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Old 8th July 2009
jdude jdude is offline
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Join Date: Jul 2009
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Default Routing internal requests to external IPs

Hey everyone. Does anyone have a good resource/idea about how to solve the following issue for a client of mine?

They have an mobile device that switches from mobile network to wifi when wifi is available. An external IP is being used as an alias on the firewall to route incoming traffic to an internal mail server. The problem is that when the mobile device is used in the office, the device tries to access the external IP and fails to connect.

I know that there is something that stops internal requests to external IPs that route back to an internal machine from working from within the network.

Right now the solution is to turn off the wifi in the office and force the phone back on the mobile network to connect.

Is there a way to analyze outgoing requests and route them directly to an internal machine?

My client would like to be able to leave the wifi setting as is and take advantage of the speed.


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Old 9th July 2009
J65nko J65nko is offline
Join Date: May 2008
Location: Budel - the Netherlands
Posts: 4,123

When the device is in the office, you could run a simple shell script that adapts it's network setting to the office situation.

Another possibility is to run a cron job every minute or so. When the device fails to connect to the external IP, adjust the network settings.

On my OpenBSD desktop, I use the following script in cron to start the ntpd daemon when I have network access, and just forget about it when there isn't .
# ---- to be used in root's crontab for machines not always connected


if pgrep ntpd >/dev/null 2>&1 ; then
    echo ntpd already running 
    exit 0

if ping -c3 $GATEWAY >&2 ;  then
   # connection to gateway exists
   echo Starting ntpd ....
   ntpd -s
   exit 0
   echo "Gateway not alive, cannot run ntpd"
   exit  1
You don't need to be a genius to debug a pf.conf firewall ruleset, you just need the guts to run tcpdump
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