/etc/rc.conf.local:
ftpd_flags="-DllUSA"
inetd=NO
echo /usr/bin/false >> /etc/shells
adduser -batch ftp -shell /usr/bin/false
chmod 555 /home/ftp
chown root /home/ftp
/etc/ftpchroot:
ftp
http://openbsd.org/faq/faq10.html#AnonFTP