#busybox --help BusyBox v1.19.3 (2011-12-13 00:47:19 NOVT) multi-call binary. Copyright (C) 1998-2011 Erik Andersen, Rob Landley, Denys Vlasenko and others. Licensed under GPLv2. See source distribution for full notice. "ip" is symlink to busybox binary #ip link set dev eth0 promisc on ip: either "dev" is duplicate, or "promisc" is garbage expected: set promisc mode on interface. iputils version of "ip" works this way.
Yes, so far busybox's ip does not support "promisc".
Fixed in git.