Apt-get :: download error with synaptic



I've installed dsl but I cannot use synaptic or apt-get. I always obtain a download error beacuse I'm behind a proxy (squid). Internet instead goes well. Where can I insert the configuration for proxy?My proxy value is like this
http_proxy="user:pass@192.168.1.251:3128"
I have seen there is in control panel "mirror list". I think that I have to change it inserting the correct proxy and the correct port. But how can I do? thanks.
The correct sybtax is :
export http_proxy=whatever

http://www.damnsmalllinux.org/wiki...._Server

original here.