用ports安装如何指定服务器

CODE:

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