On the GuestVLAN interface, apply the inbound ACL like this:
ip access-group Guests_in in
Inbound ACL should be:
ip access-list extended Guests_in
remark *** To allow traffic to proxy:
permit tcp 192.168.11.0 0.0.0.255 host 192.168.100.99 eq 8080
remark *** To allow DNS querries:
permit udp 192.168.11.0 0.0.0.255 host 192.168.100.111 eq domain
remark *** To allow IP aquisition:
permit udp any any eq bootps
To the same GuestVLAN interface, apply the outbound ACL like this:
ip access-group Guests_out out
Outbound ACL should be:
ip access-list extended Guests_out
remark *** To allow traffic from proxy:
permit ip host 192.168.100.99 192.168.11.0 0.0.0.255
remark *** To allow DNS and DHCP responses:
permit ip host 192.168.100.111 any
Comments:
1. I use something like that on a Cisco device, but good routers should allow you something similar, probabilly with slightly different sintax
2. The inbound ACL is more granular, and therefore stricter than the looser outbound ACL.
3. The order of the statements in the ACLs take into account traffic quantities: proxy traffic will be biggest, then DNS querries/answers and some light DHPC requests/responses