[c-nsp] Question on h.323 video calls through a PIX 525 with NAT

[Steven Pfister]

Yes, tcp/1720 seems to be going to the correct address. The thing I'm wondering now is this... I did the capture on the PIX itself on the outside interface. I've found at least one spot where the internal address for the unit on our side appears. I would have thought the NAT transversal setting on the unit itself would have taken care of this before hitting the PIX. And the capture being on the outside interface... would it be showing the packets before or after inspect has gotten to them. We've got one unit in the same building  as the firewall... hopefully I can duplicated the problem on that.

When I first started getting involved with the video conferencing units here, we weren't able to dial out until I turned the NAT transversal setting on. Then I found out about inspect/fixup and never understood why that setting on the unit would be needed if those commands were on the firewall config. Maybe we should try it without the inspect? No other h.323 traffic normally goes in or out of our network.

Steve Pfister
Technical Coordinator, 
The Office of Information Technology
Dayton Public Schools
115 S. Ludlow St. 
Dayton, OH 45402
 
Office (937) 542-3149
Cell (937) 673-6779
Direct Connect: 137*131747*8
Email spfister at dps.k12.oh.us


>>> Andrew Yourtchenko <ayourtch at gmail.com> 7/15/2009 2:07 PM >>>
Hi Steven,

On Wed, Jul 15, 2009 at 6:28 PM, Steven Pfister<SPfister at dps.k12.oh.us> wrote:
> I'm having some trouble with h.323 (video) calls through a PIX 525 using NAT. We can get incoming calls fine, but not outgoing calls for some reason. My question has to do with 'inspect h323' vs 'fixup protocol h323'. What's the difference between them? The video conferencing unit in question has a NAT transversal option where I can supply an address and mask.I'm wondering if I'm having a NAT transversal problem anyway. Which one would handle the NAT transversal, inspect or fixup? Currently, the PIX config has:
>
>  inspect h323 h225
>  inspect h323 ras
>
> do I need:
>
>  fixup protocol h323 h225 1718-1720
>  fixup protocol h323 h225 1720
>  fixup protocol h323 ras 1718-1719
>
> instead of the inspect commands? In addition to them?
>

"inspect" is the name of the "fixup" from 7.0 onwards - obviously as
time went, some more enhancements were added.

you can enter the "fixup" commands, but they will be autoconverted
into the respective "inspect" under "magic" default policy.

You mention that the inbound call works - so a nice way to debug would
be to grab the pcap on inside+ pcap on the outside and study them in
wireshark for both failing and working scenarios and see what is
different.

The first cutover point is whether you see the tcp/1720 in the
outbound direction or not - if not, or if it is going to the wrong
address, would mean there is an issue related to RAS signaling - else
it's something with the call signaling.

The above can be tested much easier if you are able to make the direct
calls by IP address and the other party can accept such calls without
involving RAS at all - this could be an easy shortcut instead of
staring at the sniffer traces :-) - if the direct call using IP
address works, then you can further investigate RAS. If the inbound
calls to you work, most probably it is going to be the case, but worth
doublechecking.

The inspect in the default configuration normally should be able to
tweak all the embedded IPs both for RAS and call setup, so the
endpoints would not need to have any NAT awareness nor do any special
efforts to detect/traverse the NAT.

Also might be quite useful to have a quick test with another h323
stack if you can - openh323 had a very tweakable client, and ekiga can
do h323 video as well. If those work, again you get one more baseline
to compare the sniffer traces with.

cheers,
andrew