Use a receive buffer of at least 1500 bytes.

Babel nodes are required to be able to receive 1280 byte packets.

babel.c

@@ -499,13 +499,11 @@ main(int argc, char **argv)
     }
 
     init_signals();
-    check_networks();
-    if(receive_buffer == NULL) {
-        fprintf(stderr, "Warning: couldn't find any operational interfaces.\n");
     resize_receive_buffer(1500);
+    check_networks();
     if(receive_buffer == NULL)
         goto fail;
-    }
+
     check_xroutes(0);
     kernel_routes_changed = 0;
     kernel_link_changed = 0;