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We recently got done doing a massive IP scheme change at work and since then I've noticed a few oddities. First, our setup:

IPCop firewall which has
    - Wireless network (completely segregated)
    - Firewalled DMZ
    - Internal network [eth0] (10.10.10.x)
    - OpenVPN network [tun0] (172.16.20.x)
    - Internet

Before the IP change, our OpenVPN network could talk to our internal network without any problem. Since the change we have a few servers that no longer want to talk across the boundary properly.

For example, I will connect to the VPN with and try to talk to a server at Running tcpdump I see the packets leave my machine (running wireshark), travel through tun0, travel through eth0, hit, and nothing comes back. Other servers work fine on the same network (, for example, works fine) and I cannot find a common piece between them on what would cause some servers to just stop responding to some requests.

I checked things like hosts.(allow|deny), pf/iptables, etc, that would normally filter traffic but none of them have exclusion rules that would stop traffic like this.

Any ideas on where to go from here?


The boxes that were having the problem did indeed have bad gateways that got overlooked. Made the changes in the appropriate places and everything seems to be working!

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up vote 2 down vote accepted

Is it possible you just have a bad default or specific route on Running wireshark on that machine while re-performing your test to see what happens to the return packets seems like an interesting test.

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Sounds like the remote host doesn't know how to get back to you. Check the default route (gateway). – Russ Warren Jun 30 '09 at 13:53

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