Windows Server 2008 R2 RRAS NAT Security Concerns

Question

Recently we are deploying Windows Server 2008 R2 as the NAT gateway of our private network. During the testing, we found that the RRAS was doing its job as the NAT gateway,
however it seemed that hosts in the private network were allowed to access any listening port opened on the server side (2008 R2). In the normal scenario, the server side will have the process "wininit.exe" running and listening on the TCP port 49152.
We confirmed that all hosts in the private network were be able to connect to TCP port 49152 opened on the server (connecting by using the NAT's public IP), which introduced lots of security concerns and made us nervous. Since the server is acting as a NAT,
IP packets sent by hosts in the private network will be translated and forwarded as if it is generated by the NAT server itself. Thus, the windows firewall will not block the connection at all while dealing with "local" traffic, which actually is
the traffic from the host in the private network.
What we need is a mechanism that can block the hosts in the private network to access the TCP/UDP ports opened on the NAT server side. Since the NAT server has it IP on
the public network assigned dynamically (DHCP), static IP filtering on the private NIC does not fit our needs (Or probably we may use some hidden but advanced filter settings?). Which policy or setting should be used in our case?

Answer 1

Hi Daniel,
I am aware of what you are suggesting. Actually I have active the windows firewall to protect the server.
Suppose I have a network configuration as follows:
Private Network: 192.168.149.0 / 255.255.255.0 (Private NIC on server side IP:192.168.149.1)
--------------Windows 2008 R2 RRAS NAT--------------------
Public Network: 10.1.0.0 / 255.255.255.0 (Public NIC on server side IP:10.1.0.100 )
The problem is that while the windows firewall is effectively protecting my server by filtering inbound traffic from the public network, the windows firewall will not filter the traffic from
192.168.149.0 /255.255.255.0  to  10.1.0.100 (NAT's public IP)
The reason is that the TCP/UDP connection from the private network (192.168.149.0 / 255.255.255.0) to any other networks will be NATed. Suppose TCP connection from
192.168.149.23:50000 -> 10.1.0.100:1023
It will be translated by NAT and becomes
192.168.149.23:50000 <-NAT-> 10.1.0.100:60100 -> 10.1.0.100:1023
From the windows firewall's point of view, the connection is essentially a 'local' TCP connection and should be allowed regardless of any inbound filtering rules. So vulnerability is introduced. After some research, we are almost sure that the windows firewall
does not filter local traffic. Also, we are not able to guarantee any firewalls on the client side to be installed, since the nature of a NAT server is to provide such network access ability to clients and should not require the client side to change its configuration.
I do think it is a common security concern in lots of enterprise networks where Windows Servers are deployed as NAT servers. Would you mind help us address this issue and give us some advice about best-practices related?
Thank you