version = packet[0] >> 4;
+    match version {
+        4 if packet.len() >= 20 => {
+            let dst = Ipv4Addr::new(packet[16], packet[17], packet[18], packet[19]);
+            Some(IpAddr::V4(dst))
+        }
+        6 if packet.len() >= 40 => {
+            let mut octets = [0u8; 16];
+            octets.copy_from_slice(&packet[24..40]);
+            Some(IpAddr::V6(Ipv6Addr::from(octets)))
+        }
+        _ => None,
+    }
+}
+
 /// Remove a route.
 pub async fn remove_route(subnet: &str, device_name: &str) -> Result<()> {
     let output = tokio::process::Command::new("ip")