ifdown eth0; ifup eth0
I wonder whether I need to always restart interface when I edit
/etc/network/interfaces (change IP, netmask, gateway, set DHCP),
/etc/resolv.conf (changes DNS),
/etc/apt/apt.conf (proxy for apt).
Also I think about restarting DHCP client in some situations (but not sure when this would be sufficient). Could you please shed light on this issue.