用ports安装如何指定服务器

CODE:

MASTER_SITE_BACKUP?= 
ftp://202.113.12.9/pub/FreeBSD/ports/distfiles/${DIST_SUBDIR}/
ftp://ftp.freebsd.org.cn/pub/FreeBSD/ports/distfiles/${DIST_SUBDIR}/
ftp://ftp.freebsdchina.org/pub/FreeBSD/ports/distfiles/${DIST_SUBDIR}/
MASTER_SITE_OVERRIDE?= ${MASTER_SITE_BACKUP}