function proxy_on() {
    export no_proxy="localhost,127.0.0.1,localaddress,.localdomain.com"

    if (( $# > 0 )); then
        valid=$(echo $@ | sed -n 's/\([0-9]\{1,3\}.\)\{4\}:\([0-9]\+\)/&/p')
        if [[ $valid != $@ ]]; then
            >&2 echo "Invalid address"
            return 1
        fi

        export http_proxy="http://$1/" \
               https_proxy=$http_proxy \
               ftp_proxy=$http_proxy \
               rsync_proxy=$http_proxy
        echo "Proxy environment variable set."
        return 0
    fi

    echo -n "username: "; read username
    if [[ $username != "" ]]; then
        echo -n "password: "
        read -es password
        local pre="$username:$password@"
    fi

    echo -n "server: "; read server
    echo -n "port: "; read port
    export http_proxy="http://$pre$server:$port/" \
           https_proxy=$http_proxy \
           ftp_proxy=$http_proxy \
           rsync_proxy=$http_proxy \
           HTTP_PROXY=$http_proxy \
           HTTPS_PROXY=$http_proxy \
           FTP_PROXY=$http_proxy \
           RSYNC_PROXY=$http_proxy
}

function proxy_off(){
    unset http_proxy https_proxy ftp_proxy rsync_proxy \
          HTTP_PROXY HTTPS_PROXY FTP_PROXY RSYNC_PROXY
    echo -e "Proxy environment variable removed."
}