How to configure a local source on Debian

We encounter a worst problem that we can't connect to the Internet when we want to install curl and other useful software in our server. In our campus, every machine must have an unique account to sign in, and we don't want our own account to be used on server. So the global sources such as Tsinghua's Tuna can't be used to update and install software, and a local source seems a good idea to deal with this problem.

Firstly, we download .iso format source files of Debian 10 from this website:

https://cdimage.debian.org/cdimage/archive/10.11.0/amd64/iso-dvd/

Then, create three folders (/iso1, /iso2, /iso3) and mount these .iso files to folders:

mount -o loop debian-10.11.0-amd64-DVD-1.iso /iso1
mount -o loop debian-10.11.0-amd64-DVD-2.iso /iso2
mount -o loop debian-10.11.0-amd64-DVD-2.iso /iso2

Besides, backup the sources.list file in folder /etc/apt/ and create a new sources.list file with these lines:

deb [trusted=yes] file:/iso1 buster contrib main
deb [trusted=yes] file:/iso2 buster contrib main
deb [trusted=yes] file:/iso3 buster contrib main

Finally, you can install essential tools such net-tools, curl and so on by these commands (root):

apt update
apt install curl
apt install net-tools