/etc/bash.bashrc
    /etc/environment
    /etc/apt/apt.conf
    /usr/local/etc/freshclam.conf

sudo nano  /etc/bash.bashrc


    export http_proxy=http://username:password@proxyserver.net:port/
    export https_proxy=http://username:password@proxyserver.net:port/
    export ftp_proxy=http://username:password@proxyserver.net:port/

sudo nano  /etc/environment


    http_proxy=http://username:password@proxyserver.net:port/
    https_proxy=http://username:password@proxyserver.net:port/
    ftp_proxy=http://username:password@proxyserver.net:port/

sudo nano /etc/apt/apt.conf


   Acquire::http::Proxy "http://username:password@proxyserver.net:port";
   Acquire::https::Proxy "http://username:password@proxyserver.net:port";
   Acquire::ftp::Proxy "http://username:password@proxyserver.net:port";

sudo nano  /usr/local/etc/freshclam.conf


    HTTPProxyServer proxyserver.net
    HTTPProxyPort port
    HTTPProxyUsername username
    HTTPProxyPassword password