getif
ifr = getif(s, &dst);
ifr = getif(s, &clnt_addr.sin_addr);
ifr = getif(s, &(bp->bp_yiaddr));
ifr = getif(s, &sin_server.sin_addr);
extern struct ifreq *getif(int, struct in_addr *);
ifr = getif(s, dap);