What?

You may want to use a Debian mirror different from the one live-build uses by default.

Why?

  1. You want to use a mirror that is faster for you, e.g. one that is in the country you live in, or a local Debian mirror.
  2. You have configured squid-deb-proxy to only allow access to a given set of domains.

How?

You need to set options in live-build's configuration file (/etc/live/build.conf), e.g.:

LB_MIRROR_BOOTSTRAP="http://ftp.us.debian.org/debian/"
LB_MIRROR_BOOTSTRAP_SECURITY="http://security.debian.org/"

LB_MIRROR_CHROOT="http://ftp.us.debian.org/debian/"
LB_MIRROR_CHROOT_SECURITY="http://security.debian.org/"

LB_MIRROR_BINARY="http://ftp.us.debian.org/debian/"
LB_MIRROR_BINARY_SECURITY="http://security.debian.org/"

See the lb_config(1) manpage for details.