How do I set mirrors in /etc/apt/sources.list with a Debian preseed file?

Question

I'm building a custom ISO image of Debian 10 stable ("buster") using preseeding, and my custom preseed.cfg file works perfectly except for one thing: I'd like it to configure /etc/apt/sources.list with my chosen repositories so that I don't have to do it manually every time I install a new system.

The goal is an /etc/apt/sources.list that looks like this (https is a necessity here):

deb https://deb.debian.org/debian buster main contrib non-free
deb-src https://deb.debian.org/debian buster main contrib non-free

deb https://deb.debian.org/debian-security/ buster/updates main contrib non-free
deb-src https://deb.debian.org/debian-security/ buster/updates main contrib non-free

deb https://deb.debian.org/debian buster-updates main contrib non-free
deb-src https://deb.debian.org/debian buster-updates main contrib non-free

My preseed.cfg file looks like this:

#### Contents of the preconfiguration file (for buster)
### Localization
# Preseeding only locale sets language, country and locale.
d-i debian-installer/locale string en_US

# Keyboard selection.
d-i keyboard-configuration/xkb-keymap select us
# d-i keyboard-configuration/toggle select No toggling

### Network configuration
d-i netcfg/choose_interface select auto
d-i netcfg/get_hostname string unassigned-hostname
d-i netcfg/get_domain string unassigned-domain
d-i netcfg/hostname string vienna1-preseed

d-i netcfg/wireless_wep string

### Mirror settings
d-i mirror/country string manual
d-i mirror/http/hostname string http.us.debian.org
d-i mirror/http/directory string /debian
d-i mirror/http/proxy string

### Account setup
d-i passwd/root-login boolean false

# To create a normal user account.
d-i passwd/user-fullname string theusername
d-i passwd/username string theusername
d-i passwd/user-password-crypted password $6$qVk198UWGPxpW$tzMYxQyiOrI4ClDJdDGALsyYq1j1IbXWbpem3JevFT9Krqdmt4wKdvtiY8ry3PRh277V6GHzSKP3zSI7jt04Y/

### Clock and time zone setup
d-i clock-setup/utc boolean true
d-i time/zone string US/Eastern

d-i clock-setup/ntp boolean true

### Partitioning
d-i partman-auto/method string regular
d-i partman-lvm/device_remove_lvm boolean true
d-i partman-md/device_remove_md boolean true
d-i partman-lvm/confirm boolean true
d-i partman-lvm/confirm_nooverwrite boolean true
d-i partman-auto/choose_recipe select atomic

d-i partman-partitioning/confirm_write_new_label boolean true
d-i partman/choose_partition select finish
d-i partman/confirm boolean true
d-i partman/confirm_nooverwrite boolean true

d-i partman-md/confirm boolean true
d-i partman-partitioning/confirm_write_new_label boolean true
d-i partman/choose_partition select finish
d-i partman/confirm boolean true
d-i partman/confirm_nooverwrite boolean true


# https://serverfault.com/a/622818
d-i apt-setup/use_mirror boolean false
d-i apt-setup/cdrom/set-first boolean false
d-i apt-setup/cdrom/set-next boolean false
d-i apt-setup/cdrom/set-failed boolean false

### Base system installation
# Configure APT to not install recommended packages by default. Use of this
# option can result in an incomplete system and should only be used by very
# experienced users.
#d-i base-installer/install-recommends boolean false

### Apt setup
# You can choose to install non-free and contrib software.
d-i apt-setup/non-free boolean true
d-i apt-setup/contrib boolean true

### Package selection
tasksel tasksel/first multiselect standard
d-i pkgsel/upgrade select full-upgrade
popularity-contest popularity-contest/participate boolean false

### Boot loader installation
d-i grub-installer/only_debian boolean true
d-i grub-installer/with_other_os boolean true
d-i grub-installer/bootdev  string default

d-i finish-install/reboot_in_progress note
d-i debian-installer/exit/poweroff boolean true

Answer 1

The d-i preseed/late_command string commands in preseed.cfg do what you want. I added a custom sources.list file to the iso and then copy that over after the installation completes. Add these commands at the end of your preseed.cfg.

d-i preseed/late_command string \
cp sources.list /target/etc/apt/sources.list; \
in-target apt-get update; \
in-target apt-get install -y git;

The last two commands demonstrate updating the list of packages and installing a package that isn't included on the cd (git).

Here's how I built the iso image with the new preseed.cfg and the new sources.list:

#!/usr/bin/env bash

# Install dependencies
# sudo apt install isolinux syslinux-utils xorriso

cd ~
mkdir iso
xorriso -osirrox on -indev debian-10.2.0-amd64-netinst.iso -extract / iso/

chmod +w -R iso/install.amd/
gunzip iso/install.amd/initrd.gz
echo preseed.cfg | cpio -H newc -o -A -F iso/install.amd/initrd
echo sources.list | cpio -H newc -o -A -F iso/install.amd/initrd
gzip iso/install.amd/initrd
chmod -w -R iso/install.amd/

cd iso/
chmod +w md5sum.txt
md5sum `find -follow -type f` > md5sum.txt
cd ..

xorriso -as mkisofs -o preseed.iso -isohybrid-mbr /usr/lib/ISOLINUX/isohdpfx.bin -c isolinux/boot.cat -b isolinux/isolinux.bin -no-emul-boot -boot-load-size 4 -boot-info-table iso

This script assumes that you're working with Debian 10 stable ("buster") amd64, and that preseed.cfg and sources.list files are in the home directory of whatever system you're using to build the iso.

Answer 2

d-i preseed/late_command string \
    echo "{{source1}}" > /target/etc/apt/sources.txt; \
    echo "{{source2}}" >> /target/etc/apt/sources.txt; \
    echo "{{source3}}" >> /target/etc/apt/sources.txt; \
    in-target mv /etc/apt/sources.txt /etc/apt/sources.list; \
    in-target chmod 644 /etc/apt/sources.list; \
    in-target apt update && apt-get dist-upgrade -y; \

Answer 3

The answer provided by user 507099 worked perfectly fine for me, thanks. I added the preseed late_command to the end of my preseed file, modifications as in the comments to said post were required though, and i skipped the update/upgrade at the end. Example for a standard kali /etc/apt/sources.list

d-i preseed/late_command string \
    echo "# See https://www.kali.org/docs/general-use/kali-linux-sources-list-repositories/" > /target/etc/apt/sources.txt; \
    echo "deb http://http.kali.org/kali kali-rolling main contrib non-free non-free-firmware" >> /target/etc/apt/sources.txt; \
    in-target mv /etc/apt/sources.txt /etc/apt/sources.list; \
    in-target chmod 644 /etc/apt/sources.list