net/netmon: ignore NetBird interface on Linux

Windows and macOS are not covered by this change, as neither have safely
distinct names to make it easy to do so. This covers the requested case
on Linux.

Updates #18824

Signed-off-by: James Tucker <james@tailscale.com>

net/netmon/state.go

@@ -41,7 +41,12 @@ func isProblematicInterface(nif *net.Interface) bool {
 	// DoS each other by doing traffic amplification, both of them
 	// preferring/trying to use each other for transport. See:
 	// https://github.com/tailscale/tailscale/issues/1208
-	if strings.HasPrefix(name, "zt") || (runtime.GOOS == "windows" && strings.Contains(name, "ZeroTier")) {
+	// TODO(https://github.com/tailscale/tailscale/issues/18824): maybe exclude
+	// "WireGuard tunnel 0" as well on Windows (NetBird), but the name seems too
+	// generic where there is not a platform standard (on Linux wt0 is at least
+	// explicitly different from the WireGuard conventional default of wg0).
+	if strings.HasPrefix(name, "zt") || name == "wt0" /* NetBird */ ||
+		(runtime.GOOS == "windows" && strings.Contains(name, "ZeroTier")) {
 		return true
 	}
 	return false