Apt-get in old debian version

1) watch your debian version here :

cat /etc/debian_version

2) Change your version here (such etch, squeeze  etc) :

vi /etc/apt/sources.list

deb http://archive.debian.org/debian/ squeeze main non-free contrib deb-src http://archive.debian.org/debian/ squeeze main non-free contrib 

deb http://archive.debian.org/debian-security/  squeeze/updates main non-free contrib deb-src http://archive.debian.org/debian-security/ squeeze/updates main non-free contrib

2) apt-get update

DNS tools

посмотреть МХ записи для домменого имени

host -t mx yahoo.com
yahoo.com MX 1 mx2.mail.yahoo.com
yahoo.com MX 1 mx3.mail.yahoo.com
yahoo.com MX 1 mx1.mail.yahoo.com
yahoo.com MX 9 mta-v1.mail.yahoo.com

Проверить сервер на ресолв
nslookup ya.ru 172.25.1.1 - проверить DNS сервер 172.25.1.1

Еще один способ

dig @1.1.1.1 facebook.com