[sip-comm] Firewalls and Stun


#1

I've had a lot of puzzles to solve before I could use sip-communicator successfully.
The SIP/SDP call setup seems to work well, but the establishment of the RTP/RTPC
audio streams has been very tricky. However, I've had similar problems with non-java
softphones, so I've kept banging away.

If you read my other post, you'll find out the details of the only successful software
configuration (that I've found so far).

I use a Cisco Pix firewall. I had to upgrade the software to version 6.3(3) before I
resolved all the problems with NAT and the stateful SIP firewall support. There are a lot
of claims out there that sip doesn't work properly with "symmetric firewalls", but I can
say <nervously> that it seems to work OK with mine.

I've now conducted a series of tests with sip-communicator outside my firewall... I setup
sip-communicator.xml with three different stun server settings...
1. "" (empty string) to disable STUN logic.
2. "stun01.sipphone.com" - the default setting.
3. "stun.fwdnet.net" - because I have a Free World Dialer account.

All three settings work fine outside my firewall. The 2 stun servers provide identical
responses, and these responses are effectively no-op results. As expected, I don't
need stun when my sip-communicator is using a native internet address, but it is nice to
know that it doesn't matter if I leave stun configured.

It is a different matter inside the firewall. Using the latest stateful SIP proxy, I would
expect to succeed with a no-stun configuration. The firewall ought to translate all the
embedded IP addresses and port numbers, but it doesn't work properly. However,
when I define either of the two stun servers, my calls are established successfully and
the 2-way audio traffic flows properly. (I haven't the time or enthusiasm to work out the
reasons.)

I doubt whether anyone is very interested, but I've also tried a double-nat arrangement.
This is a quite common arrangement in corporate networks. A "choke" router acts as a
crude port-based firewall and NAT's addresses between the internet and a dmz lan.
The statefull firewall then NAT's between the dmz lan and the safe internal lan. I
couldn't find a way to make this arrangement work - the stateful firewall proxy and the
stun server were always confused by the presence of the "outer" nat router and so the
media streams were not connected properly.
Regards,

Brian Burch
[http://www.PingToo.com/]

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