fix(netwatch): detect IPv6 default routes on Linux

default_route() reads /proc/net/route first, which contains only IPv4
entries. When no IPv4 default route exists it returns Ok(None), and the
code treated any Ok result as final without falling through to the
netlink-based detection that handles both address families.

On a v6-only network this made default_route_interface permanently
None, which caused iroh's has_usable_network() to return false after
network switches. The 5-second debounce timeout would expire before
the QUIC stack learned about the change, leaving connections stuck on
dead paths.

The fix narrows the early return to Ok(Some(..)) so that Ok(None)
falls through to the netlink path.

Author: Frando

netwatch/src/interfaces/linux.rs

@@ -41,9 +41,11 @@ pub enum Error {
 }
 
 pub async fn default_route() -> Option<DefaultRouteDetails> {
-    let route = default_route_proc().await;
-    if let Ok(route) = route {
-        return route;
+    // /proc/net/route only contains IPv4 routes. If it finds one, return it.
+    // If it returns Ok(None) (no IPv4 default route) or Err (file unreadable),
+    // fall through to netlink which checks both IPv4 and IPv6.
+    if let Ok(Some(route)) = default_route_proc().await {
+        return Some(route);
     }
 
     #[cfg(target_os = "android")]

netwatch/tests/patchbay.rs

@@ -76,7 +76,6 @@ async fn default_route_v4_only() -> TestResult {
 /// `/proc/net/route` contains only IPv4 entries, so the default route must be
 /// detected through the netlink fallback.
 #[tokio::test]
-#[ignore = "default_route() does not fall through to netlink when /proc/net/route has no IPv4 default"]
 async fn default_route_v6_only() -> TestResult {
     let (default_route, have_v4, have_v6) =
         with_device(RouterPreset::IspV6, IpSupport::V6Only).await?;
@@ -112,7 +111,6 @@ async fn default_route_dual_stack() -> TestResult {
 /// iroh considers a network usable when `default_route_interface.is_some() &&
 /// (have_v4 || have_v6)`. On a v6-only network this must return true.
 #[tokio::test]
-#[ignore = "default_route() does not fall through to netlink when /proc/net/route has no IPv4 default"]
 async fn has_usable_network_v6_only() -> TestResult {
     let (default_route, have_v4, have_v6) =
         with_device(RouterPreset::IspV6, IpSupport::V6Only).await?;
@@ -128,7 +126,6 @@ async fn has_usable_network_v6_only() -> TestResult {
 /// After replugging from a v4 router to a v6 router, netwatch detects the new
 /// default route.
 #[tokio::test]
-#[ignore = "default_route() does not fall through to netlink when /proc/net/route has no IPv4 default"]
 async fn default_route_after_replug_v4_to_v6() -> TestResult {
     let lab = Lab::new().await?;
     let v4_router = lab