Is it possible to perform a completely offline installation

[nadebula1984]

When I used buster installer (text mode), I was always required to set up network, and always forced to download something during installation, even if I chose not to use a network mirror. My internet connection has been extremely poor, and said downloading (maybe only a few security patches) would cost hours or days to complete. (I really don't want to try to disable all NICs in BIOS.)

I'm looking forward to weekly builds for bullseye, so I could ask my friends who have usable internet connections to download the latest DVD images and post physical media to me, so I could use them as local mirror.

[L_V]

Yes it is possible. Already done it some time ago.
But you will need to download all the necessary packages somewhere (it doesn't not solve your poor connection).

You can add CDROM to sources.list: https://wiki.debian.org/DebianCustomCD

You can create your local repository
For investigation: dpkg-scanpackages command from dpkg-dev package.

This is not an out of the box operation.

[Soapm]

Also be sure to download your firmware and drivers packages so they they can be installed during installation.

[Bulkley]

Vendors of Debian installation media

[nadebula1984]

Thanks for replies. I have found a workaround, as mentioned above.

If I disable all NICs (wired and wireless) in BIOS (or temporarily physically remove them from my computer), the Debian Installer would complain that no NIC detected. The screen (text mode) turns red and shows a list of NIC drivers. However, I could select "no ethernet" and Continue. Luckily, the DVD image contains everything I need to perform an offline installation.

When the installation finishes, I power down my computer and re-enable the NICs in BIOS (or physically re-install them to my computer). After manually editting /etc/apt/sources.list, I could download security updates from local community's repository (rather than the default debian-security repository) at an acceptable speed.