RTF_MASK
{ RTF_MASK, 'm' }, /* Mask Present -- for routing messages only */
#define PRESERVED_RTF (RTF_UP | RTF_GATEWAY | RTF_HOST | RTF_DONE | RTF_MASK)
rm.m_rtm.rtm_flags |= RTF_MASK;
#ifdef RTF_MASK
RTFLAG("m", RTF_MASK);