[Asterisk-Users] 2 Sip phones behind un-natted Asterisk

[John Fraizer]

Bruce Komito wrote:

 > In sip.conf, try setting canreinvite=no for both lines.
 >
 > Bruce Komito
 > High Sierra Networks, Inc.
 > www.servers-r-us.com
 > (775) 284-5800 ext 115

canreinvite=no will sometimes make a difference but, I believe that what 
most people are running into is what I described in my previous post. 
NAT devices track IP flows.  When you have two "flows" from the asterisk 
server that look like:

ast.erisk.serv.er:5060<->nat.wan.ip.addr:5060<->NAT<->10.1.1.1:5060
ast.erisk.serv.er:5060<->nat.wan.ip.addr:5060<->NAT<->10.1.1.2:5060

The NAT device has no way of deciding which of the two or more SIP 
devices (hardphone or softphone) on the inside the flow is supposed to 
go to.  It looks through it's flow database and sees that there are two 
potential candidates on the inside that could be the owner of the flow.

When you make sure that your SIP devices behind the NAT use unique 
source ports, you end up with this:

ast.erisk.serv.er:5060<->nat.wan.ip.addr:5061<->NAT<->10.1.1.1:5061
ast.erisk.serv.er:5060<->nat.wan.ip.addr:5062<->NAT<->10.1.1.2:5062

I guess that many people don't really truly understand how the NAT works 
on the raw level and it's a plug and play device to them.  For that 
matter, I have been running large scale networks for nearly 16 hears and 
I got bit by the same thing because I assumed that the Cisco would do 
the "Right_Thing" (TM) and choose a random source port.  Once I 
discovered that it didn't do that, it was a simple matter of telling it 
to do so.

I suppose that this is also somewhat the fault of the NAT device 
(linksys/cisco) as well since it should have noticed that there were two 
devices on the inside of the NAT trying to create identical flows.  It 
could have easilly done some mapping to change the source port on the 
WAN side of the connection of one of the flows.

OK.  This is the * list and not routing 701 so, I'll stop now.  Suffice 
it to say that it is a good idea to have unique source ports if you have 
multiple devices on the inside contacting the same device:port on the 
outside.

John Fraizer
EnterZone, Inc
http://www.enterzone.net/