$ netselect-apt testing
Using distribution testing.
Retrieving the list of mirrors from www.debian.org...

Choosing a main Debian mirror using netselect.
Running netselect to choose 1 out of 275 addresses.

netselect was unable to find a mirror, this probably means that
you are behind a firewall and it is blocking traceroute.

Choose the fastest Debian mirror with netselect

netselect-apt will choose the fastest Debian mirror by downloading the full mirror list and uses netselect to find the best one. netselect-apt writes a sources.list(5) file that can be used with apt(8).