如何从 USB 而不是 cdrom 安装 pppoe 包？

我的 debian 是从 USB 磁盘安装的——/dev/sdb。
从 cd 安装 pppoe 包很简单。

sudo vi  /etc/apt/sources.list 
deb cdrom:[Official Debian GNU/Linux Live 9.2.0 lxde 2017-10-07T13:15]/ stretch main
sudo apt-get install  pppoe

没有cdrom，我的电脑是usb的。
我想从 USB 磁盘安装 pppoe 包。

sudo  mount  /dev/sdb   /home/back
ls  /home/back/
boot  d-i  dists  isolinux  live  pool

将 USB 磁盘分配为源。

sduo vi  /etc/apt/sources.list
deb file:/home/back/ stretch main contirb

用U盘安装。

sudo apt-get install  pppoe
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  ebtables ipset libipset3 python3-decorator python3-selinux python3-slip python3-slip-dbus
Use 'sudo apt autoremove' to remove them.
The following additional packages will be installed:
  ppp
The following NEW packages will be installed:
  ppp pppoe
0 upgraded, 2 newly installed, 0 to remove and 0 not upgraded.
Need to get 424 kB of archives.
After this operation, 1,182 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
Err:1 http://deb.debian.org/debian stretch/main amd64 ppp amd64 2.4.7-1+4
  Could not resolve 'deb.debian.org'
Err:2 http://deb.debian.org/debian stretch/main amd64 pppoe amd64 3.12-1.1
  Could not resolve 'deb.debian.org'
E: Failed to fetch http://deb.debian.org/debian/pool/main/p/ppp/ppp_2.4.7-1+4_amd64.deb  Could not resolve 'deb.debian.org'
E: Failed to fetch http://deb.debian.org/debian/pool/main/r/rp-pppoe/pppoe_3.12-1.1_amd64.deb  Could not resolve 'deb.debian.org'
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?