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Thread: Help me understand how NAT and VPN tunnels work with each other

  1. #1
    Join Date
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    Default Help me understand how NAT and VPN tunnels work with each other

    Here's our setup (IP addresses changed for clarity):

    SecureClient users' Office Mode address pool 3.3.3.0/24
    Central Office:4.4.4.0/24
    Second office: 5.5.5.0/24

    Remote Access Domain on central office gateway includes 4.4.4.0 and 5.5.5.0 . IPsec tunnel exists between central office and second office and works fine.

    A hide NAT rule exists such that packets from 3.3.3.x to 5.5.5.x via the central office have their source IP address changed to 4.4.4.100 .

    Now, the following DOES work:

    Users at 3.3.3.x can ping host 5.5.5.1 . Sniffing the packets with "fw monitor" etc. shows the expected NAT taking place, and packets showing up where they should.

    But the following DOESN'T work:

    Add a NAT rule before the other one, such that packets from 3.3.3.x to host 4.4.4.2 have their source address hidden behind 4.4.4.101, while the destination is static-NATted to 5.5.5.2 . Now, 3.3.3.17 tries to ping 4.4.4.2.

    In this case, in fw monitor I see something like:

    (I) 3.3.3.17 -> 5.5.5.2 (post-inbound, destination NAT has already taken place, this is normal)
    (o) 3.3.3.17 -> 5.5.5.2 (pre-outbound, no source NAT yet, normal)
    (O) 4.4.4.101 -> 5.5.5.2 (post-outbound, this is NOT normal)

    I don't understand why I am seeing anything at the post-outbound stage. It should have been encapsulated in the tunnel by that point. That is what happens in the first case, the one that does work - I just see the packet at its post-inbound and pre-outbound stages, as it emerges from the SecureClient tunnel and before it goes into the site-to-site tunnel.

    So how come the gateway is not sending the packets down the site-to-site tunnel in the second case?
    Last edited by hammop1; 2008-04-24 at 11:16.

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