fix(tun): TUN routing failed on every connect, freezing under load

A user's log showed the same two failures on all 9 connects, 198 route errors
total:
  Added 0 bypass routes via 192.168.88.1 (if_index=11)
  Could not find ostp_tun index in routing table after 15s — traffic will NOT be captured

Two independent bugs in the Windows route layer:

1. The TUN interface index was looked up by matching FriendlyName == "ostp_tun"
   through GetAdaptersAddresses. WinTun does not set the FriendlyName to the
   adapter name, so the match never succeeded — every connect burned the full
   15s window and gave up. The tun crate hands the real index back directly via
   AbstractDevice::tun_index() (WinTun's own adapter index), which is instant and
   correct; the name lookup remains only as a fallback.

2. Every route add — the server-IP bypass and the TUN default route alike — went
   through the legacy CreateIpForwardEntry, which failed with error 160
   (ERROR_BAD_ARGUMENTS) on this machine for all of them. With the server-IP
   bypass never installed, the server's own packets were routed INTO the tunnel:
   a loop that stalls the link for seconds under load (the reported "VPN drops
   ~8s into a game" — the tunnel never actually disconnected, it froze; the log
   showed gap recovery skipping up to 402 frames with no packet loss on the
   wire). add_ipv4_route now shells to route.exe, which resolves the interface
   and validates the gateway itself and is already what the teardown path uses;
   its command form was verified to be accepted (fails only on elevation, not
   syntax). CREATE_NO_WINDOW keeps it from flashing a console per route.

Cannot be verified without the user's elevated TUN environment; the next log
will read "Added N bypass routes" and "Default route via TUN ... added" instead
of the failures.

ostp-tun/src/windows.rs

@@ -99,18 +99,34 @@ pub async fn create(opts: OstpTunOptions) -> Result<OstpTunInterface> {
 
     let current_exe = std::env::current_exe()?.to_string_lossy().into_owned();
 
-    // A freshly created WinTun adapter can take several seconds to appear in
-    // GetAdaptersAddresses (it only shows up once it has an operational IPv4
-    // binding). The default route via the TUN is what actually captures
-    // traffic, so this lookup is critical — give it a generous window (~15s).
-    let mut tun_index = None;
-    for _ in 0..75 {
-        if let Some(idx) = windows_route::sys::get_interface_index("ostp_tun") {
-            tun_index = Some(idx);
-            break;
+    // Take the interface index straight from the adapter WinTun just created,
+    // via the tun crate. The old code looked it up by FriendlyName == "ostp_tun"
+    // through GetAdaptersAddresses — but WinTun does NOT set the FriendlyName to
+    // the adapter name, so that match never succeeded: on every single connect
+    // it spun the full 15s and then gave up with "traffic will NOT be captured",
+    // leaving the default route (and, above, the server-IP bypass) uninstalled.
+    // get_adapter_index() is instant and correct.
+    use tun::AbstractDevice;
+    let tun_index = match dev.tun_index() {
+        Ok(idx) if idx > 0 => Some(idx as u32),
+        Ok(idx) => {
+            tracing::error!("WinTun reported a non-positive interface index ({idx})");
+            None
         }
-        tokio::time::sleep(std::time::Duration::from_millis(200)).await;
-    }
+        Err(e) => {
+            // Fall back to the old name lookup rather than fail outright.
+            tracing::warn!("Could not read TUN index from the adapter ({e}); falling back to name lookup");
+            let mut idx = None;
+            for _ in 0..75 {
+                if let Some(i) = windows_route::sys::get_interface_index("ostp_tun") {
+                    idx = Some(i);
+                    break;
+                }
+                tokio::time::sleep(std::time::Duration::from_millis(200)).await;
+            }
+            idx
+        }
+    };
 
     if let Some(idx) = tun_index {
         match windows_route::sys::add_ipv4_route(

ostp-tun/src/windows_route.rs

@@ -17,7 +17,7 @@ pub mod sys {
     use winapi::shared::minwindef::{DWORD, ULONG};
     use winapi::shared::winerror::{ERROR_INSUFFICIENT_BUFFER, NO_ERROR};
     use winapi::um::iphlpapi::{
-        CreateIpForwardEntry, DeleteIpForwardEntry, GetAdaptersAddresses, GetIpForwardTable,
+        DeleteIpForwardEntry, GetAdaptersAddresses, GetIpForwardTable,
     };
     use winapi::um::iptypes::{
         GAA_FLAG_SKIP_ANYCAST, GAA_FLAG_SKIP_DNS_SERVER, GAA_FLAG_SKIP_MULTICAST, IP_ADAPTER_ADDRESSES,
@@ -88,20 +88,47 @@ pub mod sys {
         if_index: u32,
         metric: u32,
     ) -> Result<(), String> {
-        let mut row: MIB_IPFORWARDROW = unsafe { mem::zeroed() };
-        row.dwForwardDest = ipv4_to_dword(dest);
-        row.dwForwardMask = ipv4_to_dword(mask);
-        row.dwForwardNextHop = ipv4_to_dword(nexthop);
-        row.dwForwardIfIndex = if_index;
-        row.ForwardType = if nexthop == Ipv4Addr::UNSPECIFIED || dest == nexthop { 3 } else { 4 };
-        row.ForwardProto = 3; // MIB_IPPROTO_NETMGMT
-        row.dwForwardMetric1 = metric;
+        // Installed through route.exe rather than CreateIpForwardEntry.
+        //
+        // The legacy CreateIpForwardEntry API was failing here with error 160
+        // (ERROR_BAD_ARGUMENTS) on every single route — server-IP bypass and TUN
+        // default route alike — which left the server IP routed INTO the tunnel
+        // (a loop that froze the link for seconds under load) and the default
+        // route uninstalled. route.exe resolves the interface and validates the
+        // gateway itself, and is what the teardown path already uses, so it
+        // succeeds where the hand-built MIB_IPFORWARDROW did not.
+        use std::os::windows::process::CommandExt;
+        use std::process::Command;
+        const CREATE_NO_WINDOW: u32 = 0x0800_0000;
 
-        let ret = unsafe { CreateIpForwardEntry(&mut row) };
-        if ret == NO_ERROR {
+        // route add <dest> mask <mask> <gateway> metric <m> if <ifindex>
+        let out = Command::new("route")
+            .args([
+                "add",
+                &dest.to_string(),
+                "mask",
+                &mask.to_string(),
+                &nexthop.to_string(),
+                "metric",
+                &metric.to_string(),
+                "if",
+                &if_index.to_string(),
+            ])
+            .creation_flags(CREATE_NO_WINDOW)
+            .output()
+            .map_err(|e| format!("could not run route.exe: {e}"))?;
+
+        if out.status.success() {
             Ok(())
         } else {
-            Err(format!("CreateIpForwardEntry failed: {}", ret))
+            // route.exe prints its diagnostics to stdout, not stderr.
+            let msg = String::from_utf8_lossy(&out.stdout);
+            let msg = msg.trim();
+            Err(format!(
+                "route add failed (exit {:?}): {}",
+                out.status.code(),
+                if msg.is_empty() { "no output" } else { msg }
+            ))
         }
     }