How to fix E: Some index files failed to download. They have been ignored, or old ones used instead in APT

First, verify that your internet connection is stable and working:

ping google.com

If the ping fails, your internet connection is down. Reconnect to your network or fix your network configuration before proceeding.

Often, this error is temporary because a mirror is temporarily unavailable. Wait 30-60 seconds and try again:

sudo apt update

If the error clears, the mirror has come back online. This is the most common resolution.

Poor DNS resolution can cause mirror connection failures. Test if DNS is working:

nslookup archive.ubuntu.com

If this fails, your DNS is misconfigured. You can temporarily use Google's DNS:

sudo nano /etc/resolv.conf

Add or modify:

nameserver 8.8.8.8
nameserver 8.8.4.4

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

If a custom mirror is causing the issue, reset to the official Ubuntu mirror:

sudo mv /etc/apt/sources.list /etc/apt/sources.list.backup
sudo cp /usr/share/doc/apt/examples/sources.list /etc/apt/sources.list
sudo apt update

This restores the default sources list. If the update succeeds, you can examine what was wrong with the original sources.list file.

If the default mirror is slow or unreachable, try switching to a mirror closer to your location. Edit your sources list:

sudo nano /etc/apt/sources.list

Look for lines like:

deb http://us.archive.ubuntu.com/ubuntu/ focal main

Replace the country code (e.g., us) with your country code (e.g., uk, de, ca, au). For a complete list of mirrors, visit the Ubuntu Mirror List (https://launchpad.net/ubuntu/+cdmirrors).

Save and run:

sudo apt update

Sometimes the cached index files become corrupted. Delete and regenerate them:

sudo mv /var/lib/apt/lists /var/lib/apt/lists.backup
sudo mkdir /var/lib/apt/lists
sudo apt update

This safely backs up the old cache and creates fresh index files. If apt update succeeds, the previous indexes were corrupted.

Clear the apt package cache, which may contain corrupted or outdated files:

sudo apt clean
sudo apt update

This removes cached .deb files from /var/cache/apt/archives/, which may have been causing issues.

If you're behind a corporate firewall or proxy, it may be blocking access to repositories. You can test direct connectivity to a mirror:

curl -I http://archive.ubuntu.com/

If this fails, contact your network administrator. If you're using a proxy, configure apt to use it:

sudo nano /etc/apt/apt.conf.d/proxy.conf

Add (replace with your proxy details):

Acquire::http::Proxy "http://proxy-ip:port/";
Acquire::https::Proxy "https://proxy-ip:port/";

If you don't have IPv6 connectivity, apt may be trying to use IPv6 to reach mirrors. Temporarily disable IPv6 to test:

sudo nano /etc/sysctl.conf

Add or uncomment these lines:

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

Apply the changes:

sudo sysctl -p

Then try:

sudo apt update

If this fixes it, you can keep IPv6 disabled or fix your IPv6 connectivity properly.