How to fix E: Could not resolve hostname in APT

Start by checking if your network is working at all:

ping -c 4 8.8.8.8

If this fails, your network is down. Check your network adapter status:

ip link show
ip addr show

If the network is up but apt fails, proceed to the next step (DNS issue).

View your current DNS settings:

cat /etc/resolv.conf

You should see at least one nameserver line with an IP address (e.g., nameserver 8.8.8.8). If the file is empty or missing, or if you only see nameserver 127.0.0.53 without a working systemd-resolved, that's your problem.

For modern systems using systemd-resolved, check its status:

sudo systemctl status systemd-resolved

Edit the resolv.conf file to add public DNS servers:

sudo nano /etc/resolv.conf

Add or replace the nameserver lines with:

nameserver 8.8.8.8
nameserver 8.8.4.4
nameserver 1.1.1.1

Save and exit (Ctrl+X, then Y, then Enter). Test immediately:

sudo apt update

Note: This change is temporary. On many systems, /etc/resolv.conf is regenerated, so changes may not persist after reboot.

For a permanent fix on systems using systemd-resolved (Ubuntu 18.04+, Debian 11+):

sudo nano /etc/systemd/resolved.conf

Find or add the DNS= line and configure it:

DNS=8.8.8.8 8.8.4.4 1.1.1.1
FallbackDNS=9.9.9.9

Save and restart the service:

sudo systemctl restart systemd-resolved

Verify the change took effect:

systemd-resolve --status
sudo apt update

Check if your system's hostname is properly configured:

cat /etc/hosts
hostname

Your /etc/hosts should have entries like:

127.0.0.1 localhost
127.0.0.1 my-hostname
::1 localhost my-hostname

If your hostname is missing, add it:

sudo nano /etc/hosts

Add a line matching your hostname (get it from hostname command).

After making changes, clear any cached DNS data and try apt again:

sudo systemd-resolve --flush-caches
sudo apt update

If you're using a different DNS system, the cache flush command may differ.

Docker containers and Windows Subsystem for Linux (WSL) have special network configurations:

Docker: If apt fails inside a container, the Docker host's DNS is likely the issue. Pass DNS to the container:

docker run --dns 8.8.8.8 --dns 8.8.4.4 <image> apt update

Or in docker-compose.yml:

services:
  myservice:
    dns:
      - 8.8.8.8
      - 8.8.4.4

WSL 2: Edit /etc/resolv.conf inside the WSL distro and add public DNS servers. Changes may not persist on reboot—use wsl.conf for permanent configuration.