Fix Ubuntu apt‑get Update Errors: Configure Local Repositories & Install Packages

Background: The author installed Ubuntu 20.10 in a virtual machine to test PCAP packets. After installation, many commands were unavailable because openssh-server was not installed, and running sudo apt-get update produced errors.

Step 1 – Verify Ubuntu Version

lsb_release -a

The command confirms the exact Ubuntu release being used.

Step 2 – Configure Software Sources

Using the USTC mirror, the author downloaded a sources.list file for the groovy (20.10) release. The original file is backed up before replacement:

cp /etc/apt/sources.list /etc/apt/sources.list.bak

If vim is installed, edit the file directly; otherwise use gedit:

vim /etc/apt/sources.list

sudo gedit /etc/apt/sources.list

Insert the appropriate deb lines, for example:

deb http://mirrors.ustc.edu.cn/ubuntu/ groovy main restricted

Step 3 – Update and Install Packages

After saving the new source list, run: sudo apt-get update When the update succeeds, install required tools, e.g.:

sudo apt install tree

Additional – VMware Tools for Bridged Networking

If the VM is not in bridged mode, install VMware Tools to enable drag‑and‑drop and proper network access:

cd /home/jacktian/桌面/123/VMwareTools-10.3.22-15902021/vmware-tools-distrib
./vmware-install.pl

Accept the defaults (answer "yes") until installation completes.

Offline Repository via ISO

When the host has no internet access, mount the Ubuntu ISO and create a local repository:

mkdir /mnt/cdrom
umount /dev/sr0
mount -t iso9660 /dev/sr0 /cdrom
# or using a loop device
sudo mount -t iso9660 -o loop /home/jacktian/桌面/123/ubuntu-20.10-desktop-amd64.iso /mnt/cdrom

Backup the current sources.list, then add the CDROM entry:

cp /etc/apt/sources.list /etc/apt/sources.list.bak
sudo apt-cdrom add

Verify the file with cat /etc/apt/sources.list, clean the cache, and update:

sudo apt clean all
sudo apt update

Offline Installation of Required Packages

If the repository is still unreachable, download the .deb packages manually. Example for net-tools and openssh-server:

sudo dpkg -i net-tools_1.60+git20180626.aebd88e-1ubuntu1_amd64.deb

The openssh-server installer is provided as a zip containing all dependency debs:

unzip openssh-server-installer.zip
sudo dpkg -i *.deb

After completing these steps, the Ubuntu VM can be accessed via terminal tools even without external network connectivity.