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I've been reading a lot of different Q&As on here and other forums about firewall positioning in relation to the DMZ and the appropriate place to put a VPN server. I've made the following logical diagram to describe what I think is a synthesis of what I've read. Can you please tell me if this design is feasible configuration and if not, where/why it's wrong?

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In the diagram the filtering router is the edge firewall. It has two externally visible IP addresses. One splits DMZ and LAN traffic. The other IP is for the VPN server. DMZ traffic is forwarded from the filtering router by port to servers in the DMZ. Requests for unknown connections not in the DMZ are either forwarded to the NAT router where they are translated or dropped.

The VPN Server also does authentication and forwards to the NAT router if the external client successfully registers on the network.

The NAT router translates internal/external traffic requests. It allows traffic to the DMZ from 'PC VLAN' but not the 'internal server vlan' servers. It blocks traffic coming from the DMZ that did not originate from a server in the DMZ.

Does this seem like a logical configuration in terms of where things should be and what the firewalls can be relied upon to do? As an aside, when I look at this I think that the DMZ and the LAN seem to be two subnets as they both split one of the externally visible IPs to enable addressing. Is this the wrong way to think about this?

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    Currently, there's no firewall between DMZ and LAN - effectively making your DMZ part of the LAN. – Zac67 Mar 16 '19 at 9:54
  • Thanks. The NAT router is a type of firewall. So it sits between the DMZ and LAN? – Jay Black Mar 16 '19 at 9:55
  • NAT by itself is not a firewall (function). – Zac67 Mar 16 '19 at 9:57
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    It's fine if it's a policy-based firewall also doing NAT. I was trying to point out that NAT alone doesn't provide proper protection. – Zac67 Mar 16 '19 at 10:06
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    @Zac67 time to make an answer based on those comments :D – user36472 Mar 16 '19 at 10:52
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The potential problems I'm seeing:

  • The NAT router needs to be a proper firewall. Otherwise the LAN isn't protected against the DMZ.
  • For Internet access, NAT is applied before the edge firewall. Make sure you apply your Internet access policies on the NAT router already.
  • VPN clients shouldn't be NATed when accessing the LAN or DMZ (this isn't clear from the diagram).
  • As a proper firewall, the NAT router might be bottlenecking client-server traffic. Alternatively, a fast L3 switch with ACLs may be used.
  • Your firewall policies should be cleanly split between client access to the Internet and DMZ (on NAT router), and access from Internet to DMZ (on edge firewall).
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  • Thanks @Zac67. That's really helpful. Do you mean the VPN should be behind the NAT? I did consider this and then instead of having a filtering router, having an edge router with a firewall doing NAT and making the DMZ and LAN two subnets. – Jay Black Mar 16 '19 at 11:41
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    For the connections VPN-servers, LAN-DMZ, PC-servers, VPN-DMZ you should disable source NAT on "NAT router". Alternatively, you could place a NAT router (without firewall) in front of your whole network (as the public-private demarcation line) and a combined firewall behind it (between the different trust zones). – Zac67 Mar 16 '19 at 12:00

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