Fix MTU detection below min IPv6 value for windows

talpid-windows/src/net.rs

@@ -333,25 +333,12 @@ pub fn add_ip_address_for_interface(luid: NET_LUID_LH, address: IpAddr) -> Resul
 }
 
 /// Sets MTU on the specified network interface identified by `luid`.
-pub fn set_mtu(luid: NET_LUID_LH, mtu: u32, use_ipv6: bool) -> io::Result<()> {
-    let ip_families: &[AddressFamily] = if use_ipv6 {
-        &[AddressFamily::Ipv4, AddressFamily::Ipv6]
-    } else {
-        &[AddressFamily::Ipv4]
-    };
-    for family in ip_families {
-        let mut row = match get_ip_interface_entry(*family, &luid) {
-            Ok(row) => row,
-            Err(error) if error.raw_os_error() == Some(ERROR_NOT_FOUND as i32) => continue,
-            Err(error) => return Err(error),
-        };
-
-        row.NlMtu = mtu;
+pub fn set_mtu(mtu: u32, luid: NET_LUID_LH, ip_family: AddressFamily) -> io::Result<()> {
+    let mut row = get_ip_interface_entry(ip_family, &luid)?;
 
-        set_ip_interface_entry(&mut row)?;
-    }
+    row.NlMtu = mtu;
 
-    Ok(())
+    set_ip_interface_entry(&mut row)
 }
 
 /// Returns the unicast IP address table. If `family` is `None`, then addresses for all families are

talpid-wireguard/src/mtu_detection.rs

@@ -57,15 +57,31 @@ pub async fn automatic_mtu_correction(
         #[cfg(any(target_os = "linux", target_os = "macos"))]
         crate::unix::set_mtu(&iface_name, verified_mtu).map_err(Error::SetMtu)?;
         #[cfg(windows)]
-        talpid_windows::net::luid_from_alias(iface_name)
-            .and_then(|luid| talpid_windows::net::set_mtu(luid, verified_mtu as u32, ipv6))
-            .map_err(Error::SetMtu)?;
+        set_mtu_windows(verified_mtu, iface_name, ipv6).map_err(Error::SetMtu)?;
     } else {
         log::debug!("MTU {verified_mtu} verified to not drop packets");
     };
     Ok(())
 }
 
+#[cfg(windows)]
+fn set_mtu_windows(verified_mtu: u16, iface_name: String, ipv6: bool) -> io::Result<()> {
+    use talpid_windows::net::{set_mtu, AddressFamily};
+
+    let luid = talpid_windows::net::luid_from_alias(iface_name)?;
+    set_mtu(u32::from(verified_mtu), luid, AddressFamily::Ipv4)?;
+    if ipv6 {
+        let clamped_mtu = if verified_mtu < talpid_tunnel::MIN_IPV6_MTU {
+            log::warn!("Cannot set MTU to {verified_mtu} for IPv6, setting to the minimum value 1280 instead");
+            talpid_tunnel::MIN_IPV6_MTU
+        } else {
+            verified_mtu
+        };
+        set_mtu(u32::from(clamped_mtu), luid, AddressFamily::Ipv6)?;
+    }
+    Ok(())
+}
+
 /// Detects the maximum MTU that does not cause dropped packets.
 ///
 /// The detection works by sending evenly spread out range of pings between 576 and the given