MikroTik

Hi all,
I have folowing situation:
primary connectivity at eth1 - public IP etc...
backup connectivity (from different ISP) at eth2 - only internal IP 10.X.X.X
LAN at eth3
Everything works OK, except one thing:
How to set webproxy to use backup connectivity only? Is it possible to mark webproxy traffic to use different routing rule? (mark packet incoming to webproxy does not work)

I think this would work: Create a loopback interface for the web proxy (and only the web proxy) to use. Set the src-address of the proxy to that loopback IP. Insert routing-marks for all connections from that IP.