2 files changed, 74 insertions, 0 deletions
diff --git a/talpid-core/src/firewall/windows/mod.rs b/talpid-core/src/firewall/windows/mod.rs
index 01c6dd4c53..4068cd0970 100644
--- a/talpid-core/src/firewall/windows/mod.rs
+++ b/talpid-core/src/firewall/windows/mod.rs
@@ -14,6 +14,7 @@ use self::widestring::WideCString;
 #[macro_use]
 mod ffi;
 mod dns;
+mod route;
 mod system_state;
 
 use self::dns::WinDns;
@@ -48,6 +49,7 @@ error_chain!{
 
     links {
         WinDns(dns::Error, dns::ErrorKind) #[doc = "WinDNS failure"];
+        WinRoute(route::Error, route::ErrorKind) #[doc = "Failure to modify system routing metrics"];
     }
 }
 
@@ -157,6 +159,15 @@ impl WindowsFirewall {
         };
 
         self.dns.set_dns(&vec![tunnel_metadata.gateway.into()])?;
+
+        let metrics_set = route::ensure_top_metric_for_interface(&tunnel_metadata.interface)?;
+        if metrics_set {
+            debug!("Network interface metrics were changed");
+        } else {
+            debug!("Network interface metrics were not changed");
+        }
+
+
         unsafe {
             WinFw_ApplyPolicyConnected(
                 winfw_settings,
diff --git a/talpid-core/src/firewall/windows/route.rs b/talpid-core/src/firewall/windows/route.rs
new file mode 100644
index 0000000000..6ecf336956
--- /dev/null
+++ b/talpid-core/src/firewall/windows/route.rs
@@ -0,0 +1,63 @@
+use super::ffi;
+use super::widestring::WideCString;
+use libc;
+use std::ptr;
+
+error_chain!{
+    errors{
+        /// Failure to set metrics of network interfaces
+        MetricApplication{
+            description("Failed to set the metrics for a network interface")
+        }
+        InvalidInterfaceAlias{
+            description("Supplied interface alias is invalid")
+        }
+    }
+}
+
+/// Returns true if metrics were changed, false otherwise
+pub fn ensure_top_metric_for_interface(interface_alias: &str) -> Result<bool> {
+    let interface_alias_ws =
+        WideCString::from_str(interface_alias).chain_err(|| ErrorKind::InvalidInterfaceAlias)?;
+    unsafe {
+        WinRoute_EnsureTopMetric(
+            interface_alias_ws.as_wide_c_str().as_ptr(),
+            Some(ffi::error_sink),
+            ptr::null_mut(),
+        ).into()
+    }
+}
+
+// Allowing dead code here as this type should only ever be constructed by an
+// FFI function.
+#[allow(dead_code)]
+#[repr(u32)]
+enum MetricResult {
+    MetricsUnchanged = 0u32,
+    MetricsChanged = 1u32,
+    Failure = 2u32,
+    UnexpectedValue,
+}
+
+impl Into<Result<bool>> for MetricResult {
+    fn into(self) -> Result<bool> {
+        match self {
+            MetricResult::MetricsUnchanged => Ok(false),
+            MetricResult::MetricsChanged => Ok(true),
+            MetricResult::Failure => Err(Error::from(ErrorKind::MetricApplication)),
+            MetricResult::UnexpectedValue => {
+                error!("Unexpected return code from WinRoute_EnsureTopMetric");
+                Err(Error::from(ErrorKind::MetricApplication))
+            }
+        }
+    }
+}
+
+extern "system" {
+    #[link_name(WinRoute_EnsureTopMetric)]
+    fn WinRoute_EnsureTopMetric(
+        tunnel_interface_alias: *const libc::wchar_t,
+        sink: Option<ffi::ErrorSink>,
+        sink_context: *mut libc::c_void,
+    ) -> MetricResult;
+}