Mirror is configured to provide rsync access for the Gentoo portage system (used when doing emerge sync). To use mirror for emerge sync and for downloading the associated distfiles add the following two lines to /etc/make.conf (most useful if done before running 'emerge sync' and 'bootstrap.sh' during installation):

    GENTOO_MIRRORS="ftp://mirror.cc.columbia.edu/pub/linux/gentoo/distro"
    SYNC="rsync://mirror.cc.columbia.edu/gentoo-portage"