f-28: add GetDefaultGateway to netif

Signed-off-by: GnomeZworc <nicolas.boufidjeline@g3e.fr>

internal/netif/gateway_linux.go

@@ -0,0 +1,23 @@
+//go:build linux
+
+package netif
+
+import (
+	"fmt"
+	"net"
+
+	"github.com/vishvananda/netlink"
+)
+
+func GetDefaultGateway() (net.IP, error) {
+	routes, err := netlink.RouteList(nil, netlink.FAMILY_V4)
+	if err != nil {
+		return nil, fmt.Errorf("list routes: %w", err)
+	}
+	for _, r := range routes {
+		if r.Dst == nil && r.Gw != nil {
+			return r.Gw, nil
+		}
+	}
+	return nil, fmt.Errorf("no default gateway found")
+}