Iptables breaks a working VoIP phone?

Kenneth Burgener kenneth at mail1.ttak.org
Mon Oct 29 22:44:36 MDT 2007

Shane Hathaway wrote:
> iptables -t nat -A POSTROUTING -s -d '!' -j

I have configured the machine to use masquerading, since the comcast IP
address is dynamic, and all of my internal machines can access the
Internet, so I think I am good on this step.

> Once you've done that, machines on your LAN should be able to contact
> the Internet through your firewall.  At the same time, assuming you have
> no other rules, no Internet machine should have the ability to initiate
> a connection with a machine in your LAN (unless someone at your ISP
> spoofs a 10.10.10.x destination address).


> 2. If you can not reconfigure your SIP ATA, the ATA is probably
> registering with your upstream provider using an internal IP address
> rather than the external IP address.  You need the conntrack_sip module
> to make your router edit the SIP stream.

lsmod shows a "conntrack_sip" loaded, so I assume it somehow magically
detects SIP traffic, or is there something more I need for the

> 3. You should only use DNAT rules if you want to run Internet servers.
> If you have a web server at address, the DNAT rule might look
> like:


> iptables -t nat -A PREROUTING -p tcp -d x.x.x.x --destination-port 80 -j
> DNAT --to-destination
> DNAT is appropriate for running an Asterisk server, but is not normally
> required for a SIP phone.
> Shane

This is basically the thought process I have gone through trying to
figure out why this setup is not working, which is why I am frustrated
that it is not working, as everything appears to be configured as needed.

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