like squid tcp_outgoing_address… a rule to match an internal ip/subnet to an external IP.
use on firewall
10.0.10.0/24 src-nat to 200.43.8.2
10.0.20.0/24 src-nat to 200.43.8.3
10.0.30.0/24 src-nat to 200.43.8.4
and in webproxy
source 10.0.10.0/24 outgoing address 200.43.8.2
source 10.0.20.0/24 outgoing address 200.43.8.3
source 10.0.30.0/24 outgoing address 200.43.8.4
now to have a single ip exit the webproxy, I have problems with authentication some sites that use Java or https, with this feature is fully resolved