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David Bullock
Asked: 2011-07-21 18:45:37 +0800 CST

Should I ever configure a host with a public IP address to accept traffic from a private IP address?

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I connect the 'net with a public static IP (my router's IP address for NAT) and a /29 subnet (for machines behind the router).

On my router, I have:

#sh ip route | inc x.x
     x.x.0.0/16 is variably subnetted, 3 subnets, 2 masks
C       x.x.196.62/32 is directly connected, Dialer1
C       x.x.206.72/29 is directly connected, BVI2


#sh run int dial 1 | inc zone
 zone-member security out-zone
#sh run int bvi 2 | inc zone
 zone-member security in-zone

The zone-pair is fairly restrictive. I would like to relax the restrictions for clients which first connect via a remote-access VPN:

#sh run int virtual-template 1
 zone-member security relaxed-zone

Now clients connecting via VPN must be assigned a private IP address. (I don't allocate public addresses to clients, right?):

# sh ip local pool

 Pool                     Begin           End             Free  In use   Blocked
 RANET100                 192.168.100.230 192.168.100.250   20       1       0

So now if I want to get a packet to x.y.206.73 from 192.168.100.230, that's fine - the router has the information it needs. But it means that the host x.y.206.73 would have to permit traffic to its public-scoped IP address from a private address ... something it would normally be configured to ignore!

So this is a dirty hack, right? What is the True Path (TM)? Should I just multi-home the hosts with the public IP addresses so that they also have a private IP address?

firewall security cisco ip-address

2 Answers

  1. "But it means that the host x.y.206.73 would have to permit traffic to its public-scoped IP address from a private address"

    No it wouldn't, it'd mean that a host would have two IP addresses, which happen to have different routing expectations to the Internet as a whole.

    There's no reason not to place more than one IP address on an interface; it's entirely possible, and in fact, IPv6, it's pretty much universal. Just make sure that your address selection rules for outgoing connections use the right address for the right task.

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  2. Best Answer

    It's entirely reasonable to have this setup. Consider it thus:

    Your 'internal network' consists of multiple networks:

    • a globally routable /29
    • a locally routable /24 for VPN users

    Your public host doesn't care that it gets a connection from an RFC1918 address unless you configure it that way. Just let it happen.

    No need to configure another IP.

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