[Asterisk-Users] Using frequent keepalives to eliminate need for NAT port forwarding?

[Tom Engleward]

--- Tim Panton <tim at mexuar.com> wrote:
> Yes. That is the way that IAX2 likes to work.
Ok.

> However, not all providers will allow it, some
> require a fixed IPaddress
> and port for them to send calls to.
Is this the reason for the recommendation I've seen in
various forums to have port 4569 forwarded to the
asterisk machine?
Do either of the providers (teliax, exgn) I've seen
recommended elsewhere on this list require a fixed
port to send calls to?

A possibly related issue:
At
http://www.teliax.com/forum/viewtopic.php?t=173&sid=bb1196132c2eee0ca4a0e09bd04d5309
in a conversation which took place in December of
2005, somebody wrote:
"Do a "iax2 show registry" from the CLI, and you will
notice what ports Teliax 'thinks' you are at... it
should not be :4569 since your NAT router has picked a
different return port for you."
But for me, the entry for "perceived" is always <my
NAT router's public IP>:4569. Somebody else in that
forum said the same is true for them too, and there
was a very interesting reply:
"I've recently uncovered a periodic problem in the NAT
kernel module in linux. It effects both 2.4 and 2.6
linux kernels. The problem is when the source port and
destination port are the same on a UDP connection
(IAX2 is exactly that). If you sniff the traffic
comming out of your router you will find that when
Asterisk can't register the source address in the
outgoing packets are still your private IP address
behind the router. This is why it never gets pasted
the first packet going out. Teliax's asterisk box will
send the reply back to a non existant IP address. 
This effects most routers on the market since embeded
Linux is the most common OS for these."

Perhaps this is the source of my (and probably some
other people's on this mailing list too) current
frustration.

Perhaps due the particular nature of this Linux bug
and the fact that PSTN origination/termination
providers normally use port 4569 on their own machines
for IAX2, a suitable workaround for an asterisk
machine behind a buggy NAT router would be to simply
use some local UDP port other than 4569 for IAX2
connections?
How do you configure asterisk to use a nonstandard
local port for IAX2?


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