Re: Unable to access smtp and pop3 services on client behind ISA 2004

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I have a SOHO router that handles the DSL connection to our ISP. The path
to the internet is essentially client to ISA to SOHO Router to ISP and
beyond. Nothing is wrong with the SOHO router but because it is already
performing NAT, I set the Internet Access rule in ISA to Route from NAT.
When I set this back to NAT, everything started working again

You have a routing problem. Based on your topology the IP subnet behind ISA
is not known to your SOHO router, because it is not directly connected to
its internal interface. Therefore not only will the SOHO router not know how
to return packets to that subnet, but it is also very possible that it
doesn't perform NAT for packets arriving from that subnet either. Any of
these two will cause traffic to not flow.

Basically if you want to continue to use the SOHO router for external
(Internet) NAT, you will have to keep the ISA in NAT mode as well, or create
a static route on your router which would forward all packets destinated to
the subnet behind ISA, to the ISA external IP/NIC. Depending on your SOHO
device this may not even be an option, or you could then run into problem #2
above (your SOHO router will not perform NAT for packets arriving from the
network behind ISA).

Virgil



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Relevant Pages

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