Error in sudo apt update

Question

I have a raspberry pi 3 b model with raspian Buster installed. I have connected it headless and am using putty and vnc viewer to access it. I am trying to update it using sudo apt-get update. The raspbi is connected to network through a router which does have internet connection.

I'm getting the following errors repeatedly:

Err:1 http://raspbian.raspberrypi.org/raspbian buster InRelease
  Undetermined Error [IP: 93.93.128.193 80]
Get:2 http://archive.raspberrypi.org/debian buster InRelease
Get:2 http://archive.raspberrypi.org/debian buster InRelease
Get:2 http://archive.raspberrypi.org/debian buster InRelease
Get:2 http://archive.raspberrypi.org/debian buster InRelease
Get:2 http://archive.raspberrypi.org/debian buster InRelease
Get:2 http://archive.raspberrypi.org/debian buster InRelease
Get:2 http://archive.raspberrypi.org/debian buster InRelease
Get:2 http://archive.raspberrypi.org/debian buster InRelease
Get:2 http://archive.raspberrypi.org/debian buster InRelease
Get:2 http://archive.raspberrypi.org/debian buster InRelease
Get:2 http://archive.raspberrypi.org/debian buster InRelease
Get:2 http://archive.raspberrypi.org/debian buster InRelease
Err:2 http://archive.raspberrypi.org/debian buster InRelease
  Undetermined Error [IP: 93.93.128.133 80]
Reading package lists... Done
Building dependency tree
Reading state information... Done
4 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: Failed to fetch
 http://raspbian.raspberrypi.org/raspbian/dists/buster/InRelease  Undetermined Error [IP: 93.93.128.193 80]
W: Failed to fetch http://archive.raspberrypi.org/debian/dists/buster/InRelease  Undetermined Error [IP: 93.93.128.133 80]
W: Some index files failed to download. They have been ignored, or old ones used instead.

Any solutions?

EDIT:

pi@raspberrypi:~ $ cat /etc/apt/sources.list
deb http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi
# Uncomment line below then 'apt-get update' to enable 'apt-get source'
#deb-src http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi

On using sudo apt-get update --allow-releaseinfo-change (suggested in raspberrypi.org forum)

Get:1 http://archive.raspberrypi.org/debian buster InRelease
Get:1 http://archive.raspberrypi.org/debian buster InRelease
Get:1 http://archive.raspberrypi.org/debian buster InRelease
Get:1 http://archive.raspberrypi.org/debian buster InRelease
Get:1 http://archive.raspberrypi.org/debian buster InRelease
Get:1 http://archive.raspberrypi.org/debian buster InRelease
Get:1 http://archive.raspberrypi.org/debian buster InRelease
Get:1 http://archive.raspberrypi.org/debian buster InRelease
Get:1 http://archive.raspberrypi.org/debian buster InRelease
Get:1 http://archive.raspberrypi.org/debian buster InRelease
Get:1 http://archive.raspberrypi.org/debian buster InRelease
Err:1 http://archive.raspberrypi.org/debian buster InRelease
  Undetermined Error [IP: 46.235.231.145 80]
Err:2 http://raspbian.raspberrypi.org/raspbian buster InRelease
  Temporary failure resolving 'raspbian.raspberrypi.org'
Reading package lists... Done
W: Failed to fetch http://raspbian.raspberrypi.org/raspbian/dists/buster/InRelease  Temporary failure resolving 'raspbian.raspberrypi.org'
W: Failed to fetch http://archive.raspberrypi.org/debian/dists/buster/InRelease  Undetermined Error [IP: 46.235.231.145 80]
W: Some index files failed to download. They have been ignored, or old ones used instead.

Answer 1

To have a complete update you need to connect to two internet addresses: to raspbian.raspberrypi.org for the default Debian cloned Raspbian repository defined in /etc/apt/sources.list and to archive.raspberrypi.org for the Raspberry Pi specific kernel and firmware in /etc/apt/sources.list.d/raspi.list. From the error messages I would suspect three conditions: DNS name resolution, internet connection and the local index cache of the packages. Because we are headless connected to the RasPi by SSH we can only use the text console. Here are some examples from my RasPi to narrow down the problem.

DNS name resolution:

rpi ~$ host raspbian.raspberrypi.org
raspbian.raspberrypi.org is an alias for mirrordirector.raspbian.org.
mirrordirector.raspbian.org has address 93.93.128.193
mirrordirector.raspbian.org has IPv6 address 2a00:1098:0:80:1000:75:0:3

rpi ~$ host archive.raspberrypi.org
archive.raspberrypi.org is an alias for lb.raspberrypi.org.
lb.raspberrypi.org has address 93.93.130.39
lb.raspberrypi.org has address 46.235.231.111
--- snip ---
lb.raspberrypi.org has IPv6 address 2a00:1098:0:82:1000:13:0:9
lb.raspberrypi.org has IPv6 address 2a00:1098:0:80:1000:13:0:8
--- snip ---

Internet connection:

rpi ~$ ping -c3 93.93.128.193
rpi ~$ ping -c3 46.235.231.111

rpi ~$ curl http://raspbian.raspberrypi.org/raspbian/
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<html>
 <head>
  <title>Index of /raspbian</title>
 </head>
 <body>
--- snip ---
  <address>Apache/2.4.25 (Debian) Server at raspbian.raspberrypi.org Port 80</address>
  <br/><address><a href="http://mirrorbrain.org/">MirrorBrain</a> powered by <a href="http://httpd.apache.org/">Apache</a></address>
 </body>
</html>

rpi ~$ curl http://archive.raspberrypi.org/debian/
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<html>
 <head>
  <title>Index of /debian</title>
 </head>
 <body>
--- snip ---
  <address>Apache/2.4.10 (Debian) Server at archive.raspberrypi.org Port 80</address>
 </body>
</html>

If this tests are successful then you can try to reset the packages local index cache. How to do it you can look at Raspberry Pi sudo apt-get update not working.